Engine oil problem

johnpstoll · Post by **johnpstoll** » Wed Jul 05, 2017 2:06 pm

I need help with an M38 engine oil psi problem. I’ve searched the forum, but my problem seems a little odd compared to most of the posts. The engine has been completely overhauled and I assembled the engine last Winter. I primed the engine and it started up and I had no problems with it.
I’ve had the engine running for about two months. Most of the time I just made short runs with it for a few miles. No problems. Oil PSI was almost 60 psi when cold and ran over 40 PSI at speed. Last week I took it on a long drive for about an hour. During the drive, the oil psi slowly creeped down until it was below 10 PSI. When I stopped and came down to idle, the oil psi dropped to zero. I shut it down and let it set for a few hours. When I started it back up the PSI was about 50 psi. I thought it maybe an oil sender or gauge problem, so I installed a manual gauge and took it for another drive. The same problem, it got down to about 8 psi at 30 mph before I shut it down. Let it set for a few hours and started it back up. It ran with 40 PSI at idle.
I’ve dropped the pan and can’t see anything obvious. I cut the oil filter apart and see no evidence of metal. The oil pump was new when installed. I know when I rebuilt the engine I installed a new .04 oil jet up front. I checked the oil pickup screen and it’s clear.
I’m going to try a new another oil pump, but beyond that I’m looking for any help about what else to check before I tear the engine down.

Post by **wesk** » Wed Jul 05, 2017 2:45 pm

What are your coolant temps compared to your oil pressure changes?

johnpstoll · Post by **johnpstoll** » Wed Jul 05, 2017 3:52 pm

The coolant stays at 180 constant with no fluctuations.

johnpstoll · Post by **johnpstoll** » Wed Jul 05, 2017 3:57 pm

I also took the oil pump apart. The rotors look new and tight. They pass all the inspection criteria. There was one shim with the spring. It was a shy short of the static length mentioned in the manual, 1.60 should be 1.64 but I don't know if that would make a big difference.

I also took the oil pickup apart. It did have some sludge in there, but I don't think it would have blocked it.

Post by **wesk** » Wed Jul 05, 2017 4:17 pm

Is the oil pump US manufacture or offshore.

Jeff_Lee · Post by **Jeff_Lee** » Wed Jul 05, 2017 4:19 pm

Given that you just rebuilt the engine, and only have a couple of hours usage on it, I am surprised by the existence of sludge in the oil pickup. That shouldn't be there in such a short period.

When the engine was rebuilt were are the oil passages flushed and the oil pickup cleaned well??

johnpstoll · Post by **johnpstoll** » Wed Jul 05, 2017 4:38 pm

The oil pump's from Midwest.

I cleaned the pickup before assembly, but I admit to not taking it apart. I just flushed it in the parts washer.

johnpstoll · Post by **johnpstoll** » Wed Jul 05, 2017 4:40 pm

I flushed the oil galley and ran a small bore brush down the galley.

danrothe2001 · Post by **danrothe2001** » Wed Jul 05, 2017 7:32 pm

What were the bearing clearances and what weight oil are you running? What was the duration of the short trips where the pressure stayed up?

johnpstoll · Post by **johnpstoll** » Wed Jul 05, 2017 7:44 pm

I ran 10w-30 oil and the short trips were about 5-10 minutes. The crank was ground and new bearings all around .10 over. I can't remember the clearances at the moment, but I went right with the manual. What are you thinking?

danrothe2001 · Post by **danrothe2001** » Thu Jul 06, 2017 6:12 am

Thinking oil clearance is too loose for the oil viscosity. Five or ten minutes is not enough time for the oil to warm up. An hour lets it come up to temperature and thin out more. Excess clearance lets the hot oil leak out and not build pressure while the cold oil will still build pressure. Driven oil has a good tech article on the relationship of engine temperatures, oil clearance and viscosity. If you can find nothing else wrong try 20w-50 and see if the pressure picks up.

oilleaker1 · Post by **oilleaker1** » Thu Jul 06, 2017 6:46 am

Most of my Jeeps will settle down hot to 9-10 lbs at a 650 RPM idle. 15-40 Rotella oil. 30-40 going down the road hot at 40 MPH. Bearing clearances are what dictate hot running oil pressure. Are you running a spin on oil filter? They have to be of the correct type filter or you will have little to no oil pressure because they have no metered flow like the original canister or Cuno type. If you aren't sure, you could block off the line to the filter and run the engine. End float of the crankshaft, rod and main bearing clearances, and a fully seated relief valve in the oil pump are paramount to having good oil pressure. As Wes asked, what temp. the engine was running hot at, that too makes a huge difference in oil pressure. 160 vs 200 degrees . When hot, oil thins and leaks faster past the bearings. I found that there was no difference between hot 15-40 and hot straight 30 wt. John

johnpstoll · Post by **johnpstoll** » Thu Jul 06, 2017 7:12 am

The filter is the original canister type filter from a good supplier. I'm assuming that the main crank shaft bearings would most likely be where the problem is. I didn't Platigauge the bearings when I put it together. I will try gauging them separately and will see what I get. If I read the manual correctly it should be no more than .003. I will let you know what I find. Thanks for the help.

johnpstoll · Post by **johnpstoll** » Thu Jul 06, 2017 4:11 pm

Well, it's not good news. I removed the front crank bearing and it looked OK. I removed the #1 and #2 rod end bearing caps and there was considerable scoring of the bearings. I took a measurement of the journal, I don't think they were sized correctly from what I can determine. I've have a better idea once I remove the engine and have a closer look. So I'm going to try and remove the crank and the piston rods and just get them redone.
I can only blame myself for not checking the clearances. Expensive lesson. What's the old saying.."Trust, but verify".

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danrothe2001 · Post by **danrothe2001** » Thu Jul 06, 2017 4:51 pm

Pictures of the bearings and crank?