: Northstar intermittent heating problem

05-22-08, 12:55 PM
I have a heating problem that has me baffeled. 1996 SLS, for no apparent reason, the car will rapidly begain to overheat then many times (not always) will start cooling right back down again. It has never lost any coolant, no coolant in the oil, etc.. Iv'e check with the dealer and they have no clue. This overheating seems to have nothing to do with driving conditions as it sometime happens a few blocks from my home and sometimes on the freeway. Some times it will go for days without a problem then start doing it again.

There seems to be no pattern to when it takes a notion to jump up there. Fans run at full speed when the temp goes up but do no good. It seems that something is shutting off the coolant flow as might be with a bad thermostat.

Here is the list of things I've done so far: new radiator, water pump, thermostat(twice), new radiator hoses, new heater core and hoses,
pressure cap, coolant is at proper ratio, water pump belt & tensioner.

Can anyone help me out? It's diving me crazy and my wife is losing faith in her genius husband!

05-22-08, 02:47 PM
How do you describe over heating? K_C

05-22-08, 04:42 PM
By over heating I mean the temp gauge which normally stays in the 12 o'clock position very rapidly moves from 12 o'clock to the first mark which is about 1:30 position then it will go to the 3 o'clock position. If it dosen't start back down I turn off the engine at the first opportunity. I never let it get into the red zone on the gauge. The car definately is getting hot. It's not just a bad gauge or sending unit. The thing that makes me think it has something to do with the coolant flow rate is that it always happens very quickly, both going up and going down. Once it starts down it will go to normal in a matter of 45 - 60 seconds telling me that once the coolant starts to flow again it cools almost too quickly! Other than the thermostat I'm not aware of any other control that would actually interupt the flow and then let it continue that quickly. Not your normal blown head gasget condition or plugged radiator etc.. I changed the heater core at the suggestion of the Cadillac dealer on the outside chance it might play some part in the situation but with no cure.

05-22-08, 08:17 PM
It sounds like air in the system, have you checked to make sure the purge line is clear? If the line clogs the system won't be able to get the air out. Also maybe the waterpump pulley or belt could be suspect.

05-22-08, 08:32 PM
Check the waterpump belt tensioner again.

05-22-08, 09:00 PM
Follow misfit & Koz's advice. If that turns up nothing, then have the coolant tested for the presence of exhaust gases.

05-23-08, 09:23 AM
What thermostat did you use, an OEM or an aftermarket? I have had lots of problems with aftermarket units in other vehicles and when I went back to the OEM problems would clear up.

05-23-08, 09:38 PM
Thanks for your help guys. I just drove the car about 45 mile freeway and side streets and it stayed at absolutely normal. Stopped to takecare of some business for a few minutes, came out and drove about two blocks and up went the heat gauge! Pulled over and shut it off checked under the hood to see if I had any leaks or could see anything wrong. Got back in and strated the engines again and the temp started right down and was back to normal and stayed there for the next 7 miles on the freeway at 65mph.

I just bought GM-OEM thermostat and an Engine Coolant Temp Sensor and will install them in the morning. The sensor of course is a beast to get to. So I'm not looking forward to changing that out just to see if it might help but at this stage I'm ready to try most anything. This model SLS does not have a purge tube or valve on it that I can find so I do have to keep massaging the top rad hose until the air is out. Some models have a valve near the stat housing but not this one. Thanks again for all your ideas. Keep them coming until we get this figuted out. I am going to put my scanner on to varify the fan speed if the above parts change doesn't do the trick.

05-26-08, 08:44 AM
I have a 98 STS with the same problem. I have changed the radiator and the therostat and had the system flushed twice. Need help also.

05-26-08, 08:18 PM
My '98 was doing the same thing. It was the head gaskets.:helpless: