Question about 1999 Pontiac Grand Prix GTP

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99 pontiac grand prix gtp overheats. installed flush kit. flush per mfg recommendations. used prestone flush 3 times. took thermostat out. currently using 50/50 prestone dex cool. overheats after approximately 45 minutes to an hour of driving in the city. When car moves around 45 mgh starts to cool down. any ideas?

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  • David Ray Nov 01, 2013

    Blockage in engine from using stop leak. Took 3 times to get it clean. This engine overheats with engine flush in engine so only use 10 minute flush or on open highway.

  • David Ray Nov 01, 2013

    Blockage in engine from using stop leak. Took 3 times to get it clean. This engine overheats with engine flush in engine so only use 10 minute flush or on open highway.

  • David Ray Nov 01, 2013

    Blockage in engine from using stop leak. Took 3 times to get it clean. This engine overheats with engine flush in engine so only use 10 minute flush or on open highway.

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Your thermostat or your coolant pump may be the malfunction

Posted on Oct 19, 2013

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  • David Ray Oct 19, 2013

    how do I test for a bad coolant pump?

  • jl rad Oct 19, 2013

    your thermostat may be bad, or your waterpump may be bad. you dont want to leave the thermostat out. also, be sure to use an air-hose and blow whatever debris may be stuck in the radiator grid, and blow it out from the engine side.

  • David Ray Oct 19, 2013

    i will try this thank you

  • jl rad Oct 19, 2013

    you can start the engine cold, then remove the radiator cap, and look down in the radiator to see if fluid is moving

  • David Ray Oct 20, 2013

    Pulled the radiator cap and observed the coolant flow thru the radiator. Flow seems to be going at a good pace. It looks in the city it peaks at 212 F and when the car starts to move it begins to cool down. I checked the fans and they are on. Thermostat is still out. Not sure what to do next.

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Replace with new thermostat

Posted on Oct 19, 2013

  • David Ray Oct 19, 2013

    I am on the 3rd thermostat now. this one has tested good (boil test). It over heats with the thermostat out or in.

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There could be several reasons that may cause an engine to overheat. It sounds like you have covered the most important ones. Thermostat,radiator and water pump. Hopefully you do not have internal damage, cracked head, head gasket, or a cracked block. I would try flushing the heater core and pay close attention to what comes out. You know, is it clogged with stop leak or rust. Then while I was flushing that I would drain the entire system and put in some Prestone super radiator flush. You just add plain water with it and run your vehicle for about 10 minutes. You then empty your cooliant system and refill with antifreeze. The flush works pretty well and if there is an obstruction from a previous owner putting stop leak in the system it generally will clean it out.
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