I'm stumped. Please help me. This problem is being seen on my 1990 Chevy K1500. I observed some leaking around my radiator cap last week. I took old cap off, and saw the rubber gasket on the cap that seals to the radiator was distorted a bit, so I replaced the cap. Now with the new cap when truck gets warmed up, the upper radiator hose collapses. While the truck is running, I remove the new radiator cap and notice that it's REALLY sucked tight to the radiator. When I pull the cap off finally, and break the seal, the upper radiator hose IMMEDIATELY fills back up with coolant. The truck coolant temp sensor on dash was approaching red. Once upoer hose filled up, temp dropped back to normal immediately. So, I check the overflow tank hose and it's not plugged. I easily blew on each side of the hose and blew air into the radiator or into the overflow tank. I next thought the thermostat might be plugged, but when I install the old radiator cap, the upper hose gets really hot and stays full of coolant. Obviously, the old cap is allowing the cooling system to pull air into the system. I can heat air gurgle sometimes in the heater core. The new cap is sealing correctly, but why is the cooling system creating a vacuum against the radiator cap and allowing the upper radiator hose to collapse??
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