99 3.4L GT overheating - Specifics

Okay, recently purchased a used 99 gt (3.4L v6) with about 160,000 miles on it. Coolant levels were jumping between overflowing out the reservoir overflow/boiling and a completely empty tank. Changed reservoir as the cap would never fully thread on completely. Believe we now have all air bubbles out of the system as coolant levels have stabilized.

Upon first heat cycle from completely cooled now, engine warms up as normal until hitting the 195 degree mark, then shoots up very rapidly into the 260+ range. Have done this 3/4 times now and car is immediately killed as it hits the 260 mark. After a few seconds of cool-down, motor can be fired up and the temp will immediately drop to standard temperature and seem to run fine but will intermittantly cycle between 205 and 230. EVERY time the car is cycled to operating temp from completely cool this occurs.

Have replaced the thermostat now, flushed coolant, and changed coolant reservoir jugs. There is no coolant in the oil whatsoever, or external leaks now as far as I can tell. Have been searching the forums up and down, but am at a loss here. Have a new thermostat on the way from GM in case the new one was faulty, but I hardly see that being the case. If it was an intake manifold or head gasket, would there not have to be an external leak, coolant in the oil or be burning coolant? I've all but completely ruled out any of the three as being the case.

Sounds like you still have air trapped in your cooling system.

Have you bled it from the bleeder valve above the water pump ?

Also turn your heater on so no air aill be trapped in the HC.
Run the engine with the resivoir cap off and dont let it go empty as it warms up.

Yup, have loosened the bleeder valve several times. It seemed to have released the air and be a reasonably constant stream of coolant, but I will have to give her a go a few more times. Whoops, I read about air being trapped in the heater core on here, but forgot to try taking care of that. The coolant level would hold perfectly still with the cap removed if air was absent from the system correct?

Just threw this together...
Bleeding Your Cooling System (http://www.automotiveforums.com/vbulletin/showthread.php?p=4813524#post4813524)

the coolant will flow in and out of the resv. through the recirculating tube lines so it should have constant flow(ing).

i just bought a 2000 grand am with the 3.4 and am having the same problem did you find a solution iv gon through all the same things you have and am at a loss

i just bought a 2000 grand am with the 3.4 and am having the same problem did you find a solution iv gon through all the same things you have and am at a loss

do not bump/hijack other's threads, specially this old, start your own thread insteard