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These days I’m also working on my 1994 Cadillac Deville (4.9L, ***NOT*** Northstar. 194,000 miles). OK here is what I have: Car overheats. Pressure tested cooling system, holds 16PSI like a champ. No evidence of any external leaks. Recently I replaced the leaky radiator, and when I did that I replaced the thermostat, and I did not install the thermostat backwards (been there, done that). While I was at it I replaced the water pump. Flushede system both ways, and there do not seem to be any obstructions. Car ran fine for several weeks then this overheat problem started. Engine runs fine while cool/at operating temp. Fan comes on when it is supposed to, and as nearly as I can tell it works OK. Anyone have any ideas? I have hoit a wall with this one. Thx ~ Pop
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++++1 on the above. check out this link to help.
http://www.ericthecarguy.com/faq/what-to-do-when-your-engine-overheats
Check the radiator cap, it may need replaced… Look for collapsed hoses as well… If not you may need to do a Block test on it…
-Karl
Radiator cap is new and checks out fine. I have done a block test and car passes with flying colors. repeated block test three times. Sorry I forgot to mention these items up front. Radiator hoses are new (replaced when I did radiator), none collapsed.
Is this problem occurring without the A/C running… I.E. does it only happen when the A/C is running… There may be an issue with the condenser…..
Just checking, but was your thermostat the proper temperature as OEM?
-Karl
Could you provide a little more info of when it overheats? Sitting in in traffic? Driving on the highway? Going up hills? When you say overheat how hot is it getting and what is the behavior when it does?
The last time I drove it it overheated after 2 minutes on the highway from a cold start. Prior to that, it required 10 or 15 minutes of highway driving before it would start to get hot. But that was the deal — sailing down the highway, and then get the HOT ENGINE message on the display. I think I am going to re-do my tests. It has been a while since I have worked on this car, as I was interrupted by more pressing issues on my BMW and my Suburban.
Speaking of re-doing testing, when you do a pressure test, how long do you leave the pressure tester on before concluding there is no pressure leak? 15 minutes? an hour? 12 hours? I left mine on for a couple hours.
Well, I have somewhat of an update: Now that I have my other vehicles back on the road, I spent some time on the Caddy yesterday. I installed a second new thermostat (some weeks ago I had a new one in there, removed it to test it, then lost it). Again I made sure I didn’t install the thing backwards. A mechanic buddy loaned me his block test kit; one that has a tube that you can hook up to engine vacuum so you don’t have top pump a rubber aiball. Per the directions I removed 3 inches of coolant from the radiator, and let the car idle for ten minutes. The directions said to then perform the block test for two minutes. Well, I was only able to go for one minute before the coolant level would surge up, out of the radiator, and contaminate the block test fluid. But in that one minute, anyway, the fluid stayed its original blue. I performed this test three times, the same result each time.
Out of ideas, I just filled and air-purged the cooling system, and figured I’d do some road tests, per requests above, to see if there were certain circumstances when it would over heat, and ascertain exactly how long and under what special circumstances, if any, the car took to over heat. Well, wouldn’t you know it, I drove the car 30 miles at highway speeds, mixed in some stop and go and long periods of idling, and the car did not over heat. The only thing I notice is there is a bit of bubbling into the overflow tank at idle after driving at highway speeds or in stop-and-go conditions. The bubbling stops after one minute of idling.
This seems to be an improvement anyway, as the last time I drove the car weeks ago it over heated within 2 miles from a cold start. Yet, that bubbling I see is still worrisome.
OK – Just finished another road test. Drove car 1 mile with the air conditioning on full blast. I stopped to check the tank, and coolant was boiling out of it. I noticed that the cooling fans were running erratically, even at times not at all despite the A/C compressor being on and the coolant boiling out of the plastic tank and onto the ground. I think maybe the next thing to check is my fan relays.
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