Problems with Coolant System

I recently purchased a ’92 Deville with the 4.9. The previous owner thought it had a blown head gasket due to excessive white smoke being emitted from the exhaust. He temporarily remedied the problem with Bars Leak. When I got the car, the heater core was not putting out heat, and I found there was no thermostat installed, so I flushed it, and installed new 195 degree thermostat. Well, now the white smoke problem is back, the coolant is pushing back and overflowing the overflow tank, and she is overheating. I checked compression, but neglected to open the throttle (oops, I hope I don’t have to do it again); lowest cylinder was 165 and highest was 200; one plug had oil on it, and other 7 were clean.
Did I buy a faulty thermostat from Advance, or did my flush free up the Bars Leak that was plugging the leak? How can I properly diagnose this? I have to fix this car, and I will do the work myself. I can get an engine guaranteed not to smoke or drink for under $400. I have the time to work on it, but not the money. However, I would hate to replace the head gaskets only to find I have a cracked or warped head. Any suggestions on how I should proceed would be great.

Throw some more stop-leak in it. Bars Leak is okay but not the best. Try going to a good auto parts place, (one that sells to mechanic's shops) and ask if they have any liquid stop-leak. Tell them you need something that is very effective.

I had an old BMW 733 which had the same problem as yours..... I suspect it was a cracked head, and not a blown HG. I used liquid stop leak which fixed the problem for years.

However, if you do have a blown HG of course stop leak will not work but it's worth a try.

Also, Cadillac recommends that all their aluminum V8 engines made up to 2002(?) should have AC-Delco stop-leak tablets (at least 2 but most people add six) added to the bottom radiator hose every time the cooling systems are flushed, whether there is a problem or not. These engines develop minor leaks etc so often, it's considered normal.
However, your problem does not sound normal. But stop leak may fix the problem for a while. And yes, a flush will knock loose old stop-leak.

Finally, cheapo used engines usually have their own problems and are not worth it. IMO its better to fix the one you have or buy a professionally rebuilt engine.
The range you have in compression and the oily plug indicates the engine has wear issues beyond the coolant loss. IMO if the stop-leak does not work, it's time for a proper rebuild.
I know this is not what you want to hear but I have learned from experience these fwd Cadillacs are really cheap to buy used, but often require expensive repairs when they get old.

One more thing, you did not install the thermostat backwards, did you? If you did, it will overheat and push the coolant out.
You can test your thermostat by placing it in a pan of water and bringing it to a boil on the stove. It should open shortly before the water boils.

Bars Leak is okay but not the best.


Also, Cadillac recommends that all their aluminum V8 engines made up to 2002(?) should have AC-Delco stop-leak tablets (at least 2 but most people add six) added to the bottom radiator hose every time the cooling systems are flushed, whether there is a problem or not. These engines develop minor leaks etc so often, it's considered normal.


Bars Leak is who makes the coolant supplement tablets for GM. And the tablets are recommended for all aluminum engines, not just up to a certain year. Also, the coolant tablets can be placed right into the radiator. They are placed into the coolant hose on the Northstars because they have the cap on the expansion tank, not the radiator.
NAPA sells a coolant test kit that confirms that you have a blown head gasket. It's about $50, and it easy to use. It might be worth it, then you'll know exactly what you are dealing with.
I agree that a used engine is cheap, but the 4.9's rarely do blow head gaskets, so a good used one would be a cheap fix!

Guys,

I really appreciate the quick feedback. I will check to make sure I installed the Thermostat correctly, and take it back out and test it, per your advice. If all that checks good, I may go ahead and buy the NAPA Test Kit (sounds like a good tool to have around the garage).

Thanks again.