I have a 99 Chevy Tahoe I replace the radiator now the heat will not get hot but it will get hot when I take the radiator cap off but as soon as I put the radiator cap on it gets cold what do I do
I have replace a water pump in the thermostat
Did you install the thermostat upside down?
You would need to bleed the air pockets out of the cooling system, by having the radiator cap off, engine running. Let it warm up to operating temp. but do not be revving it up or you will get a shower. Coolant may push out the radiator witch is fine, that would be the air pockets working their way out. Keep filling as the coolant level drops in the radiator, until full. Put the radiator cap on while the engine is still running. Then you can shut it off.
F.o.R. asked, is the thermostat installed properly?
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