89 Deville Overheating

The radiator was leaking from the side tanks so I sent it off to get new tanks put on. Now, its overheating. New thermostat was put on a few months ago. Fans ain't kickin on but they can't cause it to overheat as bad as it is now. The coolant runs back into the overflow reservoir and bubbles from time to time. If it were a bad head gasket, it would bubble in the radiator right? Could the radiator be clogged? How do you tell if the water pump is working or if the thermostat is stuck closed again? Any other suggestions? Thanks a lot!

You need to get a more accurate picture of the engine operating temp when the engine starts to push coolant into the overflow tank.

Does the dash light for overheating come on? If not, try hooking up a good temp gauge and getting a reading. If you are getting no dash light and no overheating shown on the gauge, you have a bad HG.

The alternative is to pull the spark plugs and apply compressed air to each cylinder. You have to rotate the engine so both vales are closed before air is applied. Then look for bubbles in the rad. This method not only confirm if a HG is blown, but will tell you which head is bad....... although if you change one, its a good idea to do both at the same time....... and get the heads checked for cracks and straightness at the same tie, too.

The dash says check coolant fan so I'm guessing that is the cue its getting hot. I took a infrared temperature gun n checked the radiator. It gets cooler as it goes from one end to the other so I'm taking it that the radiator is not clogged? Correct me if I'm wrong tho. Its got a sound of a miss but I'm not sure. I suppose the water pump is working bc I can put my hand on the upper hose and feel pressure. There's no temperature gauge on the dash but I checked the thermostat with a temp gun n it was around 200-210 so the thermostat should've been opened bc its a 195 thermostat. I used "AirLift" and got all the air pockets out of the cooling system. I pressure tested the system at 15 lbs and its dropped S-L-O-W-L-Y to around 10 lbs so there wasn't much of a pressure drop. I didn't see any coolant leaking externally. What you think it is?

What are the fans doing? Do they come on with the a/c on? Off? If not, check the fan relays.

I have this same problem on my vehical. If anyone could help with the problem, I couldnt thank you enough.