Posted: Sat Feb 06, 2021 10:01 am Post subject: M38A1 engine diagnostic tests
Sept 1952 M38A1 with fording valves removed. Symptoms: hard starting, heavy exhaust, plugs black, significant drop in rpm when clutch pedal released in neutral.
Diagnostic tests completed:
Compression test (throttle open, fuel off, engine warm, plugs out)
Cylinder
1. 125 dry, 125 wet
2. 122 dry, 125 wet
3. 95 dry, 100 wet
4. 85 dry, 95 wet
Vacuum readings - 20 inches steady at idle, hit throttle: drops to 8 inches and up to 22 inches
Timing - Set to 8 degrees BTDC
Carb float height - New needle and seat installed and float set at 1/4 inch (Carter YS950S). Note: Scoutpilot (Old Jeep Carbs LLC) indicated Viton tip set at 9/64 and metal needle and seat set to 1/4
Fuel pressure - 4.5 psi
Valve clearance - intake .018, exhaust .016 set cold. Note: #3 cylinder exhaust valve rotator (early engine) was down against the tappet screw. Confusing at first because I did not know what it was, but could push it up with finger to measure clearance.
This is a new to me M38A1 and trying to determine next steps. I would appreciate your experience and expertise in interpretation of these results. Thank you.
Joined: Mar 10, 2013 Posts: 105 Location: Stevensville, Montana
Posted: Sat Feb 06, 2021 11:19 am Post subject:
Several different things will cause low compression on cylinders. Worn out piston rings, weak valve springs or valves out of adjustment. The #3 & 4 cylinders being low could be the result of a head gasket issue. Did you check the torque on the head bolts? They should be between 60-65 ft lbs.
My recommendation would be pull the valve cover and check the torque on the head bolts per the sequence outlined in the manual. Set your torque wrench to 65 ft lbs to torque check. Check your valves to see if they need adjustment. If you don't find anything significantly loose, I'd consider pulling the head. This will allow you to see the condition of the head gasket, piston tops and valves in the head itself.
Are your plugs black and dry indicating carbon fouled or are they black and wet? Carbon fouled plugs are usually caused by too rich of mixture. Wet fouling could be fuel or oil. If they smell like oil you have worn valve guides. Fuel smelling would be too rich mixture and plugs not firing because they are too carbon fouled. _________________ 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
Thank you for the reply. Yes I did check head bolts and confirmed 60-65 ft lbs in order specified. All were within spec.
When spark plugs first removed, they were black with carbon. Especially #1 cylinder. I can see carbon around the spark plug holes. Cleaned plugs, but after each run they are black and wet. Difficult to determine if they smell like oil or gas.
I little confused by the vacuum readings. Seem to be stronger than expected. According to the manual "leakage of compression between cylinders is indicated by gage hand drifting regularly between 5 and 19 inches. Worn or poorly fitted piston rings or scored cylinder walls are indicated by gage hand remaining lower than normal, at approximately 15 inches". The vacuum reading don't seem to support a head gasket failure between cylinders #3 and #4 cylinders. Do they?
Joined: Mar 10, 2013 Posts: 105 Location: Stevensville, Montana
Posted: Sat Feb 06, 2021 2:00 pm Post subject:
The manuals give you and idea as to what could be wrong and where to look but, I wouldn't get stuck on the numbers they throw in there. In the meantime, there are other things you can do that the manual doesn't cover. For instance, you can do a cylinder leak down check. Since #4 is your lowest reading, I would start there. Get the piston on the compression stroke. This will close your intake and exhaust valves. Your going to put air in the cylinder with shop air through the spark plug hole using an air nozzle with a rubber tip that will seal off the spark plug hole. Just apply a little pressure at a time. You will hear air leaking and should be able to pin point the area from where the air is leaking. The areas to listen for leakage is at the oil pan, intake valve, exhaust valve or you'll see bubbles in the radiator.
If you don't know how to do a leak down check, just google it. Easier than trying to explain it.
Other things to consider:
What does your exhaust smell like and its color when its running? Black smoke will be burning rich and you will be able to smell the fuel. If its blue smoke you are burning oil and again you will be able to smell it. White smoke when its at operating temp will be coolant. _________________ 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
Wes, leak down test not yet completed.
Exhaust smokes gray and is foul smelling. Assumed it was running rich. Started with carb adjustments. Leaned idle mixture, replaced needle and seat and set float to 1/4.
Exhaust is white while warming up (WI cold and condensation). Does not smell sweet like antifreeze. Exhaust is not really bluish (like burning oil).
Exhaust seems to lessen once engine is warmed up, but it stinks.
No oil foaming or indication of water in crankcase oil.
Fuel pump pressure update. It is at 4 psi with plugs out using starter. Unclear how to measure at 1800 rpm. It is the correct 2 stage military fuel pump.
Has "primer handle". However, no fuel comes out of top carb fitting when using primer handle. Mystery?
PO installed an inline metal filter between pump and carb.
I'll wait & see the results of your leak down test. That test will more clearly define the loss of pressure routes related to your two cylinders with low compressions. If you have not done a leak down test before you will observe readings, normally with 80 PSI in on 1st gauge you should stay above 60 psi on the 2nd gauge. Static seal issues are as follows in next sentence. Then by listening for escaping air pressure at the tailpipe and the carb throat you can determine if the intake or exhaust valves are leaking. Bubbles in the top of the radiator indicate a cracked block/head or bad head gasket. As for dynamic sealing (rings) you listen at the oil filler pipe. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Mar 10, 2013 Posts: 105 Location: Stevensville, Montana
Posted: Sun Feb 07, 2021 8:24 am Post subject:
No blue smoke or oil smell from the exhaust is positive. White smoke at start up until warm up is normal. As you stated condensation. Sounds like you have a handle on your next step with the leak down test. Are you running waterproof plugs or civilian plugs with adapters? I ask because your fouling issue could be the heat range of your plugs. The fouling issue on #3 & 4 is most likely due to low compression. However, #1 & 2 have good compression and even after you tweaked the carb they are still fouling. You running points in your distributor or is it electronic? _________________ 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
It has the military 24-volt waterproof ignition system, with shielded plug wires and Autolite military spark plugs.
Forget to mention in my original post that I cleaned and set the points to .020 per specs. The point gap was significantly wider before I set them.
Distributor cap had no sign of arcing. Replaced rotor. It appears to have a newer condenser. Must have spark (perhaps weak) because it does run (poorly) and quite difficult to start.
Joined: Mar 10, 2013 Posts: 105 Location: Stevensville, Montana
Posted: Sun Feb 07, 2021 3:32 pm Post subject:
Do you have to use any choke to keep it running after its at operating temp? _________________ 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
No choke required to keep it running. Full choke to start only.
However, if you let the clutch out (in neutral) before it warms up, the rpms slow significantly and it wants to stall. Never experienced this in any Jeep before.
My first experience with 24-volt waterproof ignition. With spark plug grounded to block, what color should a strong 24-volt spark look like at the electrode gap? Bright blue?
I checked the color of spark at plug gap when grounded to block.
Before you condemn your plugs to death without a trial make sure your engine is grounded properly to the frame at the left front engine mount.
Also always have a new plug handy to check for spark in place of one that was not very good.
Also if your shielded spark plug leads are truly serviceable they alone will ground the plug for a test if the block is properly grounded. You should not have to hold the edge of the plug against the block. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
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