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Does anyone have the answer to the northstar overheating issue? There has been many post about the issue, but no answer. If you do not know the history of this issue, please read cowgirls post about the slushy sound post. All attempts to solve the problem has been taken, but no cure. I cant believe that no answer has been given because many cars have been taken to so call qualified mechanics. There should be some type of tech memo concerning this issue available. This has to be a known problem with Northstar motors, because it has happened numerous times. I have read this happening during the periods on 98-2004 northstar motors. I have a question. They say that these motors have a pressure release valve that automatically releases pressure. Is this valve replaceable? I think that it is something intermittent. Because the temp needle only moves up at certain times. This has lead people to believe that it is a thermostat, water pump, head gasket, fan, etc... issue. It has been proven that these are not the culprits. They have been changed out and have not solved the problem. If anyone know that answer to this annoying issue, please pass it on. It would be very appreciated. Thank you for your time.

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The "pressure relief valve" is in the surge tank cap. The cap is either a 16# or 18# cap. If it does not hold pressure, it will run hot or overheat, depending on how much pressure it holds.

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Your entire premise is that the Northstar has an "overheating issue"

First, the Northstar does not have an "OVERHEATING ISSUE". No tech memo?, we have helped MANY an owner with overheating, there are SPECIFIC things to check.

Are you trying to help Cowgirl here or are you trying to figure out an overheating problem for your car?

PLEASE detail your symptoms IN DETAIL and we can help, your thread is ALL OVER THE PLACE and makes NO SENSE at all.

Give us details and specifics, you are talking very general at this point and its not helpful

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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What flow does this involve? Does this hold pressure for the coolant? Also, what is the inter-workings of the coolant to the A/C? The reason I ask is because, I have and had the same issue with the over heating. I had my A/C serviced and recharged. I had my drier replaced. I was having heavy trickling sound behind the dash. It started light, then it progressively got worst. After I had my A/C serviced,it went away. It is starting to come back now, three months later. My temp gauge is rising after highway travel. After a while, the gauge will return to mid way on the gauge. This is what is so puzzling. If it is hot, it should stay hot, don't you think. That's what I think it has something to do with pressure release. That doesn't explain why it went away after servicing the A/C though. This has really been a headache. No on seems to know how to solve it either. That is also frustrating. Need help.

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Your entire premise is that the Northstar has an "overheating issue"

First, the Northstar does not have an "OVERHEATING ISSUE". No tech memo?, we have helped MANY an owner with overheating, there are SPECIFIC things to check.

Are you trying to help Cowgirl here or are you trying to figure out an overheating problem for your car?

PLEASE detail your symptoms IN DETAIL and we can help, your thread is ALL OVER THE PLACE and makes NO SENSE at all.

Give us details and specifics, you are talking very general at this point and its not helpful

It make sense, you may not understand what I am saying but its really simple.

I am having overheating issues, and all the post in cowgirls has not provided and answer. Here are the symptoms:

After driving for about 30min, the temp needle goes up 1 notch past midway. This is mainly highway driving. After a while the needle goes back to midway. In the past I had the trickling sound behind the dash, and it progressively got worse. I had my A/C serviced, and all I had was my drier replaced and freon filled. The sound and the overheating went away. Now 3 months later, its coming back slowly. I have had the radiator replaced, the water pump is not leaking, the thermostat is good. Pressure test revealed the head gaskets are good. Im looking for help. Machanics do not have the answer either.

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The AC had NOTHING to do with overheating, nothing

You need to be clear, what do you mean by, WHAT FLOW DOES THIS INVOLVE and what do you mean by DOES THIS HOLD PRESSURE FOR THE COOLANT, what is THIS, the CAP?

Ranger was speaking of the CAP...

Basically you need to inspect the cooling system fully as follows

  1. Check the coolant concentration is must be at least 50/50 but not more than 66/33
  2. Check the PURGE line to be sure it is CLEAR and flowing
  3. Check the water pump belt and tensioner
  4. Pressure test the cooling system to be sure that it holds pressure ANYTHING that leaks pressure MUST be repaired or replaced
  5. Change the thermostat if it has not been changed
  6. Change the radiator/tank cap with an OEM cap if it has not been changed or if it is not holding pressure, but TEST IT
  7. Check the operation of the cooling fans

The northstar can leak at the following locations

  1. Crossover seals
  2. Radiator end tanks
  3. Hose clamps
  4. Heater pipes behind engine
  5. Water pump seal
  6. Water pump body seal
  7. Tank can crack
  8. Cap can release pressure too low
  9. Hoses
  10. Heater core
  11. Head gaskets

To eliminate a head gasket problem the coolant should be tested for combustion by products NAPA sells a tester. In addition, a cylinder pressure test can be performed, whereas each cylinder is pressurized and pressure drop is watched and you listen for bubbles at the tank.

