overheating! HELP!

[SlickRick90TT]

I have an overheating problem that came on suddenly. It runs cold for a few minutes then all of a sudden it jumps up to normal operating temp and climbs from there all the way up. The heater continues to blow cold air, so i assumed it would be a faulty thermostat. replaced that, but still does the same thing. The floorboard on the passenger side is dry and the inside of the windshield is not accumulating moisture, so im assuming its not the heater core. It is blowing steam somewhere at the rear of the engine near the firewall as it begins to overheat. cant tell where really. I'm stumped now, and i need this car on the road ASAP as it is my daily driver. if anyone has any ideas please help!!!

[marty1mc]

I assume when you changed the thermostat you did bleed all of the air out of the system? If not, check the radiator and refill. I can take a cycle or two to get all of the air out. If you haven't bled the system of air, be prepared to dump a gallon in.

Next, start it up and look for moisture in the back of the engine. If you are losing enough coolant to overheat, you should be able to see it. There are a bunch of hoses back there for the heater core, turbos, plenum. Look it over carefully and look for leaks as it pressurizes. Also, look at the turbos themselves and make sure they aren't leaking from there. Are you seeing a puddle under the floor?

[Rich@RDZ]

I agree with Marty, purge the cooling system.

[DCaff300ZX]

I agree sounds like the purge issue...mine often takes 2-3 tries to get it all bled out after any radiator removal.
Be sure the fan is on and squeeze the hose to push the coolant to the top of the spout each time until there is no bubbles...drive around and repeat a few times and see if that helps if marty's suggestions don't help.
I had several "mechanics" swearing I had HG issues for months until I got a new radiator and did the 3-time purge, and no overheating since.