My STS ( 180000 miles) is overheating in traffic after driving on the highway with AC on high. At highway speeds, temperature gauge stays vertical. Once in town, the temperature rises slowly until I receive the "fluid hot air conditioning off" status message. Eventually the gauge reaches the bottom of the red and I get the "Engine coolant hot, Idle engine" message. If I idle the engine, the temp eventually drops and drops rather rapidly back to almost vertical. This problem repeats when I begin to drive again. When the engine is hot, the coolant reservoir overflows past the cap. There is pressure on the reservoir side of the coolant system. The upper radiator hose has no pressure. The cooling fans work in both low and high. The diagnosis seems to be head gaskets. The head gaskets were replaced once in 1991 and I'm being told that the likelihood of successfully replacing them again is very poor because of the dissimilar metal stud issues with the block and the sleeves that are placed in the block during replacement from the first head gasket repair. Any other ideas of what could be the problem or anything else I should check?
STS Overheat/Head Gasket Repair on 1998 Cadillac Seville STS
I am sorry to say if sounds like you may indeed have a "head gasket" problem again. And it is also true that repairs may not work out the second time around. If "timesert" thread repair inserts were install in the block during the first headgasket repair then there is a chance they could come out when removing the head bolts this time. From what I have seen and heard if any of the "timeserts" come loose then you are done - they should not be repaired a second time. If ALL of the head bolts come out ok - which does happen sometimes - then if the surface of the block is ok then he cylinder heads can be successfully reinstalled after they have been checked for cracks. Again, if any "timeserts" come loose it is best to replace the engine block or the complete engine.
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i have the same exact problem with my 2000 cadillac seville sts, and i have gotten it checked out by alot of mechanics and some say its the head gaskets and some say that it is the whole intake. they say the intake sometimes gets a crack in it where the coolant starts to flow through and if its cracked it could cause the coolant to get to the pistons and cause it to blow out white smoke and over heat, and of course leak antifreeze. im going to change my intake first and see before i go to deep and have something thing done about the head gaskets. well like i said i'm not sure yet but i hope its the intake
You should double check the intake on your 2000 Cadillac Seville. I don't think coolant flows through the intake manifold on that particular engine.
check your thermostat coolant