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willysmjeeps.com :: View topic - M38A1 engine diagnostic tests
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M38A1 engine diagnostic tests
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JeepdaddyRC
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Joined: Jan 10, 2020
Posts: 37

PostPosted: Thu Feb 11, 2021 11:34 am    Post subject: Reply with quote

Thank you Wes. This M38A1 is new to us. My son and I are learning, but nearing the end of our skills and tools. Unfortunately, it has been too cold to drive it (WI). The Jeep has had little use in recent years, maybe driving it more will help to re-seat the rings, reduce the foul exhaust smell and improve the starting (now there is some wishful thinking!). Even working on it in our 20 degree garage is a challenge.

Some good news, I repaired a frayed insulation/damaged wire to positive coil terminal and we have a nice blue spark now!

Will keep learning and trouble shooting. Really appreciate your help. Thanks for sharing your knowledge and expertise.
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wesk
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Posts: 15242
Location: Wisconsin

PostPosted: Thu Feb 11, 2021 12:02 pm    Post subject: Reply with quote

Are you in Southern WI? I had -17 here this morning in Holcombe, WI!
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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JeepdaddyRC
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PostPosted: Thu Feb 11, 2021 12:23 pm    Post subject: Reply with quote

Yes sir, Madison area is tropical today at zero. I spend time in Hayward (where my sister lives) and we have some land near Park Falls, not too far from you. It was -26 degrees there recently.

Will send a pm.
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cobra5
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Joined: Mar 10, 2013
Posts: 42
Location: Stevensville, Montana

PostPosted: Fri Feb 12, 2021 1:40 pm    Post subject: Reply with quote

JeepdaddyRC

I apologize for not getting back into this thread sooner. Looks like some others have stepped in and got you on the right track. Good job on getting that wire fixed and now hopefully that will put the fouled plug issue to rest. Stay warm and keep up the good work.
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1951 M37 WOW, 1953 M37 WOW, 1954 M37 WOW, 1953 M38-A1, 1954 M38-A1, 1964 M151-A1, 1967 M151-A1, 1968 M151-A1, 1978 M151-A2, 1989 M35A2C, 1965 M416, 1966 M416, 1967 M416
MSgt, USAF Retired 1/84-10/05
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JeepdaddyRC
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Posts: 37

PostPosted: Fri Feb 12, 2021 9:10 pm    Post subject: Reply with quote

Repeated the leak down test with better gauges per Wes's suggestion. Here are the results:

Cylinder.........Air in (gauge 1)......Air out (gauge 2)....Sound from
1......................80 psi.....................77 psi...............oil filler
2......................80 psi.....................76 psi...............oil filler
3......................80 psi.....................52 psi...............oil filler
4......................80 psi.....................34 psi...............oil filler

So I have 2 strong cylinders, and 2 weaker ones (3 and 4)? This seems to support the compression test. The leakage is apparently at the rings/cylinder wall. No sound from carb, tailpipe, adjacent cylinder or radiator during testing. The previous compression test results were 125, 122, 95, 85 respectively (see post 1).

Here is the leak down tester I used this time. Much better quality.





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JeepdaddyRC
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PostPosted: Tue Feb 16, 2021 9:36 am    Post subject: Reply with quote

When pressure testing #3 cylinder, the sound of escaping air seemed to change when covering the #4 cylinder spark plug hole with my finger. I could not feel pressure against my finger, so I placed a small piece of paper over the hole, I could see it flutter slightly. I thought this indicated the head gasket was compromised between cylinder #3 and #4 and could explain the lower compression in those adjoining cylinders.

To confirm a bad head gasket, I expected a similar situation when pressurizing #4 cylinder. However, no air escaped from #3 cylinder spark plug hole when pressurizing #4. No movement of paper over spark plug hole. No air in radiator for either #3 or #4.

