302 "burping" coolant
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Freshman User
Joined: Apr 2005
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From: Georgia
302 "burping" coolant
I have a 73 F100 with a 302 / C4 - very mild. I just replaced the intake gaskets due to a vacuum leak on #2 cylinder. While I had the intake off, I replaced the thermostat (195) and bypass hose. The good news is that I cured my vacuum leak. The bad news is now that the engine is "burping" coolant. I have a large radiator (same as on F100s with a 460), closed coolant system with 12# cap and recovery bottle. When I fill the radiator cold, and the recovery bottle to the cold line, drive 1 mile, the recovery bottle will be full and overflowing. It is not sucking any coolant back into the radiator as it cools down, and will be about 2 inches low after it cools (recovery tank still full). It does not seem to do this idling in the driveway.
Anyone have any ideas - I am at a loss. (I do have good compression - 155 to 170 in all cylinders, and engine runs great).
Thanks
Anyone have any ideas - I am at a loss. (I do have good compression - 155 to 170 in all cylinders, and engine runs great).
Thanks
Postmaster
Joined: Apr 2000
Posts: 2,838
Likes: 6
From: SD
Did it do this before you replaced the intake gasket? If not, then it has to do with the work you just did. Does the temp guage get hot? Does the thermostat open and coolant circulate? My '73 F100 390 has the stock radiator without recovery bottle, the coolant is always about 2" below the top of the radiator, if I fill it more it just runs out when it's hot. So I leave it there and all is fine.
Marty
Marty
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From: Georgia
No, this is an all new problem... The temperature gauge goes to the same place (bottom of the normal range). Sitting idling from a cold engine, it takes about :10 for the thermostat to open. When it opens, you can feel the upper hose get hot, followed by the right side of the top tank, moving to the left - just like it should. I've replaced the radiator cap and will see if that has an effect - hopefully the cap decided to go bad at the same time I did the work. Thanks for the replies and I'll post what I find...
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From: Georgia
Problem found - and this is one you may want to file away for future reference... Truck has factory air which includes a vacuum operated valve in the heater hose between the intake manifold and heater core. The shaft on the valve had a slight leak that was letting air be sucked into the system as it cooled down (less resistance than the radiator cap sucking coolant back from the expansion tank). As a result the system would get a big bubble of air in the heater core / intake manifold. When you started it up cold, the air bubble would expand (more/faster than coolant) and force coolant out (more than 1 quart) until the thermostat opened to release the bubble. Replaced the heater valve, and problem is gone.
At least I didn't have a cracked head or blown gasket
.
Thanks for the replies, and I hope this helps shorten someone else's diagnosis.
Boyd
At least I didn't have a cracked head or blown gasket
. Thanks for the replies, and I hope this helps shorten someone else's diagnosis.
Boyd
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