I have a 1998 STS with the northstar motor with 120,000 miles. The car runs great except for a couple of things. My problem is what everyone else with this car is experiencing it is overheating. It does not do it all the time. I have read all the posts here and on other forums and have some of the same symptoms but not the same as my problem. I drive the car daily. There is no oil in the coolant no coolant in the oil, no white smoke. no coolant leak. I recently flushed the coolant and changed the thermostat. I ordered a northstar waterpump removal tool so Ican check the impeller.
MY PROBLEM-If I leave the heater on at 80 degrees and 3 of 5 blowing, it will not overheat. If the heat is off and the temp gauge jumps I can usually turn the heat on to 90 and to full range of blowing 5 of 5 and the gauge will drop to the normal level. occasionally the gauge will jump from normal to 3/4 to hot and the usual lights come on and tell me to idle the engine. A couple times I was on the freeway and when this occurred it took me a few miles to get to a place to pull off and by the time I got there the temp was going down. When I pull over and give it 10-15 min. to cool down it will go back to the normal temp reading and I can drive another 100 or so miles home with no problems. Can anyone help me here it is driving me crazy. I would take it to the dealer but it does not do it all the time and I don't want to pay them to drive it around and wait for it to overheat or troubleshoot things I have fixed or have already checked. Any suggestions would be greatly appreciated.


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Sounds like coolant not circulating all of the time so look to the Thermostat. Often overlooked inexpensive part that can lead to overheating when going bad or gone bad.

 

changed the thermostat already.

 

The heater with air blowing thru it, acts as an auxiliary radiator , so the real radiator is not doing its job. Also the problem appears to be intermittent.

Perhaps it is a slipping water pump belt, either becaue it's worn or, more likely, due to a sticking tensioner. A few drops of oil at the pivot points can solve that.
A collapsing lower radiator hose can also lead to temporary overheating but if it's still the origianal, then that's probably not the culprit.
Wandering gobs of gunk can also be a source, but you said you'd flushed the system...

 

bad temp sending unit. the car isnt overheating the gauge is screwed becuase of the sending unit.

 

Thanks for the help. This weekend I will replace the water pump belt and oil the tensioner. If that does not help I will be shopping for a new radiator. Where is the temp. sending unit located in the car?

 

DONT REPLACE GOOD PARTS ! stop spending money on things that wont fix your problems. If a water pump goes bad it makes noises and drips water and leaves a puddle everywhere you go. the belt has nothing to do with the over heating pronlems. You have a car that I hope is holding water levels within the resivor. If you have to add water all the time and it doesnt leave puddles,you have bigger problems. I think your instrument guage is jumping around because of a bad grounding issue or a sending unit. The sending unit will be found on the intake. YOu can usually tell when a car is over heating by the smell and feel of the block. Run the car and see what happens. there is no such thing as a motor occasionally over heating. It either does it or it doesnt. Make sure your oil levels arent increasing and it isnt foamy.

 

and also make sure both fans are kicking in . check connections and clean them.

 

oiled the tensioner today. Took it on a 200 mile trip and noticed that it only gets hot on a uphill climb then downhill imediately drops to the normal level.
Any ideas?

 

lenny has some very good points. When we see the temp gage or the numbers on the DIC go high, we usually say that our Caddy overheats without really checking, even just by touching, whether it really overheats. A faulty sending unit or wiring or other electronic glitches can mimic this situation.

Also, the suggestion about the fans makes sense especially after what you've just said, uphill - high temps, downhill - o.k. The engine works harder and the car moves slower uphill, therefore more need for fan action. There can be problems with the fan itself or with fan control (temp sensor again?). Do you also have higher than expected mpg, or drivability issues? Coolant temperature plays a role in air-fuel mixing as well.

Sometimes new thermostats fail too. Do you "see" the tstat opening by watching the temps? Temperatures, at initial warm-up, usually overshoot the opening temp of the stat and than fall back and stabilize at around that temp.