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Your entire premise is that the Northstar has an "overheating issue"

First, the Northstar does not have an "OVERHEATING ISSUE". No tech memo?, we have helped MANY an owner with overheating, there are SPECIFIC things to check.

Are you trying to help Cowgirl here or are you trying to figure out an overheating problem for your car?

PLEASE detail your symptoms IN DETAIL and we can help, your thread is ALL OVER THE PLACE and makes NO SENSE at all.

Give us details and specifics, you are talking very general at this point and its not helpful

It make sense, you may not understand what I am saying but its really simple.

I am having overheating issues, and all the post in cowgirls has not provided and answer. Here are the symptoms:

After driving for about 30min, the temp needle goes up 1 notch past midway. This is mainly highway driving. After a while the needle goes back to midway. In the past I had the trickling sound behind the dash, and it progressively got worse. I had my A/C serviced, and all I had was my drier replaced and freon filled. The sound and the overheating went away. Now 3 months later, its coming back slowly. I have had the radiator replaced, the water pump is not leaking, the thermostat is good. Pressure test revealed the head gaskets are good. Im looking for help. Machanics do not have the answer either.

Pay no attention to cowgirls thread, there are thousands of threads here with lots of informtion. Successful diagnosis depends upon good communication of a problem

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Your entire premise is that the Northstar has an "overheating issue"

First, the Northstar does not have an "OVERHEATING ISSUE". No tech memo?, we have helped MANY an owner with overheating, there are SPECIFIC things to check.

Are you trying to help Cowgirl here or are you trying to figure out an overheating problem for your car?

PLEASE detail your symptoms IN DETAIL and we can help, your thread is ALL OVER THE PLACE and makes NO SENSE at all.

Give us details and specifics, you are talking very general at this point and its not helpful

It make sense, you may not understand what I am saying but its really simple.

I am having overheating issues, and all the post in cowgirls has not provided and answer. Here are the symptoms:

After driving for about 30min, the temp needle goes up 1 notch past midway. This is mainly highway driving. After a while the needle goes back to midway. In the past I had the trickling sound behind the dash, and it progressively got worse. I had my A/C serviced, and all I had was my drier replaced and freon filled. The sound and the overheating went away. Now 3 months later, its coming back slowly. I have had the radiator replaced, the water pump is not leaking, the thermostat is good. Pressure test revealed the head gaskets are good. Im looking for help. Machanics do not have the answer either.

Check your purge line to make sure that its clear and check the list I provided above. STAY OUT OF COWGIRLS thread as an example that the NS has an "OVERHEATING ISSUE" and lets deals with your issue. I AGREE mechanics can be dense and the Northstar is a unique engine that needs specific experience.

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Your entire premise is that the Northstar has an "overheating issue"

First, the Northstar does not have an "OVERHEATING ISSUE". No tech memo?, we have helped MANY an owner with overheating, there are SPECIFIC things to check.

Are you trying to help Cowgirl here or are you trying to figure out an overheating problem for your car?

PLEASE detail your symptoms IN DETAIL and we can help, your thread is ALL OVER THE PLACE and makes NO SENSE at all.

Give us details and specifics, you are talking very general at this point and its not helpful

It make sense, you may not understand what I am saying but its really simple.

I am having overheating issues, and all the post in cowgirls has not provided and answer. Here are the symptoms:

After driving for about 30min, the temp needle goes up 1 notch past midway. This is mainly highway driving. After a while the needle goes back to midway. In the past I had the trickling sound behind the dash, and it progressively got worse. I had my A/C serviced, and all I had was my drier replaced and freon filled. The sound and the overheating went away. Now 3 months later, its coming back slowly. I have had the radiator replaced, the water pump is not leaking, the thermostat is good. Pressure test revealed the head gaskets are good. Im looking for help. Machanics do not have the answer either.

Lets get something straight here, your explanation DOES NOT MAKE SENSE. I am not new at this, my chosen profession is analysis of detailed concepts and I wrote 100 page reports weekly for 15 years.

