If you were to leave vehicle in park position and increase idle rpm to about 2500 rpm, would the temp. coming from the vents increase? if yes, you have a blocked heater core.
The heating coil is probably getting plugged up. You may have heat at idle or slow speeds, but once you get going, there is not enough coolant going through it to stay warm. Get the system flushed or replace the heating coil.
SOURCE: 2004 pontiac gran prix gt
I would suggest replacing the blower motor resistor. When this component fails the blower will only work on one spped or none. This is a very common issue with all automotive models
SOURCE: location of heater control valve 2005 grand prix
i don`t think there is a heater control valve.you may have an air pocket in system.
SOURCE: 1999 Pontiac Grand Prix Overheating
I had trouble with overheating on my 2000 Grand Am, The dealer could not even resolve the issue,(they wanted to replace the intake gaskets)...I knew that was not the issue, no air leaks or water leaking there; so I sit down to think; what are the causes of overheating. Thermostat sticking - nope not it, water circulates, Water leak - nope not it, has water stays until it gets too hot and goes out overflow, fans working - yep...hummmm the only other thing I could think of was losing of water pressure, yep that's it - the cap on the coolant tank was going bad, got a new cap ($8.00) replaced the old one and no more trouble going on 6 months now.
Hope this may solve your issue.
SOURCE: Air conditioning/head lights
it is your ignition switch. Pontiac runs too much current through the switch for it to last very long. look on grandprix specific auto forums for an in depth look at this problem.
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