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99 STS overheating. pulling my hair out.


MIke D

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Hey guys, Mike D back again after a few years of being problem free. Got rid of my 93 sts and got into a 99 sts. Been great till this past weekend.

Have not ever had a problem with the cooling system but now it's behaving erratically. Started driving to austin and got about 40 min into the drive with normal temp (12 o'clock) on the gauge. Stopped at a traffic light and while waiting the temp started to rise. light turned green and got back up to highway speed thinking it just might need airflow. Nope. after about 2 miles the temp climbed up to the red arch. Pulled over and coolant was spilling out the overflow tank cap. let it cool down for about an hour and made it to a gas station. Cold water in. car is back in action. got about 15 min down the road and it started all over again.. Stop at a light and temp starts to climb... I ended up at a parts store and they said thermostat and for $6 I got an overflow tank cap just because it was the original and we thought maybe it just wasn't allowing it to build pressure and raise the boiling point of the coolant. Well neither fix worked.

Car lasted 4 days at highway and around town driving with it never budging from the "12 o'clock" on the gauge. Today came off the highway and onto the back streets. stopped at a light and there goes the temp climbing..

Here is what i have done.

Flushed the expansion tank.

checked the small tube on top of the tank running to the throttle body area.

Lower hose from expansion tank to ??? flows

upper and lower radiator hose clear.

new "failsafe" thermostat (will see if it is "locked" open when the car cools down)

just bought a new belt even though the one on there looks good.

water pump not making any noise

will check the tensioner pulley when it cools down.

Both fans run with a/c on.

When I pulled out the old thermostat there was a rubber "gasket" IN the thermostat Not the one that s sandwiched between the flange for the hose but i'm talking about the actual plunger on the thermostat.. well there was only half of that rubber gasket still attached. (where did the other half go?)

So what am i missing? where to look next?

Cop didn't see it, I didn't do it!

93 STS 225,000 Miles

99 STS 111,000 Miles

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....Here is what i have done.

checked the small tube on top of the tank running to the throttle body area.

Did you confirm there is a steady stream of coolant (at engine idle) flowing from the throttle body plumbing into the reservoir? That is your air purge line and it MUST display a constant-steady flow of liquid. Also run a length of 0.080" weed-wacker line through the nipple at the reservoir to be sure that coolant can flow into the reservoir.

new "failsafe" thermostat (will see if it is "locked" open when the car cools down)

The only thermostat you want in there is a GM replacement part. Likewise the pressure cap.

When I pulled out the old thermostat ....there was only half of that rubber gasket still attached. (where did the other half go?)

Good question. See air purge line and GM thermostat comments above.

So what am i missing? where to look next?

Autozone has a combustion byproducts tester in their rent-a-tool program; #27145 and test fluid #25739. For about $6.00 you rule out headgasket issues before you throw any more parts money at the situation. Read the tester instructions 3 times before you do the test.

Jim

Drive your car.

Use your cell phone.

CHOOSE ONE !

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I tried the weed wacker line and it is clean. I even as a test I took the purge line and ran it from the top of the expansion tank and ran it to the fitting next to the upper radiator hose line (eliminating the TB..) With the expansion tank cap off running a steady 1700rpm to purge air the purge tank it started filling up from the bottom hose.

Other trouble shooting I tried. I pulled the upper radiator hose off with car running and watched a steady stream of water come out of the block (with pressure/flow)so the water pump is working. the radiator is only 2 years old and was bought new. The thermostat cap for now is locked open just to make sure there is not a restriction there. I will switch back after we figure out where the flow problem is. It almost seams like there is flow going up the lower expansion tank hose.

So where does that lower tank hose go? I see a metal tube that branches in 2 directions...

I think what is happening is the expansion tank is filling up from the bottom and blowing out through the expansion tank cap. after a few days of this I run low on coolant and then the overheat occurs..

Cop didn't see it, I didn't do it!

93 STS 225,000 Miles

99 STS 111,000 Miles

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Now I am confused

The direction of flow is from the TB bolt with a hole in it, TOWARD the tank.

Where did you stick the weed wacker line?, into the bolt with a hole in it?,

YOU DO NOT WANT TO STICK ANYTHING ANYWHERE, IF YOU DO NOT KNOW HOW TO CHECK THE PURGE LINE, ASK. IF YOU DISLODGE A CLOG ITS A MISTAKE!!!!!! DID IT FLOW AS PER THE TEST WE SANCTION HERE?

