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99 SLS Engine Runs Hot - Looking for Advice/Recommendations


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I recently purchased a 1999 Cadillac Seville SLS. The car has only 67000 miles on it and it appears to have been babied because there is nearly no wear inside or out. The car drove beautifully for the first day or two and I became an instant Caddy lover...THEN, while on a longer local trip, I realized that the engine was running hot.


Here are the facts as I know them:


There is no smoke coming from the tailpipe


The engine idles smoothly and does not run rough at any time


The engine only runs hot when I am traveling up and down hills or if I am on the expressway above 60 miles per hour


There is no sweet smell from the exhaust


There is no indication of coolant in the oil


There is no indication of oil in the coolant


The car passed an emissions test (while owned by me) where they test admissions via the DLC port


The car goes through coolant pretty quickly once it gets hot (over the 5/8 temperature gauge range)


I have ran the car hot to the point where I receive the "idle engine" message and had to pull over and wait 5 minutes for the car to cool down


Once the car cooled down (next morning) I saw signs of a small amount of coolant leaking down into the engine compartment. Since it took approximately 1/2 of a gallon of coolant to replenish and refill the reservoir, it would seem as if the coolant is evaporating.


I have read all of the forums and talked to a number of different people, and it seems like the default cause is the notorious head bolt related head gasket failure.


However, I this is my first Cadillac, and I am having a very difficult time believing such a well taken care of vehicle can have a head gasket failure if it doesn't it exhibit all of the signs consistent with a head gasket failure. I am sincerely hoping that it is a much smaller and less expensive issue that needs to be addressed. Based on all I have read, if I do have a head gasket failure, it would appear to be the type that causes exhaust gases to enter the cooling system. Please let me know how likely this is given the age and condition of the vehicle, and what I can do to ensure that I don't spend unnecessary money on a head gasket repair. Thanks in advance!


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The correct fill level of the surge tank is 2-1/2" below the neck of the tank. If you overfill it, it will puke coolant out the overflow tube. The next thing to check is if the purge line is flowing (the small hose that runs from the waterpump crossover to the throttle body area, across the top of the engine and connects to the top of the surge tank).

Disconnect the purge line, place it into the surge tank and start the engine. There should be coolant flowing out of that line. If not, the line is blocked and that is the source of the overheating.

The car is 15 years old and it may have been taken care of but the coolant must be changed every 5 years/100,000 miles or it will eat up the headgaskets and/or cause the head bolts to pull which will allow exhaust gasses to enter the coolant and superheat the coolant, causing an overheat condition.

If the purge line flows, the next step is to perform a block test to see if exhaust is getting into the coolant. The test can't be performed if coolant was recently added or a false negative test will result.

The engine can be repaired - if the car is in good shape, that may be an option. If you can do the work yourself, the cost will be much less vs. having a dealer or shop do the job. A Northstar that has Timeserts is stronger than when it left the engine plant. My '97 STS (That I purchased with bad head gaskets) has 194,000 miles on it and runs strong as ever.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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Let the seller know it is overheating. The coolant, water pump, thermostat, and many other coolant parts might have been replaced, when the seller realized it's the head gasket, so he sold it to an unsuspecting buyer without disclosing the problem.

But i agree that the surge line should be checked. Many overlook this important component of the cooling system on the N*

Remove the radiator cap when the engine cools, and smell the surge tank for exhaust or gasoline.

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Keep us posted and even if it does need repaired, it's still a reasonable cost. After a long drive and you park the car, how does it start and idle in the am on a cold start-up?

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These engines do not like to be 'babied'. Every once and a while you should go on the highway, drop it in 2nd gear, and punch the throttle WIDE OPEN to about 5000 RPM or so. Do this a few times in a row for a nice running northstar.

Head gaskets do not fail in these engines because of a bad gasket. It is not the gaskets fault at all. They fail because the block is aluminum and the head bolts are steel. Oxidation happens when two dis-similar metals become in contact with eachother. This fact causes the head bolts to loosen and/or the threads to fail - which caused the clamping force between the gasket and the block to become out of spec - and eventually the engine leaks coolant as in a 'real' head gasket failure. The best repair for this is to have STUDS installed into the block (NOT BOLTS or INSERTS) - because studs thread deeper into the block - thus using more block area to 'clamp' the gasket. Mileage means nothing when it comes to this issue - it is just a matter of scientific fact and the time it takes for the natural oxidation to become a problem.

That being said - your issue can simply be (as mentioned) or prehaps a blocked radiator. A blocked radiator was the issue with mine recently. I just flushed it out with a hose and a bunch of gunk came out. Put it back together, dropped 3 sealant tabs in, refilled the coolant and it runs great.

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These engines do not like to be 'babied'. Every once and a while you should go on the highway, drop it in 2nd gear, and punch the throttle WIDE OPEN to about 5000 RPM or so. Do this a few times in a row for a nice running northstar.

Head gaskets do not fail in these engines because of a bad gasket. It is not the gaskets fault at all. They fail because the block is aluminum and the head bolts are steel. Oxidation happens when two dis-similar metals become in contact with eachother. This fact causes the head bolts to loosen and/or the threads to fail - which caused the clamping force between the gasket and the block to become out of spec - and eventually the engine leaks coolant as in a 'real' head gasket failure. The best repair for this is to have STUDS installed into the block (NOT BOLTS or INSERTS) - because studs thread deeper into the block - thus using more block area to 'clamp' the gasket. Mileage means nothing when it comes to this issue - it is just a matter of scientific fact and the time it takes for the natural oxidation to become a problem.

That being said - your issue can simply be (as mentioned) or prehaps a blocked radiator. A blocked radiator was the issue with mine recently. I just flushed it out with a hose and a bunch of gunk came out. Put it back together, dropped 3 sealant tabs in, refilled the coolant and it runs great.

The head bolts do not fail due to galvanic corrosion. The bolts are coated with a special coating and the threads are coaetd with a microencapsulated threadlocker/anti-seize compound. The number one cause of the thread bolts pulling is poor cooling system maintenance resulting in the coolant becoming acidic and eating away at the head bolt threads. Once that happens the threads let go and the bolts become loose and the exhaust gets into the coolant and superheats the coolant which is why the engine overheats.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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I have had many a Northstar engine where all the bolts are tight and especially 95% of the time, Dexcool will eat the head gasket away and induce coolant into the cylinder. I just finished a 98 Deville with all 20 headbolts were tight and cracked loose and #1 cylinder is filling with coolant.

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How does coolant get to the threads of the head bolt? Special coatings wear off when the bolt is 'streteched' during the process of torquing - threads heat up everytime torque is applied and the bolt is stretched. Moisture is always present inside of an engine and around the threads. This is what causes the clamping force to lessen - thus causing coolant to get to the threads and do it's magic.

Just because a bolt is tight when you remove it doesn't mean the clamping force needed to hold the combustion chamber sealed is present. Without knowing how long the bolt was before it was installed (at the factory) - as opposed to when you took it out (during a head bolt/gasket issue) - you cannot say that the bolt was tight.

There sure are lots of aluminum blocks with steel head bolts - and they may last a long time - but like I said, it's only a matter of time before they fail. It's best to install studs in any block especially a performance block/engine like the northstar. Studs are never torqued into a block. They are installed deep in the meat of a block - which uses almost 100% of the clamping area around the cylinder - and they are stretched by the nut - which pulls the head down into the block - as opposed to a bolt pulling the block up to the head....

IMO of course.

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IMO, i am thinking headgasket failure has more to do with air pockets in the block caused either by low coolant levels, or a partially clogged purge line. This causes the head gasket to rust more quickly - especially if the coolant is old and of unfavorable PH. So basically it's a little bit of everything and whatever one can do to minimize any of these, the longer the time to eventual failure. As with exterior body panels, once the rust starts, its tough to stop it. How can we slow rust down on a head gasket?

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How does coolant get to the threads of the head bolt? Special coatings wear off when the bolt is 'streteched' during the process of torquing - threads heat up everytime torque is applied and the bolt is stretched. Moisture is always present inside of an engine and around the threads. This is what causes the clamping force to lessen - thus causing coolant to get to the threads and do it's magic.

Just because a bolt is tight when you remove it doesn't mean the clamping force needed to hold the combustion chamber sealed is present. Without knowing how long the bolt was before it was installed (at the factory) - as opposed to when you took it out (during a head bolt/gasket issue) - you cannot say that the bolt was tight.

There sure are lots of aluminum blocks with steel head bolts - and they may last a long time - but like I said, it's only a matter of time before they fail. It's best to install studs in any block especially a performance block/engine like the northstar. Studs are never torqued into a block. They are installed deep in the meat of a block - which uses almost 100% of the clamping area around the cylinder - and they are stretched by the nut - which pulls the head down into the block - as opposed to a bolt pulling the block up to the head....

