I had to replace the transmission in my 1999 monte carlo 3.1, I replaced it and drove it for a while with no issues and now I have an overheating problem. I replaced the thermostat and waterpump flushed the coolant and replaced it, the heater core isnt clogged nor is the radiator, theres no coolant in my oil nor oil in my coolant. The catalytic converter isnt plugged and the air intake works fine. I want to say that its a problem with the head gasket sucking in air because the coolant is either boiling or its bubbling and its the only thing I can think of that could be causing my issues. Any ideas? Edit: took off the whole topside down to the head gasket and found coolant in there so lets hope theres not a bigger problem than just this gasket 😅

 

Are the Fans working properly? maybe they got unplugged or a relay is out on them.

 

Yes the fans work properly

 

then you got a head gasket that is bad!!! I just replace them on my suv 3400 at $1800.00 they also re-plained the heads as well. I had them put in new plug's while it was easy to do.