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97 Deville Overheating - replaced thermostat, no leaks, no smoke


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Hi guys, I've been coming here off and on for years for all kinds of help, and I must have read every post on here about 91-99 Cadillacs overhearting but I'm in a spot now. Currently driving a 97 Deville with a v8 Northstar 142k miles.

My car started overheating suddenly, had gone all summer with no problem, then recently after getting an inspection and oil change (Rotella 15w-40 diesel, to mitigate a previous oil light problem completely) I overheated both times I got up to highway speeds and have almost overheated every time I've driven through town - stop and go 30-40 mph, temperatures starting normal and gradually increasing to the 220-230 range where they begin to push into the 250s.

After check online for hours, I went and grabbed a thermostat from O'Reillys and a temperature sending unit. I was able to change the thermostat with little trouble until I broke the head off one of the mounting bolts :glare: - its now being held on by one of the original bolts and a manifold clamp after which I attempted to locate the temperature sending unit on my motor and I could not find it, nor could I find any mention of it online.

Well changing the thermostat didn't seem to solve my problems so I went around to a couple mechanics, all busy on a Friday morning until I finally found someone to look at it and they were convinced the best first step was trying a better quality thermostat, I had read to make sure to get a Delco stat but I couldn't locate one in town so I went with whatever O'reillys brand one they had. So I agree and they ordered the stat, put it in and said they went two miles and the fans were coming on at 222 and that it was maintaining temperature. They also mentioned that they tested (didn't say how and I was so relieved to hear it I didn't ask) if the head gasket was going, and said it appeared absolutely fine.

They mentioned that if the Delco thermostat didn't resolve the issue it was likely the water pump and quoted me parts and labor at $460, seems a little high but I have yet to shop around with it being almost end of the work week.

On my way home from getting the new thermostat put in, I stayed within 208 and 228 with the fan coming on (or at least I assume, its very quiet, but the temperature was coming down) during stop and go but after a couple miles started creeping up into the 230s before dipping down into the 210s, finally it started getting up into the high 230s and 240s both times coming back down eventually but this is going 30-40 mph in town. The final stretch of road before my house is about 2 miles and I drove 55 for a minute or two and the temperature quickly climbed into the 250s and stayed there till I got home and idled it where it slowly creeped back down to the 210-220s.

Theres something clearly wrong here, I'm not leaking coolant or water - visibly at least. I need to figure out what to reasonably look for before Monday, I need to get this resolved as I require this vehicle for work. Thanks for all your help if you need any more information just let me know.

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Check and make sure your purge line from the bolt to the expansion tank has good flow while the engine is running. Secondly, its not your radiator or waterpump. Third, maybe get a block test done and or see if the overflow jug smells like gas, if it does you have a head gasket issue. Sudden spike in temperature while climbing long hills, excessive pressure in the overflow tank is a symptom of a head gasket issue.

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Take the cap off the surge tank and then disconnect the purge line from the surge tank and put the end of the hose into the neck of the surge tank. The purge line is the small hose that connects to the surge tank neck near the top.

Start the engine and observe the flow - it should be a steady flow. If it is not, then there is a restriction in the purge line that needs to be cleared. Do not let someone replace the water pump unless it is leaking from the weep hole.

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

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Okay I took a few pics then disconnected the line going into the surge tank. The hose is very short and made it very awkward to observe, also made me question whether it was the right line or not but had read several places that it runs under the beauty plate and back into the engine.

Pic 1

Pic 2

When I turned it on, the hose was bent slightly upwards so I'm not sure if that impacted this at all, but it seemed to be sputtering. Was not what I would call steady.

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Take that beauty cover of and follow the hose in pic 1 and you will see where it goes. You are on the wrong hose in pic 2 . The outlet is right above that radiator hose and close to that dog bone motor mount!!

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A block test is the definitive test for HG failure and should be performed after an overheating event, not at cold start up. You can rent the tester at Autozone or O'Reillys but will have to buy a bottle of the blue test fluid.

Another indicator of HG failing is rough idle for 30 seconds or so at cold start up and then it smooth's out.

Also if you remove the expansion tank cap once the engine cools down and you get a significant pressure relief then that's another indicator of HG failure.

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A block test is the definitive test for HG failure and should be performed after an overheating event, not at cold start up. You can rent the tester at Autozone or O'Reillys but will have to buy a bottle of the blue test fluid.

Another indicator of HG failing is rough idle for 30 seconds or so at cold start up and then it smooth's out.

Also if you remove the expansion tank cap once the engine cools down and you get a significant pressure relief then that's another indicator of HG failure.

Well, thats not good then. Gotten quite a bit of the last one there this past few weeks and it does idle a little rough when cold starting it.

At this point not sure I even wanna invest in a block test. Quite a few '06 DTS and an '05 SRX for less than 7k with less than 100k miles.

Anyone have any experience with early-mid 2000 models? I've heard that Northstars from 2000 onwards are for the most part issue free.

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In 2004 the head bolt thread pitch was increased to 2.0 mm which eliminated most of the HG issues but I've occasionally read of them failing also. I think there is an ongoing thread here on a 2005 with a suspected HG issue.

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Okay I took a few pics then disconnected the line going into the surge tank. The hose is very short and made it very awkward to observe, also made me question whether it was the right line or not but had read several places that it runs under the beauty plate and back into the engine.

Pic 1

Pic 2

When I turned it on, the hose was bent slightly upwards so I'm not sure if that impacted this at all, but it seemed to be sputtering. Was not what I would call steady.

It is possible there is an obstruction in the purge line. Follow it back to the water pump crossover and disconnect it. The nipple at the water pump crossover is actually a hollow bolt. Remove he bolt and clean out the obstruction.

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

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