GMC Repair: erotic temp guage, engine coolant, radiator cap

Question
I just put a new heater core in my 1990 gmc pickup w/350 5.7 liter engine. It was leaking on the floor. Ever since then, I get about 2 miles down the road and the temp guage almost redlines. After I turn the heat on high, it goes down, then up, then down,etc... I suspect that there is air in the system. If so, how do I get it out?  Tanks for your help, Randy

Answer
Being VERY CAREFUL, Leave the radiator cap OFF and start the engine and as the engine starts to warm up the engine coolant will begin to deploy massive amounts of coolant from the radiator cap opening, just raise the idle up to around 1200 RPM'S and be CAREFUL so as not to get burnt by the deploying anti-freeze from the radiator keep adding coolant until the coolant stops deploying from the radiator as this will purge the air.


Usually if you just keep the heater ON and hold the engine at a high idle it will normally purge itself, but sometimes they can be stubborn.


Are you SURE that your thermostat is NOT stuck CLOSED ?.


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