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Okay, a little warm up to the concern is necessary. About a month ago I did my annual oil change, nothing exciting, nothing unusual and all went well. I run my truck only on warm, sunny days throughout the Spring, Summer, and Fall and because of that I use 15w-40 Rotella diesel oil. I have used that brand and weight for several years now without any issues. My truck is bone stock, straight factory/military M37 with no previous oil pressure issues.

Now onto the concern. A week after the oil change (mid July) I started the truck and it would not even out on the idle. It was acting like it was out of time and missing - very rough idle. I immediately shut it down and tried again - same thing the second and third try. I thought this was very weird as I've never had any issues like this before. I pulled the little set screw on the head over the #1 cylinder and dropped a straight piece of wire down, then exposed the distributor to make sure the two were lined up at TDC. Everything looked good, including the cap and rotor, so I buttoned it back up and tried to start again. It fired up and still had the rough idle issue but then evened out when I gave it a little gas which it did not do on the prior starting attempts.
I've run the truck several times since this incident and the idling issue has not happened again but something did happen to my oil pressure. The oil pressure on my truck has always been very good. I have a 60 psi gauge and sender and the pressure was always in the 45-50 range when driving and would never dip below 30 psi at warm idle. Since the idling incident mentioned above the oil pressure now starts out good (45-50), but as the truck warms up it drops down considerably and then barely makes it to 30 psi while driving and drops down to about 10 psi when idling. I'm very concerned by this issue and need to know what the potential problem(s) could be as I have yet to track down exactly why my oil pressure went to hell on me.

Any input on this would be greatly appreciated.

Kevin

Verify that your gauge is reading correctly. They are not particularly accurate and yours may be having issues.

Check your oil for the smell of gasoline. If your fuel pump diagram is ruptured it may be feeding gas into the oil. This will lead to low oil pressure as the oil becomes diluted.

Check for vacuum leaks, they will adversely affect idle.

But remember that those are viable readings. Pull your regulator and clean the piston and bore.

Agree with above, and trying to connect the poor idle and low oil pressure, if your mechanical pump's diaphragms are pushing gas into your oil, with that kind of dilution, it could certainly cause low oil pressure readings. And if the pump is failing to put adequate fuel flow/pressure to the carb, it could be causing the erratic idle and engine running. Engine bearings can be badly damaged very quickly by extremely thin oil, so might be best to not run the motor until the cause is found. Pull some oil out of the sump, or sniff the dipstick. And see if reading your dipstick shows your sump to have more oil than it did before that too can suggest the pump has failed.

If you do replace your fuel pump, you might change over to an electric pump, and there are lots of hints on how to do that from this forum.

HC

Gentlemen,

Thank you for your input. I've looked over the engine for possible vacuum leaks, nothing found. The oil level hasn't changed but it does have a hint of gas smell but nothing overwhelming (??). I'll be pulling the fuel pump this evening and taking a look at the diaphragms as well as cleaning the regulator valve.

I rebuilt the fuel pump back in 2003 and it's run great for 17 years, maybe it's time for another diaphragm.

I'll give an update as to what is found once I take everything apart.

Kevin

Just to add for your peace of mind-I run Rotella 15w40 as well-no issues with it but my oil pressure has never been as high as yours, likely due to the rebuild that is long overdue.
Gary

I think if one could search for the "most common thread" it would be oil pressure. I have developed a standard check of my oil pressure. Fully hot oil and engine, run 40 mph, and see what my aftermarket Autometer Gage reads, It is on my steering column, so gives a much more accurate read than trying to look down at an off center OEM gage. They are usually pretty close to each other, with differences only at lower RPM, where my Autometer dampens out pump fluctuations, and at idle.

I run 10-40 RedLine, with one qt of 20-60 per sump, and my hot oil, usually in SC ambient temps of 90+ degrees, at my standard RPM is about 37 PSI, +- a few pounds. In the several thousand miles since my motor was rebuilt first by me then more correctly by Chas T, it has not dropped or changed at all. So for me, I think all is fine. If I speed over my spec 40 mph, my pressure will continue to rise. And it can take an hour of sustained highway speed to get my oil as hot as it is going to get. A reading at idle gives much less information, because if I idle at 600 RPM, it will quite different at an idle o 750 RPM. Chilly temps, say in the 30's, definitely cools the oil, and will give higher readings, as much as 5 PSI>.

On a cold start, even with ambient temps of 90 degrees, my max PSI is 57, where the bypass fully diverts pressure over that. I enjoy watching UTube video's of first starts in years, or after a rebuild, and see what kind of pressures the gage is giving. When I see a cold motor read 40 PSI at first idle, and the proud owner comments on "great pressure", knowing what happens as the oil gets hot I kinda wonder ".... wait and see when it is good and hot before you reassure your self."

we tend to fret over a few PSI this way or that, but I suspect Dodge and the military did not intend for us to worry so. The OEM gage has a thick needle, which alone is worth 5 PSI on each side of the needle.

There is of course the old adage of "ten pounds per thousand of RPM", which is pretty safe for old simple motors, whereas modern cars can computer control their pressure via the ECU. Our engines are old, and this saying is probably all we need to know, especially with the fabulous oils we have over what was used long decades ago.

6 days out from a total knee, and working hard on my rehab, going well.

All the best.... HC
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Good info Hal! Hope your knee is progressing along!
Gary

I had a busy weekend and didn't have time to give an update.

I pulled the fuel pump last Tuesday night and gas ran out everywhere, the pump was full of gasoline and diluted oil. Now the odd part, when I disassembled the pump the diaphragm didn't have any holes or tears. There was some slight surface cracking (probably from age) but nothing that would indicate failure. At this point I called John at Midwest Military, he was a great help as usual and had everything needed to rebuild the pump.

I put the pump back together Friday afternoon and reinstalled it Saturday morning, everything went well and there were no surprises. My son changed the oil and put in a new oil filter as I was installing the pump. The used oil was definitely thin and had a slight odor of gasoline but again nothing overwhelming.

Upon starting the truck the oil pressure climbed right up to 50 on the gauge. My wife and I then took the truck for a nice drive in the country and once everything warmed up the pressure fell back to 30 and there it hovered for the rest of the drive.

I'm very relieved that it seems to be back to normal, I want to thank everyone for their time and help with this problem.

Regards

Kevin

Good to hear. By the way, when accelerating up a hill, or from a stop sight, with my oil at my idle pressure, I don't load my motor with lots of pressure and throttle, but wait a little until my pressure climbs up to a more protective psi. For instance, I don't want to go full throttle at an idle pressure. In a week or so I should be be able to climb up into my truck and head out for a drive. Left knee coming along really well. HC

Good to hear, Kevin (oil pressure), and Hal (knee)!
Gary