Oil Pressure in M38A1

[SteelTiger]

Anyone have any ideas on what is going on with my drooping oil pressure in my M38A1? When I first crank it up, oil pressur reads 40-50 PSI, then over time (10-20 minutes), it will go down to almost 0 at idle, then only back up to around 20 or so at street speeds. I am pretty certain that I have oil circulating, but this is worrisome nonethless. I like to believe my gauges.

[linx310]

It could be your oil gauge. The the oil pressure gauge, I have been told, are prone to incarcerate readings since its an early electrical gauge.

Most people go and buy a cheap oil pressure gauge and hook it up just to test and see if its the gauge.

[wesk]

Oil when you first start up is cool and thick. This combination equals higher pressure readings. Oil that has reached normal operating temps is hot and thin. This equals lower pressure readings. So the phenomena you are seeing is quite normal.

Now the manual gives you some operating ranges that are acceptable. When we evaluate a jeep's oil system we want to use the temps we find when the engine, coolant and oil are all at their normal operating temps.

No matter what viscosity oil or brand oil you choose to use these standard limits of oil pressure apply the same.

30 to 35 PSI at normal operating temps and cruising speed. (between 2000 and 3000 RPM)

10 PSI at normal operating temp at idle. (600 to 750 RPM) This is a minimum pressure

As mentioned above make sure the pressure readings you are seeing are accurate. You can mechanically apply accurate pressures to the sender and check for the correct readings on the gauge or you can tap a new known correct mechanical gauge into the system.

You must first be sure of the accuracy of your meter before you tear into the engine.

If you gauge proves accurate then your pressures are well below limits and indicate either a failing oil pump or excessive bearing surface clearance. These clearance issues that cause the loss of oil pressure can be at the main or rod bearings, the lifter bodies, the cam bearings both shell bearing front and the two non-shell surfaces center and rear. Also at fault could be the cam's rear journal welsh plug, the oil gallery plugs inside and outside of the block and the oil spray nozzle at the front of the block. Excessive wear of oil pump gears, pump case or cover and relief valve components.

[SteelTiger]

Thanks for the input. I had this engine completely rebuilt so it best be the gauge itself......though it is a new one, I suppose it could be faulty. I'll have to do a manual check of the gauge and pray for the best.

[pocat]

Can you tell me more about this oil spray nozzle or where I can find more information on it. I have a friend that had a L134 overhauled and now has no oil pressure. I took the oil filter return line off and am able to stick a wire through the fitting on the timing cover and it is clear. My guess is this spray nozzle is most likely suppose to be located inside the timing cover at this point. I feel like it is missing. If this is correct, where can I purchase one?

Thanks in advance,

Pocat

[wesk]

There is the spray nozzle and several 1/8" pipe plugs along that oil gallery. Any of them overlooked on assembly will cause a low oil pressure.

[pocat]

thanks very much for the information and the pictures are priceless.
on the oil filter canister, does the bottom hose go to the timing cover return? and the top hose to the top of the filter? That is the way this one is tubed up and it appears to me this would drain the filter housing when the engine is off.

[pocat]

Howdy,

thanks for the information so far.

I have removed the pan and timing cover.
I located one plug in place inside facing the crank.
I didn´t see any more ports inside.
I found the location for the spray nozzle. It is missing.

Can you tell me where I can purchase a spray nozzle?

thanks in advance

pocat

[wesk]

Here's the photo and measurements:

[Keyslammer]

All,
My 1953 M38A1 will lose all oil pressure climbing up an incline. It comes right back when leveling out or going down hill. This is with the crank case indicating only 1/2 qt low. Isn't there a pickup tube down in the sump? I would hate to make a protracted uphill climb with a big donut in oil pressure. A completely full crankcase might help...but this should not be so sensitive.

Thoughts?

[wesk]

You have several possibilities:

1-Pickup screen is getting plugged up.
2-Crank thrust bearing wear is excessive causing a load shift in the crank going uphill resulting in low oil pressure.
3-Pickup assembly is broken or loose.

I'd start with removing the oil pan and making a thorough inspection. Measure how much oil you drain from that pan. Confirm dipstick markings are correct.

[raven]

If you don't mind me asking exactly where is this oil orface of .040 located? Is it perhaps located behind the plug in the oil galley behind the cam gear? The reason I ask is I am helping my brother diagnose a zero oil pressure issue on a newly rebuilt engine. We have primed the pump and went as far as using an external tank pressure system to pressurize the system. While doing so I noticed the oil runs right out the holes in the crank gear so I am suspecting the builder did not install the orface. [/code]

[wesk]

I reposted my photos above. they should answer your questions now.

[raven]

Thank you, definitely answered my question. This engine however has a chain not gears. No nozzle present. During my diagnosis I did notice a significant amount of oil flow from 2 holes drilled in the crank gear opposite of each other in between the teeth. All the galley plugs are installed. I believe the flow out the two holes in the gear is excessive. Any ideas? If I had pics I would post them.

[wesk]

I looked at a hundred chain drive crank gears and never saw a single one with two holes drilled in it.