IMO of course.

WHen loosening the head bolts a pronounced "crack/pop" noise will be heard if the threads are sound. If the bolt is tight but backs out without the signature noise, the threads are compromised.

The head bolts do not stretch - they are not torque to yield bolts. The only reason the head bolts cannot be re-used is that there is no way to apply the microencapsulated threadlocker in the field so the service manual states to replace the head bolts. If the coolant gets acidic, it will eat the gaskets - finding a path to the bolt holes. If you have ever had a Northstar apart you will see that the head bolt holes are close to the water jacket.

The clamping force is the same with bolts or studs. There are many Northstars that have been repaired with the inserts that are the factory recommended repair that are just fine. The '97 STS I have has almost 90,000 miles on the repair. There is nothing wrong with using studs but they are much more expensive than the inserts that were developed by the factory for this repair. When you use an aftermarket product you are doing the validation of the product.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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I have been using timeserts for 10 plus years before the stud even came upon the marketplace. With Forums now a days its hard to push people one way or another after all the reading they have in front of their eyes.

Now a days I will install whatever the customer wants to bring me, I am bald for a reason and quite honestly sick of fighting about what stud and or product works the best. I have had no issues with my method in the past lets see 20 plus years of working on these cars.

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Whatever the mechanism, you *must* keep the coolant fresh. Once it goes acid, aluminum will suffer. This is not a Cadillac or Northstar issue, every aluminum engine will have problems if you let the coolant go too long.

Look on the Timesert web page and note how many manufacturers, foreign and domestic, use Timesert as their factory recommended repair for stripped aluminum threads anywhere on the engine block.

A simple test for coolant is to use a test voltmeter. Put one terminal to a ground on the chassis or engine, and dip the other into the surge tank. If the voltmeter reads 0.7 Volts or more, the coolant is acid and should be replaced ASAP.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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How does coolant get to the threads of the head bolt? Special coatings wear off when the bolt is 'streteched' during the process of torquing - threads heat up everytime torque is applied and the bolt is stretched. Moisture is always present inside of an engine and around the threads. This is what causes the clamping force to lessen - thus causing coolant to get to the threads and do it's magic.

Just because a bolt is tight when you remove it doesn't mean the clamping force needed to hold the combustion chamber sealed is present. Without knowing how long the bolt was before it was installed (at the factory) - as opposed to when you took it out (during a head bolt/gasket issue) - you cannot say that the bolt was tight.

There sure are lots of aluminum blocks with steel head bolts - and they may last a long time - but like I said, it's only a matter of time before they fail. It's best to install studs in any block especially a performance block/engine like the northstar. Studs are never torqued into a block. They are installed deep in the meat of a block - which uses almost 100% of the clamping area around the cylinder - and they are stretched by the nut - which pulls the head down into the block - as opposed to a bolt pulling the block up to the head....

IMO of course.

WHen loosening the head bolts a pronounced "crack/pop" noise will be heard if the threads are sound. If the bolt is tight but backs out without the signature noise, the threads are compromised.

The head bolts do not stretch - they are not torque to yield bolts. The only reason the head bolts cannot be re-used is that there is no way to apply the microencapsulated threadlocker in the field so the service manual states to replace the head bolts. If the coolant gets acidic, it will eat the gaskets - finding a path to the bolt holes. If you have ever had a Northstar apart you will see that the head bolt holes are close to the water jacket.

The clamping force is the same with bolts or studs. There are many Northstars that have been repaired with the inserts that are the factory recommended repair that are just fine. The '97 STS I have has almost 90,000 miles on the repair. There is nothing wrong with using studs but they are much more expensive than the inserts that were developed by the factory for this repair. When you use an aftermarket product you are doing the validation of the product.

Wow....

All bolts (and studs) stretch when tightened - especially head bolts. Rod, mains, flywheel, wheel studs - to name a few.

The clamping force is not the same. Bolts apply clamping force differently than studs.

If you don't know that or believe that then it is pointless to explain.

I guess I may have been a head gasket in a past life - that's why I defend them against failure -- lol.

