I have a 1998 Seville STS and it has a weird overheating problem. The car drives fine on straight road or downhill but when driving up a hill it it will start overheating and as soon as I get on a straight road again the temperature goes back to normal. I replaced the water pump and checked the thermostat and it seems fine. After driving the car at normal operating temperature I came home and took off the lower radiator hose from the radiator side and there was no coolant coming out of it, when I rev the engine some coolant will come out. Is this a clogged radiator or?? The car does not smoke, no coolant in oil and the compression was checked so I don't think it's the headgasket

 

compression test or no compression test, headgasket seal has likely failed .. either gasket or bolts, no matter .. .. you need to do a exhaust gas in coolant test but drive the car in the same mode that makes it overheat otherwise the test will be inconclusive.

 

if the headgasket was bad wouldn't the car overheat either way because I drove the car on the freeway for 80miles straight and it was fine

 

you are new here so i'll assume you have yet to read our FAQ.

the answer is no! unless the condition is well advanced, the exhaust gases seem to find their way only after prolonged heavy load which cannot be accomplished on flat road driving of any distance.
sadly, it seems that a compression test can only spot this issue after the problem is well advanced.

as time goes by, the leakage burns up the gaskets and the problem shows even at highway driving .. but at the onset, only heavy loads seem to create this condition.
apply the test kit, drive it up the hill or whatever driving style makes engine overheat - then cool it down, check the fluid for exhausts gases.
if you dont overheat it, the test may be inconclusive.


of course i also assume that you checked that the hoses, radiator, and heater and made surethey are hot therefore showing good flow of coolant.


.

 

The CO test will confirm head gasket issue, concerning radiator : you removed hose and no fluid came out? either it was low on coolant or serious blockage some where. It is possible radiator is restricted, you could have radiator flushed to see if helps....you could do it yourself but there are some epa/haz-mat issues with disposal of used coolant.

 

I guess it could be the headgasket, but what I also noticed is when I drive like from a dead stop I can hear water rushing in the heater core. Not sure what that is but it sounds like something is clogged somewhere, The car is not low on fluid it's perfectly fine. But where do I go to get that test done to see if exhaust gases are in the cooling system?? Thanks

 

Just went to test out the car and instead of driving up I just pulled over and let it idle on a hill without driving it so there is no load like while driving and is started overheating anyways. So now I am pretty sure something is clogged either the radiator or heater core. How much is it to change the heater core?

 

Hearing water rushing in heater is a sure sign of low cooalnt. drain coolant in proper container(s). Use water hose to "back flush" engine until water flows clear and it should no dribble, or take somewhere and have it done.