DID YOU SEE IT FLOW?, when you removed the purge line from the TOP of the tank, with it IDLING?, Coolant must flow OUT of the purge line FROM the bolt with a HOLE IN IT?, there is NO substitute for this, NO BLOWING INTO IT, etc to clear it, to clear it you MUST remove the bolt with a hole in it or it WILL RECLOG

EXPLAIN what you did and why

The two hoses at the bottom of the tank are related to the heater and cross the engine at the back

Is the CAP good? By the way, I totally do not agree with using an aftermarket thermostat, use OEM only, the stat is NOT causing your overheating problem

STOP TRYING TO REINVENT THE WHEEL WITH STICKING WEED WACKER AND AFTER MARKET STATS AND HOSES AT THE BOTTOM OF THE TANK AND PULLING UPPER RADIATOR HOSES WHO HERE TOLD YOU TO DO THAT STUFF?, IT PROVES NOTHING

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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Hi have you had a co2 sniff text to make sure your not getting exhaust gas, from the headgasget. I would have this done just to rule it out. Hope you sort it out and good luck.

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Now I am confused

The direction of flow is from the TB bolt with a hole in it, TOWARD the tank.

Where did you stick the weed wacker line?, into the bolt with a hole in it?,

YOU DO NOT WANT TO STICK ANYTHING ANYWHERE, IF YOU DO NOT KNOW HOW TO CHECK THE PURGE LINE, ASK. IF YOU DISLODGE A CLOG ITS A MISTAKE!!!!!! DID IT FLOW AS PER THE TEST WE SANCTION HERE?

DID YOU SEE IT FLOW?, when you removed the purge line from the TOP of the tank, with it IDLING?, Coolant must flow OUT of the purge line FROM the bolt with a HOLE IN IT?, there is NO substitute for this, NO BLOWING INTO IT, etc to clear it, to clear it you MUST remove the bolt with a hole in it or it WILL RECLOG

EXPLAIN what you did and why

The two hoses at the bottom of the tank are related to the heater and cross the engine at the back

Is the CAP good? By the way, I totally do not agree with using an aftermarket thermostat, use OEM only, the stat is NOT causing your overheating problem

STOP TRYING TO REINVENT THE WHEEL WITH STICKING WEED WACKER AND AFTER MARKET STATS AND HOSES AT THE BOTTOM OF THE TANK AND PULLING UPPER RADIATOR HOSES WHO HERE TOLD YOU TO DO THAT STUFF?, IT PROVES NOTHING

I did not put the line in the bolt with the hole in it. I put the line in the tubes on the bottom of the TB this is where the fluid heats up the TB. the thought was this is a small orifice and could be where the clog is.. When I checked this it was clean but to rule it out I bypassed it for testing purposes.

The bolt with the hole in it is down by the upper radiator hose. I ran from here to the top of the overflow tank to just rule out the clog being in the TB. This hose does have flow but it is "spurting" for lack of a better word. not a constant stream of water. could this be due to the air in the system? a clog?

Other trouble shooting was I took both the upper and lower radiator hoses off. poured water through the upper and watched it drain out the lower.. Now i know the radiator is not clogged. I also checked when the upper radiator hose was off for flow out of the engine block. It did have high flow with the engine running. This way i know the water pump is working.

While the lower hose was off I checked the thermostat and it was clean of debris.

After all this I closed the system back up except for the air purge line to the top of the overflow tank. ran the car up to operating temp (12 o'clock) on the gauge. The air pure line was still "spurting" fluid then air then fluid. Had a friend run it at 1700 rpms for a good 5 min to try and purge any more air that would be in the system. This is when all of a sudden (like in 30 seconds time) the fluid came flowing out of the overflow tank.. the only place it could come from is the bottom hose on the overflow tank. why? I thought flow would be to the engine not to the overflow tank.

Cap is good changed it 2 years ago when i changed the radiator out. and don't think it is a head gasket since no fluid out the exhaust and i also just changed the oil after the long drive trying to get the car home last weekend and there was no antifreeze in the oil. The engine is running smooth and no codes other then a/c p1343 and p1344 which has been for years...

Cop didn't see it, I didn't do it!

