Stumped by Radiator - 90 Chevy K1500

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2,789
Location
Indiana
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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Messages
402
Location
California
Radiator caps have a vacuum relief valve in them, you got a defective cap. (I would recommend buying a dealer cap, ALL the aftermarket caps are constructed very poorly)
 

Astro14

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Let's see if I've got this: 1. new cap = hose collapse 2. Old cap = hose works Conclusion: defective new cap. As to why; when the coolant heats up, there is water vapor pressure. As the system heats and cools, that pressure is released by the new cap, but the new cap isn't allowing the coolant from the overflow tank to be drawn into the system when the vapor pressure drops during a cooling cycle. So, since the vapor pressure drops considerably, the weakest point in the system (upper hose) collapses under the resulting vacuum. The key point however is that the new cap doesn't work properly.
 
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1,116
Location
midwest
It sounds like air in the system would be compressed by the water pump, which would in turn pull against a closed thermostat collapsing the upper hose, the low pressure in the rad should suck in coolant from the res through the rad cap until the system is full. How did the air get into the system? I wonder if the hose from the rad to the res is not tight and the system sucks in air instead of coolant from the res.
 
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1,555
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Oregon coast
Ive heard of plugged radiators causing the hose to collapse. If the old cap was leaking enough the hose would not collapse, but there would still be no coolant flow. Pull both hoses off and backflush with the garden hose.
 
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546
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Near Mobile
In my experience, in every case it is a bad radiator cap. A cheap item to change to find out. Over the years I've had several new parts that were bad. One was a generator on my '61 Ford that my after market amp meter showed was bad. Dealer replaced it, under protest, and all was good. I too like OEM caps and thermostats.
 
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1,116
Location
midwest
The new cap was defective? What brand was it? Did you replace it with the same brand or another? Thanks for updating, posting the solution is what makes these forums so useful.
 

Phishin

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Indiana
Originally Posted By: JamesBond
The new cap was defective? What brand was it? Did you replace it with the same brand or another? Thanks for updating, posting the solution is what makes these forums so useful.
JamesBond....yep, the new cap was defective. It was a Duralast from AutoZone. It was the kind that had the red plastic pressure relief lever on the top. I returned the cap, and exchanged it for a non-safety cap. I'm not sure, but I think it was another Duralast cap. But it didn't have any safety lever on it.
 
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