FYI - My experience with the northstar is limited, but from what I see it is a very similar design to the Ro5 NASCAR engine, which was phased out of NASCAR....My experience is with high performance 800 to 900 hp sprint car motors, and some street engines. And I will tell you, if you ever see a HP engine builder using a torque wrench to install rod, head, and main cap bolts - it shows that that builder does not understand the concept of bolt stretch and clamping force - and chances are that, that engine will not run as smooth as it could, or as long as it could....

I just bought my first northstar powered car (cheap) and I anticipate issues with it - cus I will not baby it. The knowledge I gain here will be priceless down the road. So thanks for your side of the head GASKET/Bolt over heating issue, but please excuse me if I keep it in my brain a head BOLT/Gasket issue.... lol

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How does coolant get to the threads of the head bolt? Special coatings wear off when the bolt is 'streteched' during the process of torquing - threads heat up everytime torque is applied and the bolt is stretched. Moisture is always present inside of an engine and around the threads. This is what causes the clamping force to lessen - thus causing coolant to get to the threads and do it's magic.

Just because a bolt is tight when you remove it doesn't mean the clamping force needed to hold the combustion chamber sealed is present. Without knowing how long the bolt was before it was installed (at the factory) - as opposed to when you took it out (during a head bolt/gasket issue) - you cannot say that the bolt was tight.

There sure are lots of aluminum blocks with steel head bolts - and they may last a long time - but like I said, it's only a matter of time before they fail. It's best to install studs in any block especially a performance block/engine like the northstar. Studs are never torqued into a block. They are installed deep in the meat of a block - which uses almost 100% of the clamping area around the cylinder - and they are stretched by the nut - which pulls the head down into the block - as opposed to a bolt pulling the block up to the head....

IMO of course.

WHen loosening the head bolts a pronounced "crack/pop" noise will be heard if the threads are sound. If the bolt is tight but backs out without the signature noise, the threads are compromised.

The head bolts do not stretch - they are not torque to yield bolts. The only reason the head bolts cannot be re-used is that there is no way to apply the microencapsulated threadlocker in the field so the service manual states to replace the head bolts. If the coolant gets acidic, it will eat the gaskets - finding a path to the bolt holes. If you have ever had a Northstar apart you will see that the head bolt holes are close to the water jacket.

The clamping force is the same with bolts or studs. There are many Northstars that have been repaired with the inserts that are the factory recommended repair that are just fine. The '97 STS I have has almost 90,000 miles on the repair. There is nothing wrong with using studs but they are much more expensive than the inserts that were developed by the factory for this repair. When you use an aftermarket product you are doing the validation of the product.

Wow....

All bolts (and studs) stretch when tightened - especially head bolts. Rod, mains, flywheel, wheel studs - to name a few.

The clamping force is not the same. Bolts apply clamping force differently than studs.

If you don't know that or believe that then it is pointless to explain.

I guess I may have been a head gasket in a past life - that's why I defend them against failure -- lol.

FYI - My experience with the northstar is limited, but from what I see it is a very similar design to the Ro5 NASCAR engine, which was phased out of NASCAR....My experience is with high performance 800 to 900 hp sprint car motors, and some street engines. And I will tell you, if you ever see a HP engine builder using a torque wrench to install rod, head, and main cap bolts - it shows that that builder does not understand the concept of bolt stretch and clamping force - and chances are that, that engine will not run as smooth as it could, or as long as it could....

I just bought my first northstar powered car (cheap) and I anticipate issues with it - cus I will not baby it. The knowledge I gain here will be priceless down the road. So thanks for your side of the head GASKET/Bolt over heating issue, but please excuse me if I keep it in my brain a head BOLT/Gasket issue.... lol

Yes the bolts stretch slightly but they are NOT torque to yield bolts which is what your posts imply. Torque to yield bolts cannot be reused as they are are designed to yield in the elastic region to provide constant clamping force but again, the Northstar headbolts are not torque to yield bolts. As I have explained in my earlier posts, the only reason they are not reusable is that there is no way to apply the threadlocker in the field - it is simpler to specify new bolts be used when replacing the headgaskets.

The head bolts have a torque and an angle specification - as do the rod bearings and the lower crankcase halves. The Northstar engine does not use conventional caps like a slamm block Chevy engine - the crankcase is split at the crank centerline and the "caps" are built into the halves. A torque wrench is used to set the bolts to an inital static torque and then the spec. is an angular displacement. This is the case for all the critical fasteners in the engine.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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KHE seems to reflect a knowledge of Goodwrench training at times. I'll add a little mech-geek to his information.

