Low oil pressure- need advise

[dougimes]

My Navy m38a1 has been running great as a daily driver for 6 months or so. Recently, oil pressure has been dropping from what was 10 or so idle to 30 or so at cruise. I don't trust the 0 to 120psi electrical military guage to be all that accurate, so it may be 15 to 35, or 5 to 25.

Recently, it has fallen off. a bit, most noticably at cruise- 15 maybe.

oil looks good, not burning it. Sounds good, power good, no changes in driving habits (except its warming up here in SC). Am running synthetic, was running 30 wt. Do know for sure if this occured in connection with the oil change. they were in the same time frame..


What do you think about replacing the oil pump?

The pix was on a recent run up to Davidson River NC for a few days of camping.

 

[WillWagner]

I'd check it with a known good master gauge before replacing parts. Also, since it's older it might not tolerate synthetic lube. Try going to a 20w50 petrolium based oil

 

[BIG_RED]

^ Ditto. Synthetic oil tends to be "thinner" under most conditions - could give lower oil pressure readings.

 

[JGBallew]

Seconding(thirding?) the use of a direct reading gauge. Find one that reads more in the range that F-head's pump, 0~50 PSI.

One I have in read zero on the dash gauge at idle, but with a known good direct reading gauge indicated 10 pounds idle and near 45 at cruise.

So, run the gauge first. If the pressure is low with a proper weight oil of any base, then yes you should investigate further. (Pump, drop the pan ,etc)

 

[dougimes]

Good advise.

I studied the manual on the oil pump, seems like it can be bench measured, clearences etc.

I am not sure it will come out without moving the engine off the frame.

has anyone ever taken one out with the engine installed? What are the steps?

Thanks

 

[JGBallew]

Pulling the driver's side fender would probably help, along with the header's downpipe.
Again, I'd bolt up a direct reading gauge before going that far.

The measurements you saw are intended to keep an old pump from being bolted to a freshly overhauled engine.

 

[dougimes]

Wo here is the paul harvey rest of the story


dropped the oil pan, checked out the sump and oil pickup
refilled with 20-50 petrolium based
installed a mechanical guage

When hot, shows 15 at idle, 45 at cruise.

Electrica pressure guage shows 5 and 20.

It was the guage, and the thin synthetic oil. mostly the guage

 

[jime]

Just checked mine. The electronic gauge read 10 all the time with engine running regardless of RPM. I hooked up the direct read and am getting 25 at idle and 45 at high rpms. The top end of the motor was rebuilt shortly before I got the jeep. I looked for an acceptable range in the manual but couldn't locate it. Does this range sound good?

 

[clinto]

25 at idle and 45 at high rpms.


Does this range sound good?

Oh yeah, that's fine.

A good rule to remember is that is typically takes 10psi per 1000 rpms to create the hydrodynamic wedge that "floats" a crank in the bearings.

So you're well within that rule.