Engine loosing power

[Dave W.]

Hello All, I have an M38 engine in a 1942 Willys. I've owned it for 20+ years and have put 6,000 miles on it driving the jeep trails in North Lake Tahoe. Being at 7,000 ft elevation I installed a Supersonic high compression head when I first got it to gain some horse-power back and am running Pertronics ignition. It has been running great until the last year. I checked the plugs, the ignition wires, timing, air and gas filter and all seems OK. Checked the compression and get (1) 92, (2) 80, (3) 78, (4) 83.

Any suggestions? THANKS

[parker007]

your compression is low. put oil in cylinder and retest, if same do valve job. does it smoke or use oil?

[Wolfman]

First thought.
I have a friend that owns an M-38.
It would start OK but by the time he got a mile down the road, it would hardly pull itself.
Turned out to be the valve clearance had tightened up.
When the engine cooled down, life was good but by time the engine got up to operating temp. you had to down shift to keep going. Valves were not seating.
Adjusted the valves and it was like new again.
Not sure what the normal compression pressure would be at 7000 feet. Even with a Supersonic Head. You can only squeeze the air if it is available.
The head has a smaller combustion chamber to raise the compression ratio but it is still compressing air in the same piston displacement.
Hint: The piston does not suck air in. Atmospheric pressure pushes the air in. At 7000 Ft. Less push.

[Dave W.]

Thanks for the info.
It does not smoke.
I believe the compression with a stock head is supposed to be 90 -110 PSI. Considering my lower readings (even with the high compression head), I will probably do a valve tappet check and adjustment as necessary. I don't have a M38 manual; my "The Military Jeep Complete" TM 9-803 says 0.014 for both intake and exhaust. Is that the same for the M38?

New question: At temperature cruising at 35MPH my oil pressure reads 25-30 psi, but at idle it lowers to 5 psi. Is that OK? Currently using 10-30 High Zinc oil.

[parker007]

there is many topics on oil pressure here. My opinion is 10-30 is to light. I would go to strait 30 wt, 20-50 or 15-40. it seem to me your engine is getting tired. I think 10psi is about the least when hot you want. If it was me i would check the valve lash, if it good time for a rebuild.

[Ron D]

The M38 manual says for compression test all 4 spark plugs are out and the throttle is wide open.
Normal is 105 to 115 psi.
Minimum allowable is 75 psi.
Maximum variation between cylinders is 10 psi.

M38 manual calls for .016" on the valves.
Could be mistaken, but pretty sure that Willys published a bulletin on that spec good for all L134's.

Hope this helps.
Good luck!

[Dave W.]

Thanks Ron,
Since my compression values are significantly lower, I will be ordering the necessary gaskets so I can check my tappets and be able to reinstall everything.

If it needs a valve job, I'm not really interested in removing the engine. I'm 75 and can remember my dad seating the valves with a grit paste with the head off and the valves still in place. Is that still something that can be done?

When I purchased the 42 MB in 2000, I assumed the engine was a stock original motor. Later someone told me it was a M38 motor. Anyone have the info to allow me to identify which motor I have? The numbers on the lower right side of the block are:
804380-W-11-D-P5 CUCR

Thanks for any help.
Dave W.

[sjalbert]

Per: http://www.rensjeep.com/casting.html

Block Casting 804380 is an M38 after s/n MC74419

[Ron D]

Later someone told me it was a M38 motor. Anyone have the info to allow me to identify which motor I have? The numbers on the lower right side of the block are: 804380-W-11-D-P5 CUCR
Thanks for any help.
Dave W.

Hi Dave,
I think all you'll know for sure is the casting number. Yes, 804380 is a 1950's cast number for the gear driven motor used in later M38's.
The only way to know if you have a M38 motor is if the engine serial number is still present on the boss above the water pump.
Many serial numbers got machined off over the years if the top of the block was ever decked to make it flat again.
M38 engine serial numbers are prefixed with either "MC" or "RMC" (MCxxxxxx or RMCxxxxxx).
I'm pretty sure Willys used 804380 blocks in other products (with different serial number prefixes).

Be sure to use the correct head gasket. I believe you need a Felpro 7285B.
Felpro also makes a 7267B gasket for earlier motors. They're easily confused and aren't interchangeable.
I'd have to dig it up, but Willys published a Service Bulletin describing the difference --- and problem using the wrong gasket can cause.
This information isn't in the manuals. Ask me how I know.

Good luck!

[Dave W.]

Thanks again for all the info.
I am ordering gaskets today from R. Fitzpatrick to do a tappet check and a head cleanup. I hope he is aware of the correct Head Gasket.
Dave

[Ron D]

Hi Dave,
If it helps, the Willys Service Bulletin lists the Willys part number A-8558 for the later application head gasket.
I looked at the RFJP website catalog and he also shows Willys part number A-8558 and says it's a FelPro, but curiously doesn't give the FelPro part number.
If it comes in FelPro packaging it'll be labeled with the 7285B part number so you'll know for sure.
If not, the FelPro 7285B is also embossed on the top face with the FelPro trademark and the 7285B part number, as well as "This Side Up".

The only other L134 head gasket he shows is also listed as A-8558 but it's all copper, which I believe is a Best Gasket 555C (more $ than the FelPro price, but some folks like them).

Good luck!

[Dave W.]

Thanks Ron.
Parts are on order. Will re-post when I get it apart and make my measurements. Hopefully, won't have to do a valve job.
Dave

1942 MB with M38 Engine

[Wolfman]

Lapping valves was a method used mostly back in the 50's and before. There is a valve lapping compound that can be bought.
Lapping has slowly faded away over the years. Used more for checking valve seat position and width on a valve face now days.
If you have leaky valves and the seats and valve face are in good shape, just carboned up from minor leakage, like tight valves, lapping compound will clean both up to restore the seal.
If the valve face or seat are burned, it won't work.
And if you do try to lap the valves, do a good job of cleaning up the mess. Lapping compound is pretty aggressive and will do more damage if you don't.