1998 Pontiac Transport Montana 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.

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

It is 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.

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.
1998 Pontiac Transport Heat Goes Cold Then Hot

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