Gas cap has a hole.

DanS · Post by **DanS** » Mon Dec 02, 2013 7:40 am

Probably discussed before, but, my M38 gas tank cap has a hole drilled in the top and the metal plate on the underside. I suppose the PO was trying to vent the tank. Question is, should I leave it or fill it?

Dan

Bretto · Post by **Bretto** » Mon Dec 02, 2013 9:02 am

This needs to be posted in the tech forum.
I would get the correct cap or fill it and vent the tank properly.

donthedickens · Post by **donthedickens** » Mon Dec 02, 2013 9:16 am

From Service manual SM-1002-R5 in Fuel System chapter para F-51;

"A pressure type fuel tank filler cap is used on all models. This is necessary to prevent fuel leakage from the cap vent opening when the vehicle is on a side slope. Two spring loaded relief valves which open when venting is required are built into the cap. The pressure valve opens at 1 1/2 to 2 1/2 pounds per sq. in. [.105 to .176 kg/sq. cm.] and the vacuum valve at 3/4 pounds [.053 kg/sq. cm.]. Should the pressure valve fail to open, pressure in the tank may force fuel by the carburetor inlet valve causing flooding. Failure of the vacuum valve may prevent flow of fuel to the carburetor. Should the valves fail to vent install a new cap."

Post by **wesk** » Mon Dec 02, 2013 9:46 am

The M38 uses a very different and much larger cap. The tank is vented through a tank mounted vent fitting to the air cleaner for fording purposes. The early caps used were non-vented and when the tank mounted vent valve failed the fuel would not feed. The several later style caps all had a manual vent valve which only had two positions. Open for normal driving and closed for fording. The Army superseded the early non-vented caps with the later manually operated vent caps. The Army ignored the leaking issue when the jeep was canted too far left.

M38 tank. The smaller unit is the sender. The larger unit is which has a large and small assembly on top, fuel exits through the larger section after coming up through the filter inside the tank. The tank is vented through the smaller section to the air filter.

M38 tank again. Item A is the larger assembly referred to above. The sloped top unit on it's right is for fuel to flow through the smaller square unit above the filter support bracket is for the vent line.

This is the later vented cap with manual vent valve.

The drilled cap will function the same as the manual vent valve cap when the manual vent valve is in the open position except the field modified cap with the small hole drilled in it doesn't keep moisture and dirt out.

donthedickens · Post by **donthedickens** » Mon Dec 02, 2013 7:35 pm

Wes,

Thank you for correcting my mistake.
Sorry folks.