Pontiac Repair: Temperature/Engine Problems, upper intake manifold, pontiac grand prix

Question
I have a 98 Pontiac Grand Prix GT w/ 3800 engine.  I just had both the intake manifold & thermostat replaced 1-2 mos. ago b/c low coolant light on (also ABS & trac on/off light), air conditioner didn't work & temp gauge kept spiking when idle.  It almost overheated on me & shut down, but I thought I may have caught it in time. Once back from shop, everything worked fine again...until a couple weeks ago. The heater blows cold air more often than not, when it blows warm air it's sporadic & may depend on how fast the car is moving.  Also, I noticed tonight that the temp gauge is starting to creep up again when idle.  Help!!!  :)

Answer
Hi Jen:

well a guess would be that the upper intake manifold plenium is warped and the coolant is leaking into the engine and being burned. It's pretty common on these engines especially if it has high mileage. it was probably okay for that 2 months with a new gasket on the upper manifold and it finally gave way and the new gasket can't seal up the gap cause by tha manifold being bad. The uppr manifold is made of a plastic material and after time a hundered or thousands up tempature cycles from hot to cold the plastic just warps. It will need a new one. Good luck :)