This has me confused. Any thoughts?
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wesk
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Location: Wisconsin

PostPosted: Tue Feb 16, 2021 10:35 am    Post subject: Reply with quote

One way air movement indicates that there is no bad head gasket. It would be both ways if the gasket was damaged.

Look at your readings. 3 is almost 20 PSI tighter sealed then 4. Therefore when you pressurized 3 it's ring bypass air found it fairly easy to slip right past # 4's rings but when you pressurized # 4 it was not as easy to sneak past #3's rings.

You have completed all the needed tests and the tests indicate that your only structural issue is with the rings on #3 & #4. The next step is to remove the head and pan and remove #3 & #4 pistons and visually inspect the condition of the rings, seats and cylinder walls.

The symptoms displayed by #3 & #4 are indicative of anyone or any combination of the following issues:

1 - Stuck ring (stuck in piston groove in a compressed position)
2 - Broken ring
3 - Excessive piston to cylinder bore clearance.
4 - Scored or corroded cylinder wall.
5 - Structural failure of a piston. (Hole burned in it.)

These are problems that require significant mechanical skill sets and tooling.

Had your leak down test indicated a bad head gasket or stuck/not sealing valve the solution would have been an easier path.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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JeepdaddyRC
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PostPosted: Tue Feb 16, 2021 10:55 am    Post subject: Reply with quote

Thank you Wes for the help, patience and great explanation.
Putting some PB Blaster in cylinders - a little each day for 5 days - and moving crank back and forth slightly to encourage rink soak/penetration.
I realize it's a long shot to free up the rings, but worth a try. It's really all I have left with my skill level and tools.
When weather warms will try starting and hope for some improvements (wishful thinking).
If issues persist, looks like time to hand off to a pro that can refresh this engine.
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wesk
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PostPosted: Tue Feb 16, 2021 11:49 am    Post subject: Reply with quote

We have a little trick we used with aircraft engine leakdown checks that is not difficult when you are positioning the piston at TDC compression using the leverage of the propeller. This is much more difficult to do with an automobile engine where the only place to gain leverage is the crankshaft pulley nut with a breaker bar. But it can be done.

It is called popping the rings. We advance the piston up towards TDC compression and stop just before or right at TDC. Then apply the 80 PSI. If we have a ring leaker then we pop or rock the rings in their seats by leaving the 80 PSI coming in and advance the piston slightly past TDC and then abruptly rotating it back to TDC. This will often reset or recock the offending loose or stuck ring to a better sealing angle to both the cylinder wall and the ring groove. The danger involved is letting the leverage tool slip from your grip.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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JeepdaddyRC
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PostPosted: Thu Feb 18, 2021 9:32 am    Post subject: Reply with quote

Squirted a little PB blaster into the weaker cylinders with the goal to free up any stuck rings - in case that is causing the lower compression. Wiggled the cylinders up and down for good penetration. Repeated for 4 days.
Now moving to Ford Type F ATF for a longer soak of a few days.
An older machine shop gentlemen suggested a top end soak using GM-3015.
It is a top end de-carbonizer. Apparently GM-3015 is pretty caustic and potent stuff. Need to change the oil if I use it.
All residual fluid in cylinders will be sucked out with a MightyVac (or similar) to reduce any chance of hydro-locking before attempting to start.
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JeepdaddyRC
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PostPosted: Tue Feb 23, 2021 1:04 pm    Post subject: Reply with quote

Update to free possible stuck piston rings in grooves:
2 piston soaks for 2 days each using PB Blaster (in 2 lower compression cylinders).
Then used Ford Type F ATF to soak pistons for 3 additional days.
I assumed the ATF would drain past the rings into crankcase. However, using a Mighty Vac sucked lots back thru the spark plug hole.
Turned engine over with plugs out to remove any residual to minimize chance of hydro lock.
Fired it up.
A real smoke show for 5 minutes.
Ran engine for 20 min at 2000 rpms.
Smoke ended. Ran well.
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