Torque-and-twist bolts are tension springs. The twist angle times the thread pitch defines the stretch. The stretch times the tension spring constant is the clamping force for one bolt.

There may be some gasket compression that is subtracted from the twist angle times the thread pitch to get the true head bolt stretch. Compression of the head and block is there but probably negligible in estimating clamping force.

To match the factory clamping force with a different bolt or a stud, you need to match the factors in the equation:

Force = [twist angle] X [tread pitch] X [bolt stretching force constant]

And, that's just the beginning. One of the reasons that torque-and-twist bolts are used is that as the block heats up and expands, the bolt stretches more and the clamping force increases, and the increase is the expansion amount (minus the expansion of the bolt itself) times the bolt stretching force constant. Since all these numbers are known, and in fact most of them are selected when the bolt length, diameter, alloy, thread pitch, and twist angle are selected, you control the clamping force over temperature in the design process.

I haven't seen anything in the aftermarket products that addresses control of clamping force over temperature.

More is not necessarily better for clamping force. The head, block, and gasket are designed for a range of clamping force. Too much hot clamping force and the gasket will take a set, and will leak when the engine cools off.

I haven't heard of problems with any of the popular aftermarket solutions using studs or non-GM head bolts. But if it was my engine I would either run the numbers for an aftermarket solution before I put it on my engine.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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How does coolant get to the threads of the head bolt? Special coatings wear off when the bolt is 'streteched' during the process of torquing - threads heat up everytime torque is applied and the bolt is stretched. Moisture is always present inside of an engine and around the threads. This is what causes the clamping force to lessen - thus causing coolant to get to the threads and do it's magic.

There sure are lots of aluminum blocks with steel head bolts - and they may last a long time - but like I said, it's only a matter of time before they fail. It's best to install studs in any block especially a performance block/engine like the northstar. Studs are never torqued into a block. They are installed deep in the meat of a block - which uses almost 100% of the clamping area around the cylinder - and they are stretched by the nut - which pulls the head down into the block - as opposed to a bolt pulling the block up to the head....

IMO of course.

The statement in bold indicates moisture is in the threads before the gasket has failed?

I used studs in mine because I don't/didn't KNOW timesert is as good a repair. My rational was, there is a reason the top fuelers etc. use studs and the difference in price is worth it to me. If I have to do headgaskets on my LS1, I will not be using studs though.

I am curious how I know who wins this contest of "I know more"? It is definitely entertaining. Of course a fact is a fact until it is wrong, ala the earth is flat :glare:.

Now back to the OP question....I personally would say use the test kit from post #6. I see no reason to start anywhere else. If there is combustion in the cooling, you have an absolute answer. If it passes the test as stated "with coolant that has had a few miles on it" then I would go directly to the bolt with a hole and preceed from there.

As far as the oil in the coolant and vice versa, that indicator doesn't apply to Northstar engines. I drove my car many months and many miles after I was sure the gaskets had failed. Sometimes not adding coolant for 500 miles and other times within 100 miles. I decided it had to get repaired when I couldn't make a 30 minute drive anymore.

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Top Fuelers use studs because (simply put) studs are better, and you can measure stretch much easier.

The fasteners around a cylinder (in a street engine) have to hold (or apply a clamping force) of about 42,000Lbs in order to hold the head/gasket/block around the combustion of the chamber. In a Top Fuel engine it is probably reaching 100K plus.

The Northstar has 4 fasteners around the cylinder (chevy small blocks have 5) which puts a load of around 10500Lbs per fastener in order to hold everything together. Also, a Nstar has a compression ration of around 10.3 to 1 - which is pretty high for a street engine, and it is also the reason you MUST use at least 93 octaine fuel.....

When a bolt is used two things have to be overcome before any clamping force is applied - the friction of the threads in the block and the friction where the bolt head contacts. When a stud is used (simply put) the friction of the threads in the block is illiminated. Keeping the friction/heat source out of the block has many benefits. ALSO, studs use more of the block surface to apply the clamping force because they are set deeper..

This argument can go on forever - but there is one thing I guarantee - Hendrick, Childress, Rousch, Fisher, Gaerte to name a few builders, ALWAYS use studs - the reason (apparently) is debateable....

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The information I posted was from the GM powertrain engineer who used to frequent this forum. His take on racing engines was what is good for racing is not alway good for street cars and vise versa.

