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1999 Cadillac Seville STS Overheating

Jul 21, 2009 - Update: O.K., I am concerned now. I just drove nearly 700 miles. Overheated going up steep inclines ONLY with the AC on; otherwise, car ran on level road without issue. No loss of power; no overheating. Coolant level was down after overheating incidents. Later, if I approached a steep incline, I would turn OFF the AC and could clear the incline without issue. Just took to a GM dealer for diagnostic....THEY said: Head Gaskets $3,500...possibly worse (i.e. block cannot see). First, according to repairpal the $3,500 was $1,300 higher than the highest estimate. Other mechanics have suggested that IF it were head gaskets, the car would lose power and overheat ALL the time, NOT just on inclines. Please offer your suggestions to all of this. I am a long way from home but feel I need to do something. Should I just return to the FIRST suggestion and replace the water pump...and hope? Many thanks.
Jul 27, 2009 - 27 July, 2009 Update: I limped home without repair (yet). Drove 630 miles, in 100+ temps, without incident, again, turning off AC on inclines. Then, on last STEEP incline, 10 miles from home, overheated. A head gasket repair is obviously extensive, but could use some expert advice as to whether this sounds like the problem. OR, could it still be cooling system related? The coolant is going SOMEWHERE...and no obvious, visible leaks under car, etc. IF head gaskets, why doesn't it overheat (or even RUN toward hot, which it doesn't) ALL the time? Isn't it unlikely to be a head gasket problem on a Cadillac with only 85,000 miles that has been kept in prestine condition? Returning to pippall's original suggestion, if the problem IS the water pump, would that explain loss of coolant? Please help. Thank you.

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Sounds more like an ignition control or coolant flow problem. Or, a/c fan not coming on with a/c.
Getting the water pump question out of the way, the pump either works or not. If it leaks it will cause an overheat due to coolant loss on flat ground or grades...doesn't matter. If the pump bearing or seal is bad it will still pump, it will just be loose and noisy. Coolant flow through the radiator is also essential...I don't think you have a problem there because generally this happens with less maintained higher mile cars.
A bad map sensor can cause your problem because it changes both ignition timing and fuel mixture, either of which can cause very high temps in combustion chambers especially when under a load. Mass air flow sensor can also do this but usually will cause idle problems. If fan is not on with a/c if the remainder of your cooling system may be enough to maintain decent temp control but cannot do this under a load. A/C turned off removes some load and therefore allow you to get up hills. Operating marginally, you can be pushing some coolant out of the system at times without even knowing it.
Now, about your engine: These engines are in my opinion a very poor design to begin with. Even with the best maintenance and low miles, they don't handle temperatures well. at this point, I'd be concerned that after repeated overheats and near overheats, your head gaskets may have started to fail, even though they were not the cause of the problem.Keep that in mind.
Have the items I mentioned tested. If you finally have the problem solved I suggest you change the thermostat as well...Overheating makes them unreliable.

Posted on Jul 27, 2009

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