I am new here, but have already found the site very helpful! I have a 99 Seville STS. 122,00 miles. It is over heating after about 30 min. of driving in 93 degree temps.
I have replaced the thermostat, flushed and filled. It doesnt seem to have any signs of head gasket issues, but I havent had it tested. I dont have alot of money to use the trial and error method. LOL! The fans are both working.

1. Is it possible for the fans to be working, but not turning fast enough to cool?

2. How can I check to see if it is the water pump? It does seem to be flowing, but I've noticed that I can squeeze the top hose almost closed with my hand even when it is circulating.

Any advice is appreciated!

What are your overheating symptoms? Boilover, temperature, what ?? The fans run, either one or both, fairly slow until temp gets to 234 degrees when they both go to fast (very noticeable). Slow fan operation also depends on the HVAC mode: if the a/c compressor is running, so is one fan.

If you can squeeze the top hose together, hot, sounds like the pressure cap is not holding, and this will cause coolant loss as the system heats up. There should be around a half tank of coolant, hot. Somewhat less cold.

Thanks for the iput!
The gauge goes fromthe halfway point to 3/4 then I shut it off. Then it boils over if I loosen the cap. I did notice that both fans run even if the AC is off.
The cap seems to be holding pressure also. It is overheating even thouh the coolant level is ok.

I did just notice that when I am idling the car in park, If I slowly increase the RPMs , I can hear coolant gurgling in the heater core area. Could this be a problem?

Thanks!

Yes, air in the system is causing it to overheat. The question is where is it coming from? Your head gasket may be putting exhaust gases into the system. Have the coolant tested for the presence of exhaust gases before wasting any more time and money.

I will tomorrow. I had it flushed today, is tomorrow too soon for the test to detect anything? Thanks!

if all else is well, try this. If your car is missing the fan shroud, this can cause insufficient cooling as well where as the air is dispersed rather than being pushed through the radiator fins. OR, on my car, it turned out to be a faulty coolant temperature sensor. it kept saying 240*, where it was really around 215.

I will tomorrow. I had it flushed today, is tomorrow too soon for the test to detect anything? Thanks!
Tomorrow may be too soon.Wait until when and if it overheats again.

Okay. Thanks!

Had this issue with my 95 STS. Overheating but no sweet smell out the back so I knew coolant was not getting into the motor. In fact the Head gasket was letting air into the cooling system. Happened when driving on highway mainly due to the high RPM's. End result = Game over. Ditched it and bought a 02.

Try the rad cap.... try a presure test. For me both were perfect. If I left it ideling it would not overheat. Only when driving.

DO NOT OVERHEAT THE NORTHSTAR! Sounds like you need a headgasket and depending on the year you may want to consider a newer STS.:cool2:

Good luck.

Since I flushed and filled the coolant system it only overheats when sitting still. I just noticed the AC fan is only running on low. Im going to hook it up direct to test it. Maybe I will get lucky and only need a fan or relay.:thumbsup:

Thanks for the input everyone.

Had this issue with my 95 STS. Overheating but no sweet smell out the back so I knew coolant was not getting into the motor. In fact the Head gasket was letting air into the cooling system. Happened when driving on highway mainly due to the high RPM's. End result = Game over. Ditched it and bought a 02.

Try the rad cap.... try a presure test. For me both were perfect. If I left it ideling it would not overheat. Only when driving.

DO NOT OVERHEAT THE NORTHSTAR! Sounds like you need a headgasket and depending on the year you may want to consider a newer STS.:cool2:

Good luck.
Do you realize that the Northstar was designed to run 50 miles with NO coolant (limp home mode) without any damage?

Do yourself a favor and reread post #4.

As regards your fan post, read Post #2 again.

I'm pretty much reiterating what has already been said but:
1. Have the cap pressure tested
2. Check the purge line
3. Test system for exhaust gasses
4. Feel if the passenger-side floor board is wet with coolant. If it is, you may have a leak in your heater core.

The noise you hear is air in the system. That is what is causing your car to overheat. The fans are fine so you can leave them alone. They're not putting air in the system, I guarantee you.

When I found that 1 fan was slower than the other, I switched the relays to see if the problem moved to the other fan. Now both fans run the same speed. I also put a bottle of "water wetter" in and it hasnt overheated since.

Thanks everyone for the advice.