1998 cadillac deville

My friend bought a 98 cadillac deville. Runs like a dream. However it keeps leaking coolant from the overflow cap. We replaced the entire over flow reservoir and cap, and its still 100% leaking from the cap. The lady said she was told it had too much pressure in the coolant system. It doesnt seem to have blown heads as it will drive for awhile before overheating and no white smoke in the exhaust…Any ideas? Should we do a pressure test? What could cause this besides too much pressure? It acts like it has too much pressure and is leaking through the cap. Also, you can not only see the leak, but you can hear it as well. When we push down on the cap, the noise stops. Thats why we thought it was just the cap or reservoir…Would appreciate any non-asshole replies! Thanks!

Remove the pressure cap from the coolant reservoir while the engine is cold.

Add water if the reservoir is low.

Start the engine, and watch the coolant in the reservoir as the engine idles.

If bubbles begin to appear in the reservoir, it’s an indication of a leaking head gasket.



Yup, I suspect a head gasket repair is in your future. Normally as the coolant heats up it will expand into the overflow tank and then return when the coolant cools. Excess pressure is from the combustion gases getting past the head gasket and into the cooling system. Alternating temp spikes and cool downs on the temp gauge are clear symptoms. You don’t necessarily have to see the white smoke depending on how bad it is. A shop can diagnose it with a test kit or pressure test but likely will not be cheap.

How long a drive does it take before it starts overheating?
If an engine overheats at all, that can be a sign of an ominous problem–such as a breached head gasket–or it could indicate the need for a radiator flush, a check of the cooling fan’s function, and/or a new radiator cap/thermostat.


Those Northstar engines had lots of problems including head gaskets. Do as @Tester recommends.


New reservoir. New cap.
Coolant level rises/pushes out of reservoir.
What does coolant system pressure tester say when motor is running?

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also check to see if the radiator fans are coming on. if not, the engine will overheat.

As I said, it’s my brother in laws car not mine…I’m just trying to help them figure it out. We know quite a bit about cars…we’ve actually done heads before on a dodge charger…not super hard. Was just irritating not being able to figure it out with 100% certainty…

ALOT of people prefer 24 year old cars. Not sure what u mean bro. I have a 93 towncar and I turn heads everywhere I go. It’s 28 years old. I get compliments from literally every single person who I’ve passed in it. Young and old. It doesnt matter. Older cars are easier to fix, they have more class. And they are just better. Period.

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Why are you upset with folks trying to help you? You’ve gotten good advice, try it out.


Oh oh. I didn’t realize this was a northstar engine. We put about 150K on ours and a pain to work on. I didn’t really have any major trouble with it but I have read horror stories on trying to do a head gasket repair. If I remember right, the replacement head bolts would simply not stay seated in the aluminum block. I had mostly coil and egr problems. I just got used to the light being on after the dealer tried to clean out the EGR passages but couldn’t get everything. Said they’d have to pull the transmission to go any further. Nice when they were new and would get up and go, but I’d never buy a used one.


I used timeserts on my caddy. Guy online has oversized stud kit that has good reviews. But, motor has to come out. Any motor out repair is deathknell for old cars. Just ain’t worth it.

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Older cars easier to fix ( it all depends on what is wrong ) Class ( not all of them ) and no , older vehicles are not better .

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You might try a different pressure cap, maybe you just got unlucky, the original was bad, and the new one is bad too. I’m talking about the cap that regulates the cooling system pressure. That may or may not be the cap that fits the overflow bottle. The regulating pressure cap usually has the psi rating written on it (14 psi or whatever it is designed for.)

Note that you could still have a small head gasket leak even with no white smoke out the exhaust pipe and no overheating. If the pressure cap is proven to be ok, shops have various ways to test for that problem.

I appreciate anybody here who legit answered my question. Thank you…no need for the rude comments though. We’re all adults. Act like it. This isn’t middle school

@rieflez27_180316 Stop with the profanity . Also this is an open form so not all posts are going to be just what you want .

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UPDATE: Was a clogged metal hose…talked to a guy at an autopart store and tried cleaning that specific hose…no more problems…heads are fine. Thanks to everyone who wasn’t rude!! Appreciate yall

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I can tell you dont work on cars…they actually are ALOT easier to replace. Clown.

And ya…they’re better bud. Lol I don’t agree with u …at all…

Some years ago one of my coworkers drove some version of this car. He was a tech scientist, but a physically large person (6’5") from Arizona, cowboy hat/boots, the works, and a car that size fit his physique better than smaller cars. Two of my coworkers decided to play a practical joke & figured out how to open the hood and mounted a texas longhorn shaped ornament in place of the Cadillac hood ornament. Looked pretty good. OP, when you get the cooling system problem under control, consider a change of hood ornament.