The combustion chambers of the Northstar engine were redesigned for the 2000 model year with computer simulation software that was not available in 1990/1991 when the Northstar development began. The result was a better combustion design that did not require the use of premium fuel. The 2000+ model year cars did not have the PREMIUM FUEL ONLY on the dash.

All the Northstar engines have a knock sensor that would retard the timing if spark knock was present before the driver even knew it. The same GM Engineer also stated regular unleaded fuel could be used but you might not obtain the fuel economy or performance as you would with premium.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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Compression ratio dictates what octane your fuel has to be. Gasoline will combust, without a spark, at a certain compression (which is exactly how a diesel engine works). The higher the compression, the easier gasoline will combust so, the higher the octane has to be. Higher octane (premium gas) is less volatile than lower (regular gas). The tick or ping you hear when a lower octane fuel is used in a high compression engine, is the slamming of the piston pin, rod bearing, and main bearing against their respective clearances - because the piston "fires" before it is supposed to - to early before top dead center - with no spark. If this happens too much there will be catastrophic failures in the engine. This is called 'detonation' and rightfully so.

Compression ratios for the Nstar are - prior to 2000, 10.3 to 1. After 2000, it went to 10 to 1. @ 10.3:1 you need minumum 93 octane. @ 10:1 you need minimum 92 octane. Where I come from regular unleaded is 87 octane, which would set the knock sensors off and probably shut down the engine. There is NEVER a performance edge from using a higher octane fuel in an engine that does not require it with repect to the compression ratio, and it is hard to believe that an engineer said that...No wonder the Nstar is famous for leaks....wow

It is true that what is good in racing isn't always good in in the street - but using 4 fasteners to clamp a combustion chamber, IMO, requires the strongest type of fastening, which is a stud....

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The Northstar originally had a 10.3:1 compression ratio and the use of premuim fuel was recommended. The 2000+ had a redesigned combustion chamber and a slightly lower 10.1:1 compression which allowed the engine to burn regular fuel as outlined below from the GM Powertrain engineer who was directly involved with the Northstar engine:

"Engine test data determines how much compression an engine can have and run at the optimum spark advance.

For emphasis, the design compression ratio is adjusted to maximize efficiency/power on the available fuel. Many times in the aftermarket the opposite occurs. A compression ratio is "picked" and the end user tries to find good enough fuel and/or retards the spark to live with the situation...or suffers engine damage due to detonation.

Another thing you can do is increase the burn rate of the combustion chamber. That is why with modem engines you hear about fast burn chambers or quick burn chambers. The goal is the faster you can make the chamber burn, the more tolerant to detonation it is. It is a very simple phenomenon, the faster it burns, the quicker the burn is completed, the less time the end gas has to detonate. If it can't sit there and soak up heat and have the pressure act upon it, it can't detonate.

If, however, you have a chamber design that burns very slowly, like a mid-60s engine, you need to advance the spark and fire at 38 degrees BTDC. Because the optimum 14 degrees after top dead center (LPP) hasn't changed the chamber has far more opportunity to detonate as it is being acted upon by heat and pressure. If we have a fast burn chamber, with 15 degrees of spark advance, we've reduced our window for detonation to occur considerably. It's a mechanical phenomenon. That's one of the goals of having a fast burn chamber because it is resistant to detonation.

There are other advantages too, because the faster the chamber burns, the less spark advance you need. The less time pistons have to act against the pressure build up, the air pump becomes more efficient. Pumping losses are minimized. In other words, as the piston moves towards top dead center compression of the fuel/air mixture increases. If you light the fire at 38 degrees before top dead center, the piston acts against that pressure for 38 degrees. If you light the spark 20 degrees before top dead center, it's only acting against it for 20. The engine becomes more mechanically efficient.

There are a lot of reasons for fast burn chambers but one nice thing about them is that they become more resistant to detonation. A real world example is the Northstar engine from 1999 to 2000. The 1999 engine was a 10.3:1 compression ratio. It was a premium fuel engine. For the 2000 model year, we revised the combustion chamber, achieved faster burn. We designed it to operate on regular fuel and we only had to lower the compression ratio .2 to only 10:1 to make it work. Normally, on a given engine (if you didn't change the combustion chamber design) to go from premium to regular fuel, it will typically drop one point in compression ratio: With our example, you would expect a Northstar engine at 10.3:1 compression ratio, dropped down to 9.3:1 in order to work on regular. Because of the faster burn chamber, we only had to drop to 10:1. The 10:1 compression ratio still has very high compression with attendant high mechanical efficiency and yet we can operate it at optimum spark advance on regular fuel. That is one example of spark advance in terms of technology. A lot of that was achieved through computational fluid dynamics analysis of the combustion chamber to improve the swirl and tumble and the mixture motion in the chamber to enhance the burn rate"

One of the reasons race car engines use studs is that the heads are removed and replaced several times over the life of the engine - that is not the case with a street engine. If the heads are removed once during the engine's lifetime, that would be about it. The small block Chevy engine was designed back in the 1950s when there weren't the computer aided engineering tools that were available in the 1990's so things were over-engineered. That can be said for a lot of products, not just cars.

