2002 Pontiac Montana Minivan (3.4L GM 6-cyl). 3 different mechanics, can’t find where air is coming from. All of them say it’s just residual air from recent work (intake gasket replacement). No other symptoms other than air in system & overheating. No smoke, no leaks, no oil in coolant, no coolant in oil, etc.
Initial diagnosis was faulty intake manifold gasket. However, after taking it apart, our mechanic discovered gasket was fine, but some previous person had worked on manifold and failed to tighten any of the bolts!! They had worked their way completely loose, no wonder air was leaking in. Went ahead and installed new gasket.
Also had coolant system power flushed (to try to push out any air pockets, as well as any gunk). But problem persists - if the engine is warm, I need to bleed the air every 2 minutes, or it will overheat.
Finding most of the air in bleeder valve for the upper hose (return hose). I point the nose of the vehicle in the air with ramps (the bleeder valves are pretty low in the engine, I try to force the air to the front). Then I bleed the air (takes a good 5 minutes, lots of air). Then try to run the vehicle. 2 minutes later, more air clogging the system, overheats again, another 5 minutes of bleeding the air.
Have repeated this process 50 times, yet problem persists. I’ve burped the system, massaged the hoses, cracked the radiator cap - each time lots of air. Seems like a massive amount of “residual” air. I believe it’s taking on air somewhere, but no symptoms. No coolant leaks, etc. I’ve been parking the vehicle on a clean sheet of cardboard, so I can monitor any leaks whatsoever. But there are none. Also no symptoms of head gasket (no smoke, no coolant in oil, no oil leak, etc). I also replaced the bleeder valves themselves.
My guesses: The interior heat is barely luke warm, I’ve been wondering if one of the heating cores is cracked (the vehicle has two). I’ve been told that if there was a crack I’d see coolant leaks, but maybe the crack is on top? If so, I may rework the hoses to bypass both heater cores. I also wonder if the water pump is sucking in air somehow.
This is our previous family minivan, now I use for work. We’re done with the vehicle, I was ready to sell before this started. I’ll have a much easier time selling it if I can get this problem solved first.