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99 Cadillac Seville STS overheating

I just returned from a 185 mile drive and had to navigate a relatively small mountain pass. I had gone over this pass on the trip down without issue. I also recently came back from a 2,800 mile trip, over the Rockies, without issue. Engine overheated warning came on, so I pulled over immediately. Anti-freeze and oil levels appear fine. Recently, intermittently, I have had a POOL of water on the floorboard, passenger side; it is clear, has no odor (like anti-freeze) but, being intermittent, have not determined the cause of that, and not sure if it is related to overheating issue. Not sure where to start with the overheating problem. Your help and advice appreciated. Thanks.

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  • Jeffrie Bond
    Jeffrie Bond Jun 29, 2009

    Thank you. Appreciate your suggestions.

  • Jeffrie Bond
    Jeffrie Bond Jul 10, 2009

    Update: Took car to shop who hooked up the car to the computer for diagnosis. Thermostat, water pump, sensors, etc., checked o.k....could not get the car to overheat in the shop (of course). I have been driving the vehicle now about the last 10 days without issue but last night, AGAIN after going over a small incline, it overheated again. I pulled over, let it cool down, and drove the remaining 40 miles home (including over more inclines) without issue. Incidentally, I was told if it happened again to leap out of the car and shine a flashlight under the hood to assure BOTH fans were working, including the supplementary fan; they were. Obviously, this is an intermittent thing, which is my next question. Can a thermostat check o.k. but only work intermittently? What about a water pump, or sensors...can THEY work intermittently, or do they just go out completely? In other words, can some of these things check o.k. on the computer but they, in fact, only are functioning intermittently? What about something associated with the cooling system (although I see no evidence of anything leaking, as one would think would be the case with a faulty cap). This is a Cadillac and I cannot really afford a fishing expedition so am looking for the path of least resistance and cost in this remedy. Your continued suggestions are appreciated. Thank you.

  • Jeffrie Bond
    Jeffrie Bond 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.

  • Jeffrie Bond
    Jeffrie Bond 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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Change the thermostat.if still the same change water pump.

Posted on Jun 29, 2009

  • Philip Long
    Philip Long Jul 27, 2009

    Yes the thermostat can have an intermittant falt, and so the temp switch cam.So i would change the thermostat firstly.Then get back to me if you still have the problem.

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Its your heater core

Posted on Aug 11, 2009

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The thing is if they have tested the compression it will clearly indicate a bad head gasket but if the compression is good then the gasket is fine. But I have had this exact thing happen was a bad cap this is easy to tell after warm up the top radiator hose will be limp no pressure or very low this indicates a bad cap which every hill will cause the overheating. Hope this helps

Posted on Aug 10, 2009

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Check for coolant flow at the Purge line,this is a common problem on the Northstar.
There is a small diameter hose coming from near the top of the coolant tank,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.If the hose is clear, check for blockage at the nipple on the tank.

Posted on Aug 11, 2009

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I have a 1999 Casillac STS with about 184,000 miles on it. Purchased brand new and oil changes and other maintenance done religeously. About 2000 miles ago it started hearing up, but only when going up...


Well, you are not going to like the answer. First check for leaks, and there is a small bleed line that runs from the engine to the coolant tank that can plug up.

BUT much more of an issue is head gasket leakage. This is caused by steel studs being bolted into the aluminum block. This is extremely expensive to fix correctly. Just replacing the gaskets will have the condition return soon. The correct fix involves time-certs being installed in the block, this is a bare block on the machinists bench process. A time cert is like a heli-coil, but more solid.
My wife's car did this at 50,000. And vehicle was extremely well maintained. You would think GM would have figured this issue out, but no.
Typically takes 5 to 7 grand to fix.

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I have a 96 Cadillac Seville STS the engine is strong with low miles. The ignition was replaced about 6 months ago at the dealer because the car power would not go on. Well, same problem. I will be...


Nothing is more frustrating then an intermittent problem (to both the mechanic and the car owner) Your description leads me to think that there is wiring that is overheating and is causing an ignition breaker to trip. Or maybe there is a related ignition wire that is loose, corroded, or broken. Good luck but let me tell you that you may not find the answer until the problem becomes constant.

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I need a cadillac seville sts 1998 4.3L & 32


Check for coolant flow at the Purge line,this is a common problem on the Northstar.
There is a small diameter hose coming from near the top of the coolant tank,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.If the hose is clear, check for blockage at the nipple on the tank.

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2 Answers

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do you have the 4.9 in it they are pron for there heads cracking with out you knowing or any smoke coming out you tail pipe.

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99 cadillac seville continues to overheat even though i put a new water pump


maybe the water pump wasnt the problem to begin with. if the engine temperature only goes up when you stop the car then most likely it is the FANS that are not engaging to cool the radiator which is a problem with the fan sensor, computer that controls the fans, relay/fuse. I had the same problem on my 1999 cady STS coming back from my vacation. the engine temperature would be normal only when I drove the car above 70km/hr because the air passing through the front of the car would cool the radiator. when I slowed down the fans wouldnt start and no air was passing through to cool the rad, hence the engine temperature would start to go to HOT. good luck.

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2000 cadillac seville sts- it is overheating only when I drive it. Changed the water pump, two therosdats and flushed it three times. Then I put block seal it. but no luck still overheats when I drive it....


check to see if cooling fans are working. if not replace fans or fan sensor. check oil. if there is coolant in the oil, you may have a cracked head or a defective head gasket.

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