overheating still

98 pontiac transport has some issues. I replaced the head gaskets and since then i have not been able to keep this van from over heating. one day its fine the next overheating. Right before it overheats i lose my heat. Pressure tested ok, fans operate, replaced water pump, radiator,thermostat(2),flushed heater core, upper hose lower hose and all gaskets top to bottom. Flushed system twice works for a day. Any thoughts or ideas would be helpful thanks.

Welcome to AF.
Thats a tough one........... possibly an air pocket in the cooling system......... or loose/defective impeller on waterpump?

I will check the water pump monday morning. If its not that i think the head gaskets are leaking some where and letting the air in. Cant figure the air would be coming from anywhere else. With the van elevated in front end it will run for hour and not overheat put it on the ground and overheating again. I figured air but wasn't sure how or where? Hopefully its the waterpump! Thank u!!

There has been a case on this or other forum with same engine block having all pump impeller blades rusted off... I wish that would be your explanation..the front end up, by your sayng, causing enuogh differential gravity to cool the engine ..Keep us informed of developments....

Might have got a bad water pump im hoping if not its got to be air getting in at the heads somewhere. Theres no leaks and temp guages gos up and down more than normal. Replaced everything but heater core which i dont think would make the van overheat. However the heat will go away and then the van overheats. The van then will drop back down on temp guage and heat returns. Dont get it?

.....ahh... I just noticed you did have replaced the pump already....:headshake.. : a pump with impellers does pump, period..yeah..I'd have hoped for that simple solution....but I'm inclined to consider the head gasket issue was indeed not resolved, however, as all the symptoms are matching....may be one head was warped in the first event....very annoying...it takes a very small leak to let exhaust pressure invade the cooling system on the power stroke when engine is under load....

Might be cracks too..

From a cold start, check the coolant for air bubbles with the engine running & rad cap off. May take 5 or more minutes..

no success yet pretty sure its air now. the vans rear heater core and lines i think is where the bubble is. The rear heater core has quick connect fittings hard to "burp". any old school tricks on 3.4 to get all air out of these vans? i can get the engine air out of lines then in a day its back. appears like its coming from rear lines. 99% sure heads and gaskets are ok. had machined and pressure tested before replacement.

no success yet pretty sure its air now. the vans rear heater core and lines i think is where the bubble is. The rear heater core has quick connect fittings hard to "burp". any old school tricks on 3.4 to get all air out of these vans? i can get the engine air out of lines then in a day its back. appears like its coming from rear lines. 99% sure heads and gaskets are ok. had machined and pressure tested before replacement.

Any possibility of bypassing the heater cores with hose, to prove that there is air being trapped in the heater cores? :frown:

ok here we are last two days drove without thermostat in. Van didnt overheat at all never came off cold line. It cant have head cracks or gasket issue or should have overheated in the 50 miles i put on it. i reinstalled a super flow thermostat and drilled a 1/8 hole in it. drove it ten miles left it sit then drove it 3 hours later and it overheated. Gotta be air locked somewhere???? Does this make sense to anyone?