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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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Well, I went ahead and pulled all three cooling fan control relays and tested them using the relay test procedure in ETCG’s relay test video. All three relays tested out fine, but I noticed that the contacts on the relays were fouled with that green scale/gunk. So I cleaned the contacts and re-installed the relays. Then I let the car idle up to temp, and re-drove my 1-mile test with the AC on. When I got back and opened the hood, there was no bubbling in the reservoir, and the fans seemed to be operating normally — definitely not erratic as before. I plan to make a longer more rigorous road test tomorrow.
OK — over the past few days I have been able to spend a bit more conecentrated time with this car:
The easiest/quickest way to get this thing to overheat (actually what happens is coolant boils out of the overflow tank) is to drive it one mile starting with a dead cold engine with the AC on full blast. If you drive the car with the heat on full blast, you can drive it for 20 miles or more. But even when you do that, once you park the car and let it idle you can see bubbles coming up into the overflow tank. And, it will bubble for a while after shutdown. But when you drive with just the heat on the tank may not necessarily boil over at all even though it bubbles a bit.
I have performed several block tests for head gasket leaks. It checks out just fine, the fluid stays perfectly blue. I have done several pressure tests, and the system holds pressure just fine. The cooling fans do come on, but I do wonder about them. My understanding is that the fans should both be running if the AC compressor is running. But this is not always the case. Sometimes one or both of the fans will slow down or stop even while the compressor is running. This happens with a warm or cold engine. Yesterday I pulled all three fan relays and tested them by applying voltage to the relays’ coil contacts and then checking the switch contacts for continuity. All three relays checked out fine as nearly as I could tell, though the contact blades at the base of the relays all had that green scale-buildup on them, so I cleaned that off as well as I could and reinstalled the relays. This didn’t help, the car still boils after a mile with the AC on.
The radiator is relatively new (replaced last spring), water pump is new, thermostat as been replaced, radiator cap has been replaced. There is no debris at all between the radiator and condenser. I am using 50/50 premixed coolant.
Any further thoughts/ideas you might have would be appreciated!
The fans should come on with the ac on. Pull the coolant
sensor connector and bridge the connector with a small paper
clip.fans should come on full speed. you may get an engine light
which we can reset later. If fans come on and stay on drive the car
see if you over heat.If not I would suspect the ect.OK this morning I did as collegeman said and jumped the temp sensor plug with a paper clip. I started the engine and both fans came on full blast. I did the same test as before, drove the car hard for one mile with the AC on full blast. The car “boiled” the coolant out of the surge tank, exactly as before.
Am I right to strongly suspect that this is in fact a false boil, caused by an exhaust leak into the cooling system, even though I can’t get it to show up on a combustion leak test?
Wow. both fans on and still an overheat. I’m stumped
for the moment. With both fans running does a piece
of paper stick to the condenser or fall off?Gentlemen, we finally have an answer. I bought a brand new block test kit, made sure I had fresh fluid in the kit. Today I was able to rev the engine and get controlled bubbling in the surge tank. While it was bubbling I did the block test with the tube in the neck of the surge tank. I pumped the bulb about a dozen times, and the test chemical turned from blue to green. Definitely looks like I have an exhaust leak intot the cooling system. I don’t know why I had so many false positive tests. Being an inexperienced DIYer might have something to do with it.
I really do appreciate you guys’ help with this, and I appreciate your patience with the rabbit trails I have gone down on this thing.
Stay Dirty, guys. You are the best.
I think, since this engine has almost 200,000 miles on it, I am going to swap the engine out for one I have in my garage on a pallet that has 75,000 miles on it. I just hope I can get the engine out the top because I don’t want to mess with the whole dropping the subframe rigamarole.
Glad you found the problem! Don’t worry, it’s sometimes very difficult to find a combustion leak like that with one of those testers. The most accurate method that I’ve found is by using an emissions testing machine. You remove the radiator cap and put the tester probe near the opening. If you see HC, then you have a combustion leak.
I remember you saying you don’t have a Northstar engine in that vehicle. That’s a good thing because it’s not the head gaskets on those that go so much as the head bolts pull the threads right out of the block. You need to re-drill all the head bolt holes and put new thread repair kits in them to properly fix them. It’s a real pain.
Having that engine is pretty handy. Hopefully you can R&R it through the top like you want. Good luck and keep us posted.
Hey thanks, Eric.
I have been doing a lot of research on swapping the engine and it looks like dropping the subframe is reallly the way to go. The Haynes Guide says you really need a car hoist to do this, and some web threads I have read say that not using a car hoist you run the risk of warping the body of the car, because it does not have enough bracing with the subframe out.
Is there any way to mitigate this risk of ruining the body using jacks/jackstands? I do not have a hydraulic car lift in my garage.
Honestly, you may be in over your head on this one. That’s a LOT of weight and you run the risk of serious injury if you don’t have the right equipment. Dropping subframes is no joke. If you don’t have a lift and a brace for when you drop it out, you could get into serious trouble.
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