I now am a victim of the dreaded “Overheat”. ’97 Deville, 4.6 N*
I have extensively searched and read every thread here that mentions anything at all about the cooling system or overheating, but alas, I am left “uneducated”.
It started last Wednesday afternoon, driving the X-way home from work, went about 25 miles when suddenly got the “Turn of A/C” message, I knew from reading here what was happening. I slowed from 75 to 60 mph but the temp kept climbing and messages appearing. When it got to 260 , I pulled over. Of course coolant was coming out the overflow.
I let her cool down long enough to get the cap off and had to add about ½ gal Dex-Cool mix. Started her up, checked the coolant level and went on my way. About 6-8 miles further, I again got the “Turn off A/C” message as I was exiting the X-way, temp started falling as I coasted up the ramp and was down to around 235 and falling so I easily drove the remaining 9 miles to my house while the temp fell to the usual 203- 217 degree readings.
When I popped the hood to check it out, I noticed that the water pump belt was quite stretched to where the tensioner was nearly bottomed out. I parked it then and went under the hood on Saturday after picking up some more Dex-Cool, distilled water, new Thermostat & gaskets, new belt, new water pump, and Bars Leaks powder (2 tubes).
Drained out all the old coolant, checked the water pump, (quite snug with no play, not too tight but did not spin too easily either. Only slightly less tight than a new one I have, so I didn't replace it), installed new belt, checked and put a drop of oil on tensioner, being careful not to get any oil on the pulleys and belt, and installed the new thermostat. I then proceeded to flush the block and the radiator. Old coolant from block was fairly clean BUT, when I put the water hose in the upper radiator hose, it seemed that the water from my hose didn’t flow as easily as I thought it should, the stream out was very weak. I then proceeded to back flush through the lower hose and still it seemed the flow just wasn’t as good as I thought it should be.
I then proceeded to blow all water out of the block and radiator with compressed air (<15 lbs.) on one end and a Shop Vac on the other. Then a refill with 50/50 Dex-Cool and distilled water, two tubes of Bars Leak, new belt, then did the usual routine of letting engine come up to temp and topping out the tank, let it cool down then a test drive and again topped off the pressure tank.
The temps on the dash seemed to be all over the place while driving in town(but not over 227 degrees) so I parked it for the night. Next morning I checked the tank, still full with a slight vacuum on the cap when removing it.
I then proceeded to start the engine and quickly check for air bubbles in the tank, only one good burp on startup, with no sign whatsoever of ANY bubbles all the way up to operating temp (197 Degrees), temps stabilized while driving around town.
No sign of water in the oil nor oil in the coolant, however, when I put it back on the interstate the next day at 65 MPH, temp got up to 240 degrees in 12 miles before I shut it down. Coolant was splashed out of the overflow hose. Let it cool down, topped off the tank and proceeded home on the highway at 55 MPH, went about 6 miles then before temp climbed back to 240, again shut it off, topped the tank and made it the rest of the way home.
I have an appointment on Saturday to have the radiator “Boiled out”, and “Rodded” by my local radiator shop, along with a pressure test of my cooling system, they do great quality work! I hope this will solve the problem as I have read here that a clogged radiator could cause overheating at highway speeds.
Anyone think the radator is my problem?
I really don’t believe and this point it is the Head gasket(s), even though she has 195K on the clock, she was religiously maintained at the Dealer by the lady who previously owned it. Not sure how the coolant was maintained, but it was a proper 50/50 mix and, like the rest of the car, it was clean as a pin!
Any ideas guys?

Anyone else had these symptoms? How did you fix it?

Yes, several people here had those symptoms.

Change the head gaskets.

Ask your auto repair shop to run a cylinder leak down test to prove or disprove a blown head gasket(s). IMHO the cooling system pressurization test doesn’t always prove a blown head gasket on an N* engine because this test doesn’t recreate the cylinder pressures that the head gasket endures as it starts to leak. The cooling system test is a low-pressure test less than 25 psi while the cylinder leak down test will be closer to 120 psi. You may as well find out before you throw a bunch of parts and labor at it and in the end still need the head gaskets replaced.

I have the same problem with my 94 sls, high way speeds, high revs or under some load. it blows the coolant out the coolant tank.. from what I read, time for head gaskets.
anyone can say what they think

HEAD GASKET HEAD GASKET.......

Sorry Man... It's the HEAD GASKET..

Even with the weird thing about the coolant not flowing very well from the radiator hose.. Very Sorry. Your wonderful 195K Deville HEADGASKET lasted quite long, but it's time to change her out..

I just got done with my change 1 month ago, believe me I wanted to believe it was something else.. It's the Headgasket.. Exact same symptoms you had, in a nut shell it runs perfectly except it overheats on the HYWY due to the coolant overflowing. Basicly it's cylinder pressre that is pressueizing the coolant system and yout 15lb cap can't handle the pressure.. ANd you really only have higher cylinder pressure under load like WOT or on the hywy. Short stabs here and there to 40MPH will not really cause your coolant to overflow..

Get ready to change out the headgaskets..

Sorry and good luck..

Clint

Make an adapter out of an old spark plug shell so that you can pressurize the combustion chamber with 120 PSI shop air with the piston at TDC. Watch the cooling system for bubbling. That is the sure test for head gasket integrity.

Are you sure the stat is working correctly?? Even with a new one, it would still be suspect from what you describe. Put it in pan of water on the stove and turn on the burner. You should see the stat open just before the water boils.

What about the vapor vent line?? Disconnect the 3/8 hose at the surge tank. Start the engine momentarily. Coolant should stream from the hose toward the surge tank. If not, the vent line is restricted somewhere and that would lead to overheating since the water pump cannot purge vapor.

What about the pressure cap? Is it sealing and pressurizing the system. Without pressure the system will build vapor and boil much easier. If the cap is not sealing well or has failed then it could also cause the same symptoms.

There is a strong possibility it is the head gaskets starting to fail..but it is funny that you went from no problems to an overheating condition that quickly. Typically head gaskets start to slowly fail and you will start to notice signs much sooner than a sudden, catostrophic overheat like you describe. Just seems odd from your observations.


Not sure about your "flushing" procedure with the hose. The stat was in place when you did that from your description. If the stat was in the system then I would expect very little flow per your observations. The stat is on the inlet side of the water pump. When you are putting the hose in the upper radiator hose fitting as described you are back-flushing. There will be little to no flow out the lower hose fitting on the water pump if the stat is in place as the stat is closed. So...your description of the flush sounds perfectly normal if I understand you correctly. To get a lot of flow thru the engine like that you would need to take the stat OUT and then flush with it out of the system.

In older cars, a highway overheating problem usually meant a clogged radiator. So when my '97 DeVille showed the exact same symptoms yours did, I sent the radiator out to be reworked first. But alas...no change. Not to sound like a broken record here but, it's head gaskets if the radiator work doesn't fix your problem. Everyone tries the radiator trick first, but I have yet to read a thread on here that didn't end up being head gaskets! I would go ahead and check the things Bbob mentioned just to be sure, as I did on my own engine. Having done that, combined with the measures you have already taken, you have eliminated all other possibilities and head gaskets are the only possible cause left for your overheating problem. As was stated, the thermostat must be removed in order to back flush. How can you get a flow through a closed thermostat? Answer: you can't.

If you really got 200K miles before teardown, that's about as good as it gets. Consider yourself fortunate!