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Discussion Starter #1
I just replaced my 2 core stock rad to a 3 core aluminum rad, waterpump, harmonic balancer,thermostat, fuel pump and went from a 12 lb rad cap to a 13 lb rad cap. When I drive around for a few minutes and shut the car off I get a knocking noise from the cooling system. The upper rad hose actually moves a bit when it's knocking. The car is not overheating and I have heat in the car. If I leave the rad cap loose then it does not make the noise. Any idea what is causing this knocking noise and how do I get rid of it. I have a 1968 Mustang, 289, air, power brakes, power steering. I'm thinking maybe I should look through the garbage and put my 12 lb cap back on. The last thing is when it's knocking if I open the rad cap a bit then i can hear the a/f rushing through the top hose. My only other thought is my thermostat is faulty but I can't understand why I'm not overheating.
 

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Try putting the old cap back on. If it's still doing it, I would suspect a bad head gasket maybe. Could be letting exhaust gases escape into the coolant causing bubbles and the noise.
 

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What you are hearing sounds like water hammer. A glob of water followed by hot steam pushing it along. I would say you have air trapped in the top of the engine and or coolant hose. Did you have the heater on when you filled the system?
 

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Discussion Starter #4
I forgot to turn the heat on when I was filling it but since this has started I turned the heat on and I have good heat. Any idea how to get rid of this air. I've let the car run without the rad cap on , drove around with the rad cap loose, not sure what else I can do.
 

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park the car with the nose uphill making the rad cap the highest point, on the steepest hill you can find. and just let it idle with the cap off. it should chug a few times then let out the bubbles. then you can top off your coolant and you're good to go.
 

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Discussion Starter #6
Got it fixed

I let it run forever and finally got the air bubbles all out and it's all good. Thats guys for the advice.
 
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