93 STS 225,000 Miles

99 STS 111,000 Miles

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Here is how to test the purge line

With engine COOL, LIFT HOOD, Pull purge line off the TOP of the coolant TANK, NOT the overflow hose, the purge line hose

Let is hang, go start the car and let it IDLE

Go and look at the purge line that is HANGING

Is coolant flowing from the purge line, YES OR NO? That is the ONLY test I know of to test the AIR PURGE LINE to RID AIR from the cooling system.

The 2 hoses at the BOTTOM of the tank are related to the HEATER core and route themselves back to the water pump crossover, THEY will NOT cause overheating,

The BOLT WITH A HOLE IN IT is located near the THROTTLE BODY, see a photo of it here, it is located at 2 o'clock in this photo next to the upper radiator hose. You do not want to dislodge a clog if there is one, you remove the bolt and remove the clog otherwise it will just clog at some time in the future again

WaterPumpandRadiator022-1.jpg

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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Yup, do the block test. The '99's seem to be the worst for HG failures for some reason.

The bolt with the hole in it is down by the upper radiator hose. I ran from here to the top of the overflow tank to just rule out the clog being in the TB. This hose does have flow but it is "spurting" for lack of a better word. not a constant stream of water. could this be due to the air in the system? a clog?

Putting that line into the top of the tank just routs the coolant back into the tank without making a mess. As long as there is coolant flow, it's not clogged. Flow will increase with Rpm.

So where does that lower tank hose go? I see a metal tube that branches in 2 directions...

post-9-130948051419_thumb.jpg

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You need to do a combustion test for exhaust gas in the overflow tank...

I would go with Jim's advice, and have Exhaust gas test done to see if the is any Gas in the coolant header tank. My last car before my CTS i had a issue that sound's the same. i was sure it was not the headgasget but lack of flow, as the was no oil mixed with the coolant. And two weeks later the headgasget let go big time, ok it was a XJ Jag. But for peace of mind have a sniff text or what ever its called state side.

Cheers Pete

Edited by btx5mg pete
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  • 2 weeks later...

Sorry, been a while. between trying to diagnose the car more, getting called away for work then forgetting my password... I'm back... without a car still..

I tried to to the gas test using the tester parts from autozone. The problem I keep running into is that by the time the car starts to come up to temp it is overflowing out the surge tank. Strange because it is down below the full line when I start the test....

after talking to a caddy mechanic for 30+ years he said try changing the water pump. it's only $50 for an autozone lifetime warranty. duramax??? After I swapped it out it continues to overheat. After I purged all the air out and am running 50/50 mix... Both fans work.. This last drive it came up to normal temp fast and stabilized at the "12 o'clock" but I didn't even make it 5 min down the road before it overheated. so it's getting worse..

I'm thinking between a 2 year old radiator, new thermostat, water pump, and belt.. No leaks to let it run low on fluid... Could it be a bad temp sensor? I'm thinking it is just the head gasket going... cause why else would it be getting worse every time i run it?

Cop didn't see it, I didn't do it!

93 STS 225,000 Miles

99 STS 111,000 Miles

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you prolly need head gaskets and when the car is cool pop off the surge tank cap and stick your nose to it and see if it smells like gas.

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You do NOT need a water pump, its a waste of money and unless it was leaking its amazing that such an experienced 'cadillac' mechanic would suggest that, its NEVER the water pump's problem. Its a shame you didnt think to ask, you wouldnt have wasted $50.

You never answered the question as to whether the purge line was clear AT THE TANK, I am still waiting for that answer, CLEARLY (note to self, something was stuck into the bolt with a hole in it to clear it in error, potential problem).

Did your mechanic pressure test your cooling system? Does it hold pressure, good mechanics do that first to test for system leaks, what was the result?

Did your mechanic pressure test your cap? Does it hold the rated pressure and is it the right cap, did he determine that? If he is experienced he has a CAP tester.

What is your coolant to water concentration as a percentage? Did he use strips or a refractometer?, experienced mechanics like he would test that also so that he knows the BOILING POINT.

Radiator new, water pump new, stat new, water pump belt and tensioner good (note to self, aftermarket stat was used and subsequently overheated)

How is the tank, holding pressure?, oh that's right, a bad tank would show up under a proper pressure test that your mechanic certainly did if he was experienced. Experienced mechanics have pressure testers with adapters to fit all cars in their tool box. What was the result of the pressure test, does it hold pressure at 18 PSI without dropping?

Stick your nose in the tank and take a deep smell, smell exhaust fumes?

