Overheating mystery
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Northstar Engines and System Technical Discussion Discussion, Overheating mystery in Cadillac Engine Technical Discussion; Hey guys, long time no post...good thing I guess. I've got a '99 STS that has an overheating problem...first I ...
  1. #1
    blocker11 is offline Cadillac Owners Member
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    Overheating mystery

    Hey guys, long time no post...good thing I guess.

    I've got a '99 STS that has an overheating problem...first I was losing coolant so I decided to have it checked out & in the process replaced the thermostat....turns out it was a hairline crack in the surge tank.

    Replaced that too, so for 3 days everything worked great. I then had the car overheat again today....it's a nice day in the Chicagoland area finally so I turned the heat off & that's when it began to overheat! Fans seemed to kick on when the heat was turned on but not when it was off...I'm not expert on any of this of course, so I hope I'm explaining it all ok. Coolant levels seem good, upper radiator hose is hot, etc.

    Is this indicative of anything in particular that I should be looking for that I'm missing?

    I would greatly appreciate any help I could get regarding this guys...thanks & if you need more info on anything, let me know.
    kb

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    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: Overheating mystery

    Do a search for purge line and check it. You may have air in the system that is not being purged.

  4. #3
    blocker11 is offline Cadillac Owners Member
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    Re: Overheating mystery

    I believe I've checked the purge line and can feel fluid passing through it...would it be better to take both ends of the line off?

    An update...this morning temperature started to climb again with the heat/cooling system off. I turned it on and the needle went back to 12 o'clock...I even ran the a/c to 60 degrees & could feel the extremely cold air coming out and the needle never budged from TDC....then turned the cooling system off & the engine temperature rose slightly again....

    I've got no ideas!!!!

  5. #4
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: Overheating mystery

    Have the coolant tested for the presence of exhaust gases before wasting any more time or money.

  6. #5
    Submariner409's Avatar
    Submariner409 is offline If it won't run, stick on fender vents
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    Re: Overheating mystery

    If the engine temperature is running anywhere from 1/8" to the left of the 12:00 tick, or halfway to the next, upper, tick, you're OK. One fan should come on at 224 degrees two at 236. You say the upper hose is hot, the big, long one to the passenger side of the radiator, right? How about the hose on the driver's side which comes out of the radiator about halfway down the tank. CAREFULLY test it. It should be somewhat cooler (relatively) than the top hose. Just sitting, idling, watch the temp gauge and the fan(s). The temp should start up past 12:00 a bit, a fan should come on, and the temp goes back to around 12. If you have the nerve, block the front of the radiator with cardboard. One fan will come on as above, but the blocked radiator won't cool, so the temp should go to 236 or so. Both fans go high, pull the cardboard. Other than that, I have no great ideas. You've covered purge lines and surge tanks. Be careful of fast idling too long in P. The cat may tend to overheat.

    There's a temp gauge pic in a thread in here within the past 48 hours. ("Temp gauge fluctuating" - Deville forum, yesterday eve.) Is the w/p belt OK? Just over the thermostat housing, under the stamped steel guard....

  7. #6
    RickyHenry's Avatar
    RickyHenry is offline Cadillac Owners Fanatic
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    Re: Overheating mystery

    I have a 1999 Seville STS, same engine. I had a very similar problem. I replaced the Thermostat, replaced hoses, replaced the Radiator... I even thought the fan wasn't working. The car has been drivable but often I will over-heat, for no reason at all.

    Crank the Heater to full blast as a temp driving solution everytime I would use the car just to get somewhere in it. Bring some coolant or water with you on the road if you are going anywhere. From all of my months of research, it is known that there is a problem with this car and the N* where either the heads will crack or the heads will become loose and you have a leaking gasket. Hopefully it doesn't turn out that way for you. If it does, you may want to check into the Recall program that Cadillac has for that year car regarding head bolts.

