Oil pressure question

Hi everyone.

A few weeks ago a friendly Cj was found and it decided to follow my truck home. I know its not a true classic jeep, but it can do highway speed and has disk brakes so It can be driven on any road and keep up with traffic. It makes the girlfriend happy and it looks and acts close enough to a classic cj for me so all is well.

Well almost. Its 258 runs great, Engine compression is between 148-154 psi. It had lots of oil leaks, so I assumed the low oil pressure was just leak issues or a bad guage at first.

It now has a new mechanical oil guage, no oil leaks and a new oil pump. I filled it with 5 quarts 5w 30 and a little bit of Marvel Mystery oil to top it off. Oil pressure cold is 75-80psi. When Warm the oil pressure drops to about 20psi and 8-12 sitting at a light. No valve noise or any bad sounds.

The motor is far from new but when I changed the oil pump there was a coating of silver Bearing material in the bottom of the pan, but the bearings did not have any play that I could feel.

So my question is:

A do I ignore it and replace the 5w-30 with straight 30w and maybe a can of restore or some other temporarry engine junk and drive it for the summer hopefully with out problems.

B. Do I replace the bearings and bottom end and if so I think it can be done with out removing the engine. But I am curious if anyone has done that.

C. Break out the engine hoist, pull the motor and rebuilt everything now?

I really don't want to spend my weekends this summer under the jeep. I just want to drive it around and have some fun with it.


Thanks for your help.

Oh btw the picture if it made it? has nothing to do with the oil problems, but thats the new toy. Its been cleaned up and had the cheap chrome bumper removed as well. I'm building some new bumpers soon.

 

In general, 232s and 258s do not suffer from low oil pressure problems. 20psi hot seems pretty low though - ideally you'd like to see about 10 psi per 1000 rpm. The pressure should be lowest at hot idle, and 8-12 psi is borderline - ok, but no reserve. It's surprising that the pressure does not increase to say, 30 or 35 psi at cruising speed when at operating temperature.

The first thing I'd do is replