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'81 CDV, '64 SDV, '73 MB 450SL
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Discussion Starter #1
Today, sitting in traffic for a VERY breif period of time, my 98 SLS overheated. The needle started climbing, and the "ENGINE HOT AC/OFF" warning appeared on the DIC. Shortly thereafter, it changed to "ENGINE HOT IDLE ENGINE" So I pulled over, turned the heat on and revved it up until it cooled down to normal temp.

It got warm again driving, but I left it in 3rd and it calmed down and didnt get hot again the rest of the day.

The car was probably only sitting in traffic for 5 minutes. I've driven this car 40K+ miles in the last year, mostly highway, but plenty of sitting in traffic jams too. It never overheats. Very occasionally, sitting in a drive thru at the bank or whatever for a few minutes, the needle will jump a slight bit (just barely right of center) but as soon as you give it a little gas it drops down to where it normally sits.

This car has never gotten hot, or overheated before. Does anyone know what it could be?


Coolant flush was 10K miles ago, (and is full cold, I checked) The thermostat was changed when the coolant was flushed. Fans work, with the AC, and you could hear them changing speeds (speeding up) as the engine got hot.

This one baffles me. There is no evidence of air binding, coolant and it was like a fluke thing. It will not overheat no matter how long it sits in the driveway with the AC on now.

Maybe it just got jealous, it must have known that I was on my way to testdrive a 75 CDV. I was cheating on it lol.

Any ideas would be greately appreciated.
 

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2009 CTS4 2007 STS
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298 Posts
at best if you have no coolant loss check the cap on the coolant tank if its bad what happend to you is one of the symptoms
 

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94 Eldorado, and a 99 ETC
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3,887 Posts
Maybe the fan relay got stuck, then after a little bit it freed up?... Kinda hard to diagnose something like this. Would have to replicate it while monitoring what the fans and engine are doing and whatnot.
 

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'81 CDV, '64 SDV, '73 MB 450SL
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224 Posts
Discussion Starter #5
Who knows. Today it was fine. And I sat and sat in traffic, AC on, and it was 88 degrees out, yesterday it was barely 75. Maybe just a fluke, or something stuck the thermostat, and now its free.
 

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Who knows. Today it was fine. And I sat and sat in traffic, AC on, and it was 88 degrees out, yesterday it was barely 75. Maybe just a fluke, or something stuck the thermostat, and now its free.
I am willing to bet that your fan relay took a dump on you yesterday.
 

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1992 STS / 2005 MB G500 / 2003 STS / 2006 XLR-V
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11,694 Posts
Hopefully its not H/G related, because of the unusual nature of what occurred Id agree with the relay thing or possibly a malfunctioning coolant temp sensor.
 

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'81 CDV, '64 SDV, '73 MB 450SL
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224 Posts
Discussion Starter #8
Re: 98 SLS overheated --- MORE.....

When I began this post, it was the beginning of what is becomming a more normal problem. I have some (I think) more useful information for diagnosing the problem.

I'm thinking the fans are fine. You could hear them running, and you could see them spinning (fast) when the hood was open and the overheating was going on.

It has now become a chronric problem. It was around 92 degrees today.

Start it up for the day, it works fine when its cooler out, in the morning. I travel for work. I drove the car about 50 miles and when I began slowing down (Low RPMS) the temp began climbing.

Cruising about 60 (also low RPMs) with the A/C on, the engine begins to heat up also. It doesnt seem to heat up slightly. The temp guage climbs steadily, it probably takes a minute or maybe slightly longer to get to the red zone and the DIC says "idle engine." If you pull over, and idle it at about 3000 RPMs, it cools right down to the half way mark where it should be very quickly.

Once it overheats, its going to overheat in traffic no matter what.

At 60 MPH, I noticed that if I just shift into 3rd and elevate the RPMs (about 2500 or so in that gear at 60 with the cruise on, it cools off, and stays at the half way point (normal) for the remainder of the trip.. Thank goodness the northstar is fuel efficient.

I'm guessing water pump. But I'm not really sure. This is a strange one to me.

What I can say, is that the Coolant level is good, and fresh (coolant flush over the past winter) I maintain the oil, and the fans work.

I need to figure this out soon, this is my work vehicle :helpless:

Thanks in advance for all your help.

Aaron
 

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White Diamond '03 DHS (with DTS floor shift)
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86,979 Posts
Alright, I'll say it. Have the coolant tested for exhaust gases (head gasket).
 

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'81 CDV, '64 SDV, '73 MB 450SL
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224 Posts
Discussion Starter #11
Should I have posted this in the northstar forum?

I'll have to get the tests done, but every search I've done on overheating that involves head gaskets seems to be the exact opposite conditions.

The head gasket ones seem to overheat when under load, or acceleration, overeheats when its under easy conditions.

Example. If it starts heating up, I can mat the pedal and accelerate to 100 MPH and it'll cool off under load.
 

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1997 Eldorado, 2003 Audi RS6
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1,320 Posts
No, the overheating from head gaskets comes from pressure being driven into the system displacing the coolant and causing it to be dumped out temporarily counteracted when RPM is raised, even at high throttle. After you let off, the temperature will climb, probably with a 20 second or so delay first.
 

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'81 CDV, '64 SDV, '73 MB 450SL
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Discussion Starter #15
I think it's the head gasket :(

I started it cold this morning, with the "radiator" cap off, I guess its not really a radiator cap since its in the recovery tank, but anyhoo..

If you gas it some, you can watch the coolant sink, and when you drop the throttle, the coolant level increases.

I let it warm up with the cap off, and it never overheated.

Once it was warm, I did the throttle thing, and it brought the level up enough to run some of the coolant out.

It doesnt lose, and has not lost any coolant, but the coolant does look a little contaminated. (It is only 10K miles old).

Once I put the cap back on, the car began getting hot and I shut it down.

This sucks. I really like that car, and I cant believe the amount of problems it has. I've always owned a Cadillac since I got my license. The old ones were fantastic cars. The new ones stink.

I ran my 93 Deville with the 4.9 into the ground until the undercarriage fell apart around the engine that never gave me a day's trouble, and was still getting upper 20s fuel economy with over 200,000 on it.

I cant believe these Northstars are so notorious for this HG problem. And for what it's worth, I HAVE maintained the coolant, and the dealer did it for me.

UGH.

Aaron
 

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White Diamond '03 DHS (with DTS floor shift)
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I guess its not really a radiator cap since its in the recovery tank
Actually, it is a surge tank.

If you gas it some, you can watch the coolant sink, and when you drop the throttle, the coolant level increases.
That is why it is called a "Surge" tank. THAT is normal. Do you see bubbles in the tank when cold? That is not normal.

Once it was warm, I did the throttle thing, and it brought the level up enough to run some of the coolant out.
That also is normal. With the cap off it cannot pressurize and thus the boiling point is not raised.

It doesnt lose, and has not lost any coolant, but the coolant does look a little contaminated. (It is only 10K miles old).
Can you describe "contaminated"?

Have the coolant tested for exhaust gases.
 

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'81 CDV, '64 SDV, '73 MB 450SL
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224 Posts
Discussion Starter #18
Sorry I didnt reply sooner. Was working on the 81 CDV I just bought.


The coolant does not bubble at all.

DUH surge tank, I knew that's what it was called. I couldnt think of it and didnt feel like looking in the FSM.

By contaminated I mean it just doesnt look like I thought it should.. Maybe brownish-greenish.

But then it dawned on me that before I took it in several months ago, I put in some leak check that you use a black light with to see if I could figure out why the heat didnt work. There were actually no leaks. But that substance was kind of a strange color, and it looks similar to that. Perhaps the color is fine. I'll try and take a picture, but there are no guarantees it'll come out well. The heater core was blocked at the time. I had the dealer flush it then. I never looked at it after, but the heater has worked fine since, and the coolant level still remains normal.

I checked the waterpump belt and tensioner. To me, they both seem fine.

I'm interested in this exhaust test. Does the car have to be present for it, or can I bring a sample to my dealer? I'll call them on Monday.

I'd rather not drive it at this point, but I do not want to tow it there for a simple test, if I can bring a sample.

-Aaron
 

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White Diamond '03 DHS (with DTS floor shift)
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Forget the dealer. Any radiator shop can do it or you can buy a kit from Napa or such. I think you can use a sample , but not positive as I have never had the need to do one.
 

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'81 CDV, '64 SDV, '73 MB 450SL
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Discussion Starter #20
Well, here is the update. Today, I went to Napa and Advance auto parts, and they all looked at me like I had three heads when I asked about anything used to test coolant for the presence of exhaust. OK, fine. So I figured at the very least, I'd grab a new cap, and I got home and replaced it.

I checked again with it cold, to be sure, that there was NO bubbling in the surge tank. I revved it up, watched the coolant, and there was never any bubbling, and the level still has not changed. I replaced the cap, and I let the car idle for a solid hour, and it did not overheat.

Next, I had to get gas anyway, so I took the car to the gas station, and left it running (AC on) filled it up, and drove it around some. Temp never moved from 1/2 way point on the guage.

Finally, I drove it down to a back road where I know I wont get busted, and did 3 sprints to 100 MPH, full throttle, and just let the car coast down from there. Never got hot.

When I got home, I left the AC on and allowed it to idle for another 30 minutes or so, the needle went up a tiny bit, the fans kicked on, and it returned to 1/2 on the guage.

Could my car be fixed?

The big difference will be that today, it was kind of rainy, and not very hot out. Around 65 degrees, and it has been quite a bit warmer, so I'm interested to see how it behaves when it gets hot out again.

I'm praying that the cap was the only problem!

Thank you, as always, to everyone who has chimed in and tried to help with this problem. I think if I dont have to pay for head gaskets, I'll have to become a supporting member :)

Plus, I can work on my 81 CDV.

-Aaron
 
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