The knock sensor does not "shut down" the engine - it feed a signal to the ECM to retard the timing until the spark knock is gone - all unnoticed by the driver.

The microencapsulated threadlocker on the head bolts acts as a lubricant and a high strength threadlocker. This substance is also under the bolt heads as well. The bolts are torqued to a static torque of 22 ft-lbs. and then an additional 180° in three separate increments. This provides the most uniform clamping pressure of the heads to the block.

A Cadillac 4.9 engine which required premuim fuel or it would knock, did not have a knock sensor. The engine could be made to run on regular or midgrade fuel by retarding the ignition timing. Performance and/or fuel economy would have a slight loss.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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It is obvious you are clueless when it comes to compression ratio, gasoline, and octane.

And you can't read between the bullshit lines the GM Engineer is feeding you.

Regular unleaded is 87 or 89 octane. It will detonate in an engine with a compression ratio of 10.3:1 or 10:1. The timing event that the computer controls will have no effect on the combustion of the gasoline, thus rendering computer aided retardation of the timing (knock sensor included) USELESS.

GM thinks 92 octane fuel is regular? Not in the real world....

The type of engine has NO BEARING on the fact that gasoline needs to be 'tamed' (so to speak) inside of the combustion chamber. The cheapest way it is tamed is by adding octane. The more octane, the more tame, and the more likely it won't detonate in a high compression engine like the Nstar - or ANY OTHER high compression engine.

Do you know what causes the "knock" that a knock sensor is searching for?

I remember when engines (even fuel injected ones) actually ran without the use of computers. What the heck, Sprint Cars don't even have a battery..

It matters not how many computer controlled devices are attatched to an engine - or how many computer designs are rendered. The mechanics of the crankshaft, rod, piston, piston rings, and the explosive power of gasoline remain the same.

I can't believe I'm still replying to this topic....I hope mostfocused' 99 sls overheating problem is solved

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"It is obvious you are clueless when it comes to compression ratio, gasoline, and octane." - and you are what kind of engineer to convince me of your brilliance?

"thus rendering computer aided retardation of the timing (knock sensor included) USELESS."- so every can maker has included a "

useless" knock sensor to ? raise the price of the car, make useless parts to replace??

"I remember when engines (even fuel injected ones) actually ran without the use of computers. What the heck, Sprint Cars don't even have a battery..

It matters not how many computer controlled devices are attatched to an engine - or how many computer designs are rendered. The mechanics of the crankshaft, rod, piston, piston rings, and the explosive power of gasoline remain the same."-which is why todays cars get more gas mileage and make more power???

"I can't believe I'm still replying to this topic"- that makes two of us, though I imagine there are many more thinking the same, so you better get your next unsubstantiated smart remark on here quick before they shut this thread down. Speaking of, you had no reply to "The statement in bold indicates moisture is in the threads before the gasket has failed?" that would be my quote of your quote, in case you can't read between the lines of a bullshit member that gets on here to learn and be entertained, rather than spout nonsense.

You are obviously on the wrong forum

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Where there is air - there is moisture. The air/moisture corrodes the steel bolt, and over time the clamping force, required to hold the 10500lbs of lifting force applied on the head everytime the cylinder fires, is compromised to the point of letting coolant enter the combustion chamber and the bolt threads - then everyone blames the head gasket...

I am no engineer, I'm just a guy who learned from all of his failures, and learned why the failures happened. I don't google my knowledge, I lived it.

Knock sensors and computers can do nothing about fact that gasoline (and ANY liquid) will combust under a certain pressure. When it happens at the wrong time inside on ANY engine, all they can do is sense it and try to adjust for it.

If you can't (or don't) understand how gasoline and octane work, I suggest you learn about it. And try to read and comprehension things before comment on them....

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