Trying to decide what to do with my motor

1952 - 1968, M38A1, M170, questions, discussions, regarding anything related to the M38a1 series.
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IdahoTory
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Trying to decide what to do with my motor

Post by IdahoTory » Mon Mar 13, 2023 8:33 am

First post! Hello everybody, my name is Tory.

I apologize in advance this is going to be a long one...

I recently acquired an M38A1 (Date/Del. 10/52). The previous owner had only driven it a couple of times and it overheated. That was a few years ago. He swapped the head gasket, no joy. He pulled the head and had it milled, no joy. Then he parked it (pre-Covid) in his heated garage and left it there.

I bought and paid for it as a non-runner last week. I took the risk because it appeared to have most of its original parts and was for the most part unmolested. Virtually rust free even in the usual trouble spots. The battery box had some light surface rust underneath the trays and even the floor boards were solid. Its a good rig from that respect. Still has a strong 24 volt system and it likely had some refurbishment in the past.

As per tradition... it also came with several boxes of what appear to be original parts and NOS still wrapped in the original paper. A set of original rims with the pin holes. An original canvas top, a nicely done roll cage (not installed) and an M100 Trailer that looks to be unmolested as well.

I did all the usual hasn’t been started in years things. Pumped out all the old fuel, blew out the fuel lines. Lubed up the cylinders. Resurrected the batteries and after about 20 cranks she blew her smoke, and away it ran.

The F134 ran OK cold with around 20 PSI of oil pressure at idle and around 35PSI at higher RPM’s. I let it heat up to 180 indicated with the radiator cap off until the thermostat opened and I could see coolant flowing. It idled nicely for about 30 minutes. Drove it back and forth in my driveway. The brakes work, the clutch felt almost new, it shifted.

All the gauges work = Temp, Oil pressure, volts, speedo, fuel.
Charging System = Pumping out 28v+
Tires = Very good. No cracks, still had whiskers.

Took it for a short drive and it felt low on power. It would struggle to make power in anything but first gear, but it still sounded smooth. I ran it for about a half mile on flat road and managed to make it into 3rd and then like a light switch, it started running poorly all of a sudden (the real issue!). I limped it home and used the throttle lock to keep it running. It sounded like it lost a cylinder so I decided to pull some plug wires while it was running. The first wire I pulled was number 4. Nothing changed. At the very least cylinder 4 has a problem. Sometimes you get lucky on the first try...

Cylinder 4 diagnosis

Wire and plug test. I pulled the plug assembly (its that oddly satisfying OEM screw type submersible cable and plug) and replaced it with a spare spark plug to fill the hole so fuel wouldn't spit out while I tested it. As soon as I cranked it back up I could see a strong spark. I put it back together thinking maybe it was just a bad connection but no. Cylinder 4 has spark. I tested the other cylinders in the same way, they all have similar voltage readings, all were good.

Compression test. I started from the front and worked my back on a warm motor.

Cylinder 1 = 90 PSI
Cylinder 2 = 75 PSI
Cylinder 3 = 70 PSI
Cylinder 4 = 0 PSI

It was for sure running on all 4 cylinders when I idled it before taking it out for a drive. It was smooth. When it got warm, everything went to crap. I suspect if I tried again tomorrow it would behave the same way. It would likely run on all 4 cylinders just fine until it warms up and then go to crap.

It could be a sticky valve on 4, bad gasket, or a crack. Maybe its starved for oil someplace seizing up a valve… I have no idea. As for the rest of them, I can’t think of any way it would be so low on the other three cylinders except for normal wear. I suppose it could be a bad head gasket or even a crack between 2 and 3 in addition to whatever problem there is on 4. I’m no F134 expert but I think 90 PSI on cylinder 1 on a warm motor is low no matter how you slice it.

Its also puking oil out the rear of the motor someplace, so much so that its hard to see exactly where its coming from but I suspect a rear main seal or maybe a problem with the labyrinth of oil lines. I don't have a lift, and crawling around on the floor cleaning oil off of everything is not something I'm going to do unless its absolutely necessary.

Questions???

1. I'm assuming those compression numbers on the "good" cylinders are not very good. Is that correct?

2. Is there a concept of numbers matching engines to frames to tubs on the M38A1? I plan to make this a daily driver (more or less) and I'm not specifically interested in doing a restore at this point. However, I don't want to swap motors if it would impact its value (collectability?) since I believe its such a good example. I can get a rebuilt long block or I can rebuild the engine that's in it. I'd prefer to just purchase a rebuilt quickly, drop it in there and be done with it, but I am interested in keeping it true to period if that's a thing with these.


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dpcd67
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Re: Trying to decide what to do with my motor

Post by dpcd67 » Mon Mar 13, 2023 5:29 pm

Welcome to the forum!
90 PSI compression will run fine.
Finding a rebuilt block will be very hard. Why not rebuild yours? Or just fix cylinder #4? You already know what to look for. Have you adjusted your valves?
Engine serial numbers have a gap ahead of vehicle numbers, and on M38A1s; no one cares about them.
Now I see that it is 75 on the other cylinders; you need to rebuild it.
Last edited by dpcd67 on Mon Mar 13, 2023 7:09 pm, edited 1 time in total.
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Kendall
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Re: Trying to decide what to do with my motor

Post by Kendall » Mon Mar 13, 2023 6:08 pm

What is the serial number on the engine (located on the flat behind the water pump)?

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Re: Trying to decide what to do with my motor

Post by parker007 » Mon Mar 13, 2023 6:25 pm

pull both valve covers and inspect it may be just a stuck valve

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Re: Trying to decide what to do with my motor

Post by IdahoTory » Thu Apr 20, 2023 9:23 am

I disassembled the engine and it was very apparent that the engine was oil starved. Hard to say when but it had oil and good oil pressure when I ran it.

All of the bearings were horribly gouged. The cylinders were all scored and more problematic is that they were already at .060 over. The crank was also ground close to maximum tolerances already and the there was some jankiness with the main journals (hard to explain). I brought it to my local machine shop and they did some more measuring, about the only good thing left was the fly wheel and the head.

In the meantime I received a package from the previous owner. It had paperwork back to 2002. This is not the original block, and I would have known that sooner if I paid closer attention to the leading letters and less to the serial number (I'm not a classic Jeep guy yet). It was a replacement SD block sourced and rebuilt in 2018.

Fast forward a couple of weeks...

The original block (the engine that was in it when I bought it) would have cost a hundreds of dollar more to fix the journals.

I tracked down another engine locally. It was locked up, but it was also really cheap at $100. I popped the head and its got a holed piston and the pistons are seized in the cylinders. I managed to disassemble it leaving the pistons in place. Enough to see that the crank was good. I then sent the whole thing to my local machine shop.

The shop extracted the pistons and magnafluxed the block. The new block had .060 pistons and a previous crack repair (well done). The cylinder walls were in bad shape but the crank, the flywheel and the rods were very nice and still had life left to machine them. I am going to have sleeves put in it but this will be the base for the rebuilt engine.

I should have a fully assembled long block any day now. I am also expecting to get the freshly rebuilt transmission, xfer case and over drive back in a week or two. I'm hoping for many years of trouble-free driving starting as soon as June.

This will work for now, but I will keep an eye out for a rebuildable MD block. One day somebody may care about those things, but for now, I'm gonna drive my M38A1 every day that its not raining.


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