M38A1 Gas Cap Question

1952 - 1968, M38A1, M170, questions, discussions, regarding anything related to the M38a1 series.
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USASA
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M38A1 Gas Cap Question

Post by USASA » Tue Jul 27, 2021 11:43 am

So, a while back I replaced my original (rusty) gas cap with the new one below and, once I opened the valve, everything worked fine.
Fast forward several months and we're on a convoy over the Santa Fe Trail, up to 10,000' with ambient temperatures all over the place. Several vehicles were suffering from vapor lock and, at one point, it looked like I'd fallen prey to the same ailment. As a group, we'd let things cool down and then all would be well (until the next incident). My jeep was running at a rock-steady 170º the whole time.
So - here's the question what does the "Vapor-Lock" on the gas cap mean when the valve is in the CLOSED position? If I have vapor lock, does closing the gas cap valve help alleviate it? I just happened to notice the embossing this afternoon.

And yes, I know M38A1's don't suffer from vapor lock, but mine - to coin a term - definitely "failed to proceed" at times.

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Gene
'54 M38A1
'67 M416


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Miles Through Time
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Re: M38A1 Gas Cap Question

Post by Miles Through Time » Thu Jul 29, 2021 10:56 am

Most on this forum have forgotten more about military vehicles than I will ever know, but since no one has responded, I'll at least keep the thread alive. I may have the wrong understanding of the term vapor lock in your reference, but in the vintage vehicle hobby, vapor lock is described as the liquid fuel turning into vapor by an external heat source, usually an exhaust manifold. If this occurs on one of these vehicles, either stopping and waiting for the heat sorce to cool down and then proceeding, or pouring cold water on the fule line or carb bowl that is causing the issue. So, with your trail ride, when vapor lock occured, a respite from the group would cause the heat source to cool down until you proceeded and the heat source boiled the liquid into vapor again.

By referencing the vented fuel system you may be referencing an occurance like a fuel pump loosing it's prime or a pressurized system loosing it's pressure. I'm not aware that the A1 had such a system, but perhaps it does and I just never had ours do that.

I'll follow this post to see if I'm leading you down the wrong trail on vapor lock.

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Re: M38A1 Gas Cap Question

Post by dpcd67 » Thu Jul 29, 2021 7:10 pm

No, you are right; vapor lock in the engine compartment fuel lines, carb, or pump, is not related to the vacuum created by a non vented gas tank cap or failure of the gas tank vent system if it has one; that should not be called "vapor lock" because is isn't. The valve on the gas cap is only for fording; once you get out of the water, you open the valve. Vented field tanks and gas caps are one system; the phenomenon called vapor lock is not related to them.
So, the question, "If I have vapor lock, does closing the valve on the gas cap help"? NO.
Two different things with two different causes.
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USASA
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Re: M38A1 Gas Cap Question

Post by USASA » Fri Jul 30, 2021 2:00 pm

Thanks - that's what I thought but, left alone in a hot garage sniffing gasoline fumes :mrgreen: and reading gas caps, makes me come up with these questions. Case closed.
Gene
'54 M38A1
'67 M416


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