Its REALLY SIMPLE because its in YOUR head, and you have not communicated it well. I am sure my fellow members will attest to that.

Before this post of yours your posting was vague, you had the preconception that the Northstar overheats, and you had a chip on your shoulder or an attitude.

You are coming here for help, when you come for help, try to be OPEN to our suggestions and recommendations, NON JUDGEMENTAL, and carry no attitude because you have CONCLUDED to NEGATIVE ideas about the NORTHSTAR. This is an enthusiasts site, we approach things from a POSITIVE perspective NOT a NEGATIVE one.

I compare diagnostics to an INVERTED CONE, there are LOTS of potential causes at the beginning of an analysis but you ELIMINATE each cause ONE BY ONE through testing and verification until you are left with the CAUSE. BUT, you approach this board with a negative attitude or preconceptions and you will get less help.

Negativity flows freely in the world today, try to be positive and communicate clearly and we can help. Your basic mechanic out there sucks because they are judgemental, have a negative bias, parts replace, and are driven by money. WE ARE NOT

WE CAN HELP YOU. We have helped hundreds of owners with overheating problems.

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Pressure test revealed the head gaskets are good.

What kind of pressure test? A cooling system pressure test? That will not detect a bad head gasket. A cylinder leak down test where each cylinder is pressurized will. Which one was done? You can also get a block test kit and sample the air in the surge tank for exhaust gases.

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Your entire premise is that the Northstar has an "overheating issue"

First, the Northstar does not have an "OVERHEATING ISSUE". No tech memo?, we have helped MANY an owner with overheating, there are SPECIFIC things to check.

Are you trying to help Cowgirl here or are you trying to figure out an overheating problem for your car?

PLEASE detail your symptoms IN DETAIL and we can help, your thread is ALL OVER THE PLACE and makes NO SENSE at all.

Give us details and specifics, you are talking very general at this point and its not helpful

It make sense, you may not understand what I am saying but its really simple.

I am having overheating issues, and all the post in cowgirls has not provided and answer. Here are the symptoms:

After driving for about 30min, the temp needle goes up 1 notch past midway. This is mainly highway driving. After a while the needle goes back to midway. In the past I had the trickling sound behind the dash, and it progressively got worse. I had my A/C serviced, and all I had was my drier replaced and freon filled. The sound and the overheating went away. Now 3 months later, its coming back slowly. I have had the radiator replaced, the water pump is not leaking, the thermostat is good. Pressure test revealed the head gaskets are good. I'm looking for help. Mechanics do not have the answer either.

Lets get something straight here, your explanation DOES NOT MAKE SENSE. I am not new at this, my chosen profession is analysis of detailed concepts and I wrote 100 page reports weekly for 15 years.

Its REALLY SIMPLE because its in YOUR head, and you have not communicated it well. I am sure my fellow members will attest to that.

Before this post of yours your posting was vague, you had the preconception that the Northstar overheats, and you had a chip on your shoulder or an attitude.

You are coming here for help, when you come for help, try to be OPEN to our suggestions and recommendations, NON JUDGEMENTAL, and carry no attitude because you have CONCLUDED to NEGATIVE ideas about the NORTHSTAR. This is an enthusiasts site, we approach things from a POSITIVE perspective NOT a NEGATIVE one.

I compare diagnostics to an INVERTED CONE, there are LOTS of potential causes at the beginning of an analysis but you ELIMINATE each cause ONE BY ONE through testing and verification until you are left with the CAUSE. BUT, you approach this board with a negative attitude or preconceptions and you will get less help.

Negativity flows freely in the world today, try to be positive and communicate clearly and we can help. Your basic mechanic out there sucks because they are judgemental, have a negative bias, parts replace, and are driven by money. WE ARE NOT

WE CAN HELP YOU. We have helped hundreds of owners with overheating problems.

Thats not what I meant. I did not come here with a chip on my shoulder. I came for answers. Sorry if you took it that way, but that was not my intention. I apologize if it offended you in any way. If i came with an attitude, I wouldn't have added you as a friend. Again, I apologize for the miscommunication. Thank you for the info that you provided. I hope that I can tap into your expertise in solving my issue.

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No problem, just check and eliminate each item in the list I provided. If you don't understand something ask a question. It is also a good idea to search. Example, search PURGE LINE and you will come up with a whole bunch of information and PHOTOS as to the "bolt with a hole in it" is located, etc.

Remember it is summer and temps are hot now, so you may run a bit warmer. If you need any seals (crossover), radiator end tanks, water pump body seals, etc, fix them, fix all items that leak pressure

Do not confuse AC system noise with cooling system noise,

Stay positive you will fix this, these cars run cool when the cooling systems are kept up to spec

The first step should be checking the purge line, then coolant concentration, then pressure test for leaking pressure. You WILL NOT see coolant leaks that leak under pressure, the coolant immediately evaporates.

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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  • 2 weeks later...

I have had two Devilles that have had overheating problems. The check list that BodybyFisher provided is comprehensive and if followed should isolate the problem. Given my experience I would surely do the test that shows if there is combustion gases in the coolant. Combustion gases in the coolant is an indicator that there is a head gasket problem.

My 1989 Deville with 80,000 miles overheated and after a lot of run around the "block checker" test was run and showed a leaky head gasket. I replaced the engine with a Jasper engine with a 3 year warranty which started overheating after 3 years and 2 months.

My 1999 Deville with 78,000 miles started using coolant, about 1 quart per 100 miles, it never overheated.

Two different "block checker" tests showed a leaky head gasket. The only option my Cadillac dealer gave me was to replace the engine for $7000-$8000. My car is in excellent condition and only worth about $3000-$3500 so a new engine was out of the question. A local shop that works on North Stars would fix it for $2300. He said he removes the engine and buys a kit that replaces the stripped bolt holes in the block. He gives a 1 year warranty.

My other option was to add a sealer product to the coolant. These sealers only work if you get all the antifreeze out of the system so I took it to the shop that had flushed my radiator. They flushed out all the coolant, replaced it with clear water, added Bar1100 block seal, let it run about 1 hour, drained out all the coolant and let it dry for 24 hours, then refilled with 50/50 coolant. Cost was $150. Now I have a car that both uses coolant and overheats.

You would think that a $35,000 automobile should last longer than 78.00 miles. No more Cadillacs for me.

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Sealers DO NOT work in a Northstar, so that is NOT an option

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Sealers DO NOT work in a Northstar, so that is NOT an option

Hey BodybyFisher, do you know why sealers do not work in a Northstar? I have already gone down the path of no return by using a sealer and I would like to inform the shop that did my work as to what happened and why. It may keep someone else from doing what I did.

Advanced Auto has a product called Blue Magic and they clearly state that it works on Northstar engines. Best I could tell this is a Sodium Silicate product as is most other sealers. This is the one piece of information that gave me false hope.

As you know a 11 year old Caddy has little trade in value and if I spent $2300 on it would not improve the trade in value. I gambled and lost.

Do you have any suggestion as to what to do with a car in excellent condition that has a blown head gasket?

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Yes, the reason head gasket sealers do not work on the Northstar is because typically the bolts PULL OUT of the block as you noted, the bolt holes need attention either timeserting or studs. We have debated the reasons why the bolts pull out, but the original reason was that the coolant becomes acidic, rots the head gasket, gets into the head bolt threads and damages the threads. There are other theories that we have discussed here also

Here is a link to a photo of a blow head gasket, click on the photo to enlarge it, the blow out is between the cylinders. I do not believe that sealer will work in a head gasket blow out like that, if the blow out is between the cylinder and a coolant jacket it may be able to be sealed however temporarily with the sealer you are talking about...

http://holdenpaedia.oldholden.com/Blown_Head_Gasket

Typically those head gasket sealers fill a BREACH in the head gasket itself and can work. But because on the Northstar the bolts PULL OUT of the block, when the piston fires it has the effect of pushing up and MOVING the head upward. That movement precudes the use of sealer as the movement creates problems for the sealer setting and maintaining integrity.

I hope that helps.

The only fix is to timesert the block or to stud the block and we can provide lots of information for both methods

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Listen, many times owning a car is a passionate decision and not a good financial decision nor should it be considered one. You are talking about UTILITY here, the best car you can drive for say, $3000 (the cost of your engine repair)....

If you like the car, its in good condition its worth fixing. Keep in mind that the DEVILLE will probably be the LAST big car that GM will make, you have a car that will never be made again (at least until the current administration ends, :lol:). Long trips are amazing in a big Deville. Just yesterday I was thinking, I want to get a big Deville, JUST because they are going the way of the dinosaurs

If you get the engine repaired by a reputible mechanic and it is timeserted or studded, you will have an engine that lasts a long time

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Makes sense, thanks you provide a valuable service.

Well thank you, if you keep your Caddy, consider joining as a supporting member, its inexpensive, there are benefits and you support this terrific resource.

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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GEEZ, Mr Slimm doesnt even know who I am, and he gets smashed by you guys. SO he has the same problem as I do, is that a first time, cars do have issues. AND MINE IS ONE> not a chip on the shoulder, just a pain in the butt. Just to update you in my thresd----I have the same issue -still- however, I have made 3- 300 mile trips, hot weather, no air conditioner on and it has never overheated, but let me make a 30 mile trip with air conditioner on and it spikes right up. Thought it was OK, but I feel it may have a small compression leak somewhere, just not ready to pay a fortune to have it found, so I live with it. Sorry MR SLIMM for getting beat up by mentioning me.

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GEEZ, Mr Slimm doesnt even know who I am, and he gets smashed by you guys. SO he has the same problem as I do, is that a first time, cars do have issues. AND MINE IS ONE> not a chip on the shoulder, just a pain in the butt. Just to update you in my thresd----I have the same issue -still- however, I have made 3- 300 mile trips, hot weather, no air conditioner on and it has never overheated, but let me make a 30 mile trip with air conditioner on and it spikes right up. Thought it was OK, but I feel it may have a small compression leak somewhere, just not ready to pay a fortune to have it found, so I live with it. Sorry MR SLIMM for getting beat up by mentioning me.

He did not get SMASHED by "us guys" because he mentioned you, he got smashed because he made a lot of false assumptions/conclusions, and USED you as support. As I said, the Northstar does not have an overheating 'issue'

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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  • 3 weeks later...

GEEZ, Mr Slimm doesnt even know who I am, and he gets smashed by you guys. SO he has the same problem as I do, is that a first time, cars do have issues. AND MINE IS ONE> not a chip on the shoulder, just a pain in the butt. Just to update you in my thresd----I have the same issue -still- however, I have made 3- 300 mile trips, hot weather, no air conditioner on and it has never overheated, but let me make a 30 mile trip with air conditioner on and it spikes right up. Thought it was OK, but I feel it may have a small compression leak somewhere, just not ready to pay a fortune to have it found, so I live with it. Sorry MR SLIMM for getting beat up by mentioning me.

He did not get SMASHED by "us guys" because he mentioned you, he got smashed because he made a lot of false assumptions/conclusions, and USED you as support. As I said, the Northstar does not have an overheating 'issue'

I have two questions. Is there anyone out there that have solved this problem? I see a lot of to-do's, which is great information, but where are the comments of people that have solved the overheating pain? Where are the testimonials of the people that fixed this issue and what they did to fix it? That's what Im waiting on to read. That way, a lot of money is not spent chasing the wrong issues. I have another question. What do you do next after you do a NAPA test and it doesn't show any signs of exhaust, whats the next step?

Edited by Mr Slimm
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No problem, just check and eliminate each item in the list I provided. If you don't understand something ask a question. It is also a good idea to search. Example, search PURGE LINE and you will come up with a whole bunch of information and PHOTOS as to the "bolt with a hole in it" is located, etc.

Remember it is summer and temps are hot now, so you may run a bit warmer. If you need any seals (crossover), radiator end tanks, water pump body seals, etc, fix them, fix all items that leak pressure

Do not confuse AC system noise with cooling system noise,

Stay positive you will fix this, these cars run cool when the cooling systems are kept up to spec

The first step should be checking the purge line, then coolant concentration, then pressure test for leaking pressure. You WILL NOT see coolant leaks that leak under pressure, the coolant immediately evaporates.

If you have a coolant leak that leaks under pressure, how is this identified, 1... 2, what is the symptoms of?

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If you have a coolant leak that leaks under pressure, how is this identified

Huh? It's identified by the leak, steam, telltale wetness or drips.

what is the symptoms of?

It's a symptom of a leaky gasket, seal or failed component.

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Leaking under pressure cold is different than leaking under pressure HOT. When a radiator shop pressurizes your cooling system up to say 16 to 18 PSI, cold, you will see seals, hoses, hose clamps leak, radiator end tanks, coolant tanks, actually weep or leak.... it will DROP pressure on the pressure gage over time and you will see coolant leak. You could get fancy and put dye in the cooling system and look for leaks with a disclosing light to see leaks

However, when the coolant is HOT and under pressure, it typically will vaporize when it hits atmosphere from a leak and you wont see it but will probably smell it

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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