I've been noticing the loss of power in going up residential hills in my neighborhood. So I warmed up the 1952 M38, 24 volt, all stock Jeep. Pulled all the plugs out and inserted the compression gage, opened the throttle and cranked till the needle stopped. What I found was 80,80,0-5,55 lbs. On number three there just was no resistance. .A little back story..... I asked a Gentleman to take a look at it about 5 years ago. He said he put in new rings and gaskets in it and it ran what I thought was great. Never faltered any unless I did something wrong to cause it.
Now here we are. I'm not sure what to look for or what to recognize when I open the motor up. I have read about broken rings, valves not closing. What should my next step be?
Thanks in advance.
Rusty, I moved your post here to our technical board since it is not considered an ongoing project jeep record type post. Wes K
It's time for you to open the engine manual and follow the troubleshooting charts. It is not time to pull the head yet.
As a starter the next thing I do is a differential compression test. If you do not know what this test does, or how it is done or what tester you use then it's time google "Differential Compression Test" and follow the data you find.
It's quite simple and will point you at the culprit quickly.
This is what I have done and found out. I didn't have a differential pressure tester. I put the cylinders at TDC when I checked each cylinder with air. What I did find is the air blows back into the intake side of the motor on Cylinder 3 and 4 with little or no resistance. Cylinder 1&2 leaked the air so little I couldn't tell in which direction. I read Page 72 in the TM 9 8012, I have more than 20 psi variation. It didn't give any direction to go other than to notify ordnance maint. personnel. Which Manual to next?
You have either two stuck open intake valves or a head gasket leak between those two cylinders. First pull the side valve cover and compare all four pairs of valve operations. Then pull the head.
Get yourself a copy of the civvy service manual which offers more than a referral to the motor pool. SM 1002 is a good start:
That's what I did this afternoon. I've done some reading in a civvy manual and followed the trouble shooting area. I removed the intake/exhaust manifold, valve cover. I was hoping for a broken spring or something obvious, nothing jumps out at me. Tomorrow I'll remove the head. I did notice some water corrosion between cylinder 3 & 4 between the head and block. Didn't see that till I removed the intake and exhaust mans.
Sure glad I used antisieze on the intake/exhaust nuts.
What about the 80 PSI from cylinder 1 & 2? 80 PSI it's a far cry from 135.
Before I removed manifolds I did manage to get 25 psi on cylinder 3 and 75 psi on cylinder 4. 1 & 2 are still close in psi.
More to come tomorrow.
Thanks
Pressurize #1 & 2 cyl's and see where the air hisses from before you pull the head. Don't forget to listen for air blowing into the oil sump past the rings. Just pull the dipstick out and place your ear close to the top of the tube. If you do hear a loud hiss in the sump then squirt oil on top of the pistons and recheck compressions. If they improve (air hiss is lower) then the 80 is from worn rings/bores.
Compressions tests are meant to be done with the engine at normal operating temp and the carb throttle valve & choke plates wide open.
You could have carbon build up on the valves. In the past I've poured water (slowly) down the throat of the carb on a fast idle running engine to steam clean the valve and pistons, hard to do now that it's apart.
Mike B.
Mike B
1953 M38A1 Brush Truck
1952 M38
1951 M100 Trailer
Air coming from crankcase, pulled head off. Carbon build up on pistons not much carbon build up on valves. No.3 cylinder valves looked a lot cleaner than the rest ( less carbon). So since I'm here, I've not rebuilt this motor so I'm going to do it. Got the civvy manual out going line by line on disassembling. It just kept getting slower and slower down the road. Best I could do on the flat is about ~45.
I have a picture of the pistons but can't remember how to get it here on this post. I have it in my album, I tried opening the album in another window and then tried to copy it. Just not sure now.
Looks like two head gasket leak areas and a head & block that need resurfacing. #3 looks like it has been water injected for a while looking at the piston.
For various reasons the mating surface of the head and block deform over the years and once the head gasket lacks a uniform sealing pressure across it's entire surface it will allow leakage of coolant, compression and intake/exhaust airflows to bridge parts of the gasket. The machinist resolve this issue at overhaul by reurfacing (re-machining) (or as often referred to as Decking) the mating surfaces to true them up.
If your block serial number is still stamped on the block by the water pump
and you have the block decked, be sure and make a note of the number
and get it restamped after machining if they remove it during the process
Ya thanks, It's been decked before cause the serial number was gone when I got it. I hope there is enough to work with. Do you think that the head gasket being bad would cause the oil to always be dark. It seems like the oil was always dirty.
I have another block that still has the serial number on the water pump boss. It's from a 48 - 49 willys. Haven't opened it up, but the whole block is there minus the valves. Hopefully I won't need it. I saw they have NEW blocks for sale at Kaiser Willys for 1600 and change.
.A little back story..... I asked a Gentleman to take a look at it about 5 years ago. He said he put in new rings and gaskets in it and it ran what I thought was great. Never faltered any unless I did something wrong to cause it. 
I got here using an outside way. My kid helped me so I don't know. But here is the block. You can see I rubbed the valves with my finger.
