Why is my radiator knocking

Foster03GA82Foster03GA82 Member Posts: 3
edited February 2019 in Pontiac
Please help! I have an 03 Grand Am 3400 that continues to run hot. Over a period of time I have replaced the exhaust manifold gasket, head gasket, timing cover gasket, water pump, and now the thermostat, coolant sensor, and radiator. After replacing the radiator, coolant sensor, and thermostat, I refilled the system (it only allowed 1/2 gal of coolant. I started the car an let it come to operating temp. Still could not add more coolant. I opened the bleeder screw but it never ran coolant. After 45min I started to hear a knocking sound near or from the thermostat housing almost like the sound of positive pressure engaging then release pressure. After a few more minutes the temperature red lined and the car started to over flow from the reservoir. Also cycled the heat in and off once car came to temp. Why is this happening?

Best Answer


  • Foster03GA82Foster03GA82 Member Posts: 3
    Hi imidazol97 thanks for the advice, that does make sense. I wonder if the knocking could be the result of the pressure build-up from the steam briefly forcing the thermostat open?

    The design of the thermostat housing makes it impossible to put it on backward and the location makes it a headache to remove because you have to remove the throttle body and almost everything else on the right side of the engine.

    It is horizontal so no bleed hole on the thermostat itself, but I am starting to wonder if autozone is giving me the correct thermostat. This is the third one. It is the one with the rubber ring around it, so I think if I ran it without a thermostat it would just leak from the housing. Once the car got to temp the upper hose was warm and the lower hose was cool, so I think there was some flow at least. Also, the car ran for nearly 30min before it redlined.

    This engine is the 3400 SFI that has the historically problematic cooling system. There is no cap on the radiator. It has the pressurized expansion tank. While trying to fill it I left the cap off and had the bleed screw open. Unlike some others I see, mine doesn't have a bleed screw at the thermostat housing. It has a connector for the return line. In addition to having the cap and valve open, I pulled the return line from the reservoir so I could see when coolant started to flow. It never did. Only steam

    I think I'm gonna try removing the hose from the thermostat housing and pouring coolant directly into the radiator that way. I also considered disconnecting the heater core hoses and pouring coolant directly into the heater core. Maybe this will help to displace any air pockets. Other than that, I am at a loss. Thanks for the advice and please let me know if you think of anything else.

  • imidazol97imidazol97 Crossroads of America I70 & I75 Member Posts: 26,506
    edited February 2019
    That's an awkward setup for the 3400.

    Have you seen these videos on the 3400 engine in various applications and filling them to get the air out?


    2014 Malibu 2LT, 2015 Cruze 2LT,

  • Foster03GA82Foster03GA82 Member Posts: 3
    Thanks for those vids. I haven’t had a chance to try that but I will try it tomorrow and let you know how it goes. A friend of mine thinks that the head could be warped but not cracked since it doesn’t seem to be mixing oil and coolant. I’ll will keep you posted. Again thanks for the advice. 
  • thecardoc3thecardoc3 Member Posts: 5,559
    Here is the best way to fill a cooling system.
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