Expert: Rob Painter - 6/5/2011

Question
I have a 1999 Cadillac Seville STS.
Recently (last week) I had to have the engine replaced due to a crack in the old engine's block.

For two days after getting the vehicle back it ran fine, (Approx 200 miles). Then it over heated. I am not losing any coolant this time. However, the cooling fans will not run unless the AC is on??

What should I look for to determine the root cause of this problem.
I am thinking relays, I just thought I would ask the expert first.

Thank you.

Answer
Hello,

That last engine was junk from overheating last time and you may not have fixed the root cause and you might be back to square one. Junk yard engines are VERY problematic, because most times they were overheated and not any good. 9 out of 10 junk yard Cadillac aluminum engines are junk!

Now, relays rarely fail. They can, but it is not common place. You have a problem because you don't know the history of the replaced engine. Coolant MUST circulate to get the fans on. If the cooling system is not circulating for numerous reasons, plugged radiator, plugged water jackets, stuck thermostat, the fans will never come on due to a excessive rise in the engine temperature. Your relays are obviously working because the fans come on with the a/c.

Even something as minor as an old RTV water pump gaskets, thermostat gasket etc, is enough to plug the small water jackets and keep everything from circulating throughout the engine.

I really appreciate your situation, my guess is the coolant sensor to turn the fans on is not seeing that the engine coolant is even hot, due to lack of circulation.

I will explain what I mean with circulation. Picture a circular pipe with an inlet and outlet going into a radiator. Free flow would be where the water/coolant whatever is circulating from the inlet to the outlet and through the radiator. Now, lets throw a thermostat into the picture which restricts while cold and as it warms up the thermostat opens and you have free flow again. Let's say the thermostat sticks closed when warmed up. This creates a restriction, so much that on one side of the pipe leading to the thermostat is now backed up and there is no flow to the other side of the pipe to run to the radiator. In other words, the water/coolant supply is choked off affecting the coolant sensor we just threw in. The sensor has not warmer/hot water to sense and will not turn on the fan. As far as the sensor is concerned, its not even warmed up to trn a fan on to cool the water/coolant. This probably is not the best example, but it gives you an idea of what you are facing.

Besides, other than the a/c mode and defrost, coolant fan only will work for you in the city. If you are overheating on the highway, my guess would be a plugged radiator.

Bottom line, I think you need to have this all checked out as to the circulation. If by some chance the engine was good, it may be as simple as replacing a stuck thermostat to get everything flowing and get the fan to come on as normal.

It could be much more complicated and turn into Pandora's box.

I would love to give you a simple absolute answer, but there are far too many variables here.

As I stated about old gasket material or RTV silicone I have run into these where a the gasket material had no effect in the water jackets, then became lodge blocking off the cooling system circulation. Possibly this happened in your case, where coolant is in one side of the engine and not on the other side causing the engine to literally melt.

I have had questioners come to me with over heating issues, so many times running into these old gasket issues, finding them and correcting the problem with my suggestions.

With the replaced engine, we don't know if a water pump was changed, which is very common for old gasket to enter the engine. We don't know if the thermostat was replaced.

Please forgive me if it took a long time to find the solution, but I think while I write and to go over these letters and edit with the short time restraints I have in volunteering these answers.

Here is what I would do: Go to the parts store. By a flushing T. Install. Connect a garden hose. Remove the radiator cap and turn on the hose. Put on the radiator cap. See if you can see a pipe plug on the front of the engine. It will screw into the block. You should see more than one in the block. Once you find a plug and use an adjustable wrench to remove this square head fastener. Once removed, turn your water back on. Is water flowing freely?
Do the same on the rear side of the engine. Plugs should be opposite across from each other. Turn water on with that plug removed as well. I believe you are going to see a major difference in flow showing you roughly what side of the block has the restriction. Then you can trace flow and attempt to find the problem.
If you have a heavy restriction on the front plug area. I would be considering a plugged radiator.

If you are forced to go this far, don't forget to run 50/50 anti-freeze.

All I am saying I have been dealing with the aluminum Cadillac engine and overheating issues since 1982. Cubic Centimeters have changed. Engineering concepts have changed, but the problems remain the same.

What if the owner of this engine used stop leak and the sensor to turn on the fans lost it's sensitivity by being coated and coolant flowing past it was 230 degrees and it did not pick this water temperature up to turn on fans?

I have a love hate relationship with these engines. When running cool, they are powerful and get great mileage.

Go for the thermostat first. Feel the radiator with your hand with engine warm. Is it equal heat? Cold?-No circulation!

Are the upper and lower hoses warm or cold? Should be equal heat. Any type of this testing you need to be extremely careful!


As you already know, you CAN"T overheat the great aluminum Cadillac engine!

Good luck and I hope you understood my little story. If I assisted you, please give the answer a rating. Thanks

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