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I bought a rebuilt fuel pump for my M38. In the body where the vent line should go is a vent cap that looks the same as the ones fitted to the axle differential housings.

My questions are
1 - Did the Army / Willys delete the transfer case to fuel pump line in late model M38s with sealed bell housing? There’s nothing in 8012 or 1804B to suggest they did.
2 - Did they do so for the M38A1?
3 - Could this be a field modification?
4 - Will it affect how the pump works?

If this is very unlikely to be a military modification I’ll install correct vent lines.

The M38 had that line thru the end of production in 1952. The M38A1 had that line until the end of the Army contract fording equipment at MD23359. It was at that point in 1953 that the M38A1 switched from the line to the transfer case to the small vent fitting on the pump. Your replacement fuel pump is obviously originally for the 53 A1.

Thx Wes

I’ll fit a proper vent line to keep it original M38

Remember, even with the proper vent line you need the proper elbow and connector on the center of the pump body. The connector has a check ball in it!






These are hard to find but there is a Canadian Member that was selling them.

Hi Wes

I’ve been reading more about the vent system and wanted to clarify a couple of things if that’s ok:

1 M38 without deep fording and waterproof bell housing the pump port was plugged. In this configuration the pump middle section would be at atmospheric pressure or maybe very slightly above as crankcase pressure would be exiting through the PCV and the oil filler tube.
2 With the check valve fitted and with fording valves open the check valve also closed off the port as the valve was closed by the spring and ball
3 When the fording valves were closed and the bell housing drain plug installed crankcase pressure would open the ball check valve at 2 psi and through the vent line would pressurize the bell house, transmission and transfer case

Are these points correct?

If they are then if there was no check valve but the lines were installed per 9-8012 and the fording valves were open and bell housing drain plug removed wouldn’t the only issue be a small escape of crankcase gas through the vent line to atmosphere via the bell housing? Wouldn’t that be pretty much the same as pre waterproof condition in point 1?

I’m looking for options to look right and still work if I can’t source the correct check valve.

As always many thanks for your help, I still have a lot to learn about my M38!

The MC-38 Maintenance Manual has a good description of how "Crankcase Ventilation" works on page 33 and 34. When describing the crankcase ventilation valve (commonly known as the PCV valve) it states that "When this valve operates correctly a slight vacuum is present in the crankcase..."

This contradicts a portion of your point number 1 "In this configuration the pump middle section would be at atmospheric pressure or maybe very slightly above.."

1 M38 without deep fording and waterproof bell housing the pump port was plugged. In this configuration the pump middle section would be at atmospheric pressure or maybe very slightly above as crankcase pressure would be exiting through the PCV and the oil filler tube.

1st both early & late M38's had deep water fording systems. The late system just does it better. (Find yourself a copy of the Early M38 Operator's Hdbk TM 9-804 dated Sept. 1950) As Bob mentions in normal operations with ventillation valves in non-fording positions the crank case exerts a negative pressure or vacuum on the body of the fuel pump


2 With the check valve fitted and with fording valves open the check valve also closed off the port as the valve was closed by the spring and ball

Yes, meaning things would operate just like when the pipe plug was installed with fording valves open.

3 When the fording valves were closed and the bell housing drain plug installed crankcase pressure would open the ball check valve at 2 psi and through the vent line would pressurize the bell house, transmission and transfer case

Yes

If they are then if there was no check valve but the lines were installed per 9-8012 and the fording valves were open and bell housing drain plug removed wouldn’t the only issue be a small escape of crankcase gas through the vent line to atmosphere via the bell housing?

To some degree the arrangement you describe can cause a wee bit of grief in that crankcase being able to operate at a slight vacuum under normal dry operations would be a good thing however using your thoughts then the crankcase could never develop any vacuum.

Wouldn’t that be pretty much the same as pre waterproof condition in point 1?

With the blinf pipe plug installed the crankcase can develop some degree of normal operation vacuum. Leaving that plug out it cannot.

The bottom line here is do you want a stock correct M38 based on which fording system plumbing belongs on it (early vs late) or not? If not rather than try to use a made up work around simply delete the two fording valves and the plumbing and keep the simple civvy style PCV system only.

Thx Wes. As it turns out my distributor has the cap type valve too not a vent line so I guess it’s probably M38A1 too. I plan to strip it and do a full paint job in the spring so I’ll make the vent lines then to bring it back to M38 spec. The hard part I think will be finding the check valve. Any ideas where I can find one? Tried internet search but nothing came up.

There were only 3 distributors used during M38/M38A1 production. The Army supply system allowed the first to deplete and leave their system. The first two distributors (early & late M38) and the M38A1 distributor all used the plumbing for venting the distributor housing. None came with a axle housing style check valve fitting. So if you have one with

my distributor has the cap type valve too not a vent line so I guess it’s probably M38A1

it was modified by someone who either did not want the two pieces of tubing or wasn't able to locate or fabricate two.