1998 Pontiac Transport Heat Goes Cold Then Hot

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1998 Pontiac Transport Montana.
The thermostat was replaced but problem still there. Question: Hi, I have a 1998 Pontiac transport Montana. The heat will go from hot to cold then back from hot to cold then the temp will rise to hot then drop down to about 60 d then to about 100/110 and the heat will blow hot for about 5-10 min then same thing all over again i replaced the thermostat and the problem still there any help will be appreciated thank you.

Answer:
Sounds as if you are getting air in the cooling system. Some common causes of this are leaking lower intake gasket, or blown head gaskets. The rear headgasket is prone to this condition. From under the van, the pass. Side of the rear cylinder head may show signs of leaking externally. If that is the case, it may also be pushing compression into the cooling system. Air in the cooling system will affect the heater in the manner you have described.

You may want to let engine run with radiator cap off and look for bubbles. Or remove the spark plugs and inspect for signs of burning coolant.

It is also possible that the replacement of the t-stat fixed the heater problem, but you have air in the system from that procedure. This would act as if you still have the same problem, but in fact just need to bleed system.

I have a 1999 Pontiac Sunfire that just started overheating. I replaced the thermostat and water pump so far but it still over heats as soon as you start to drive it. The radiator doesn't seem to get very hot and it doesn't blow warm air in the car. What do you think I should try next?Sent from my iPad.

Answer:
Sounds like a blown head gasket. When the head gasket is broken or cracked, it will usually cause an overheat condition very quickly once the car is started. You can remove the spark plugs and inspect them to see if they've been burning coolant, or look into the cylinders while the plugs are out and see if you can see or smell coolant. The engine type in your vehicle has been known for having head gasket problems and warped cylinder heads.

Unfortunately, you'd have to disassemble the top half of the engine to correct this and get the head checked out for cracks or being warped.