Purge line clear to the tank, oh thats right we don't know, need answer, clogged purge lines are famous for overheating problems.

What was the result of your mechanic's test of the coolant for combustion by products does he have an electronic sniffer? How about the NAPA/Balkamp combustion by products tester did he use it?

Did he do a compression test to see if the head gasket is breached?

Did he pressurize each cylinder at TDC to see it EACH cylinder holds pressure?, while listening at the tank for bubbles?

I am sure this experienced mechanic who replaced the water pump, has done all of this right?, what were the results?

Is your idle rough in the morning?, are you getting misfires in the morning?

Diagnosis is pretty clear but specific to the Northstar, its either the head gasket or its not the head gasket, testing is the only way

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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These types of statements confuse me

AFTER I PURGED THE SYSTEM OF AIR

You do not need to purge the system of air, it happens on its own, what did you do?

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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BBF you sure are condescending... having a bad day? or you just changed your attitude from when I was here years ago??? I said I talked to a mechanic... you ASSumed I had him work on it.. I also said I did NOT put the line in the bolt with the hole it it.. so CLEARLY the "note to self" was wrong... So much of the other posts of your are so inflaming that i am not going to waste my time on you/this thread... I am asking for advice not sarcastic rants on your behalf... As far as the other (civil) people on the board. I thank you for your help.. But this BodybyFisher has made me a true believer of the "other caddy board" as being a better and "more professional" board. Good luck with this tyrant.

Thanks again (others) for your help.

Cop didn't see it, I didn't do it!

93 STS 225,000 Miles

99 STS 111,000 Miles

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Believe what you like, and YES I am being condescending to you. I do not like your tone, your experimentation, the weed wacker statement, your incomplete and difficult to understand writing style, your 'experienced' mechanic of 30 years who led you totally wrong, one who did not give you better advice, your using untested diagnosis methods with aftermarket stats with switches on them and that you needed to mention that your mechanic is so experienced in the first place, and I could go on.

The fact is, yes, I have been edgy because of you and this thread, I get that way when I really dont like what I read. Your writing style is difficult to follow and choppy. Your thread totally rubbed me wrong and yes, I get PO'd when I read it, THAT is what you see and hear and NO I dont normally get that way.

These are statements that bother me

Cold water (great way to crack a block, never use COLD water or pure WATER if you are having overheating problems)

This is vague and hard to understand >> "When I pulled out the old thermostat there was a rubber "gasket" IN the thermostat Not the one that s sandwiched between the flange for the hose but i'm talking about the actual plunger on the thermostat.. well there was only half of that rubber gasket still attached. (where did the other half go?)"

new "failsafe" thermostat (will see if it is "locked" open when the car cools down) (Spend any time here and you know to use GM validated stats, but now you are re-inventing diagnostics)

This is vague and was never answered >>> "checked the small tube on top of the tank running to the throttle body area"

No reply to this >>> "The only thermostat you want in there is a GM replacement part. Likewise the pressure cap"

This is vague and have NEVER been mentioned on this board before >>> "I tried the weed wacker line and it is clean" what does that mean?

What the hell does this prove or mean, and did you ever hear this described here? >>> "I even as a test I took the purge line and ran it from the top of the expansion tank and ran it to the fitting next to the upper radiator hose line (eliminating the TB..) With the expansion tank cap off running a steady 1700rpm to purge air the purge tank it started filling up from the bottom hose"

At that point you are all over the place and I am already aggravated with what has been done and want to walk away and should have stopped looking at this thread but then

Where has it ever been said to do this on this board >>> "Other trouble shooting I tried. I pulled the upper radiator hose off with car running and watched a steady stream of water come out of the block (with pressure/flow)so the water pump is working"

You are not asking for help, you are telling us what you are doing, and what you where doing has never been discussed related to overheating like this "So where does that lower tank hose go? I see a metal tube that branches in 2 directions"

All of a sudden YOU are coming up with this "I think what is happening is the expansion tank is filling up from the bottom and blowing out through the expansion tank cap. after a few days of this I run low on coolant and then the overheat occurs" I was like HUH?

Then you mention your 30 year experience cadillac mechanic who told you to change the water pump, wow

Mike if you come for help, come but you didn't come for help you came to disrespect what this board is about, to go off the reservation, not to answer questions clearly, to come up with half baked theories and diagnostic methods, quite frankly if I didn't know better I would doubt that you have spent any time here at all. That is NOT the MIKE D, I remember

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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