    Anyways... look at my current thread and my current project that is happening in my garage at this very moment:
    http://www.cadillacforums.com/forums...s-project.html

    I would put my money on it that you have something wrong with your cylinder heads, and was caused by the boring of the bolt inserts on your head block. It seems to be very common with this car.

    Ricky

  8. #7
    blocker11 is offline Cadillac Owners Member
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    Re: Overheating mystery

    Not sure I'm ready to jump all the way into a head gasket issue just yet.

    Doesn't it seem odd that the engine runs at normal temp when the heat or A/C is on and then starts to overheat when they are turned off?

    Is there something that turns on when the HVAC is on and is "supposed" to turn on as well when the system is off, to keep the radiator & engine running cool?

  9. #8
    JimD is offline Cadillac Owners Connoisseur
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    Re: Overheating mystery

    Rather than guess at whatever is "normal" or "overheating" for your engine, take a peek at this link and tell us approximately what temperature you are calling "overheating".

    http://i70.photobucket.com/albums/i1...ithnumbers.jpg

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    blocker11 is offline Cadillac Owners Member
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    Re: Overheating mystery

    The car has gotten to the point where the needle is over 235 for sure, probably 250 too.

    The DIC says "engine hot - a/c off" and a few others that I cannot think of right now.

  11. #10
    zonie77 is offline Cadillac Owners Connoisseur
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    Re: Overheating mystery

    Quote Originally Posted by blocker11 View Post
    The car has gotten to the point where the needle is over 235 for sure, probably 250 too.

    The DIC says "engine hot - a/c off" and a few others that I cannot think of right now.

    If it got that far and you see nothing else you should test for exhaust gases in the coolant immediately. If it is HG's you don't waste time and money on other stuff. If it's not HG you search for an overheating problem.

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    JimD is offline Cadillac Owners Connoisseur
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    Re: Overheating mystery

    After reading your posts again, several times, I am hearing you describe what sounds like a cooling fan control problem.

    Will the engine overheat at idle sitting still with A/C OFF? If so, pop the hood and see if both fans spin up when the temperature gauge reaches approximately 223 degrees. You have to look; you cannot hear the fans.

  13. #12
    blocker11 is offline Cadillac Owners Member
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    Re: Overheating mystery

    Sorry for the delay....I was working all weekend & never got to idle the car for an extended period of time.

    However, this morning with no a/c on I drove my daughter about 9 miles to daycare & everything was fine....got back into the car after dropping her off & about a mile later the engine started to overheat (over 250 deg per your picture)...I turned the heat on & everything went back down to normal temp.

    Got back into car after making another stop & it started to get hot again (b/t 235-250)...pulled over & started the heat but it took much longer to feel warm air coming out as opposed to the first time.

    This is killing me, I love the car but don't want to sink 4G into a 10 year old car with 94,000 miles on it.

    Any additional thoughts would be great.

  14. #13
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    krimson_cardnal is offline Cadillac Owners Enthusiast
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    Re: Overheating mystery

    The fans being referenced are the radiator cooling fans, not the cabin heat blower fan, just to clear that point. Have you check the purge line for flow? On a a cold start pull the end at the tank and be sure you see flow. Could be a coolant flow issue causing problem. Are you loosing coolant?? K_C

  15. #14
    blocker11 is offline Cadillac Owners Member
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    Re: Overheating mystery

    my coolant level is good, just a touch low but I don't believe the car has fully cooled down yet.

  16. #15
    blocker11 is offline Cadillac Owners Member
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    Re: Overheating mystery

    Here's the million dollar question:

    What would cause a car to overheat when the A/C or heat is not turned on? Is there something that is supposed to fuction when those 2 items are turned off (water pump? I don't know)...the cooling fans operate properly when car is in idle and the temp starts to rise, the fans kick on a bring temp to TDC.

    I really feel like there should be a fix for this that doesn't involve head gaskets but I'm no mechanic and just trying to make myself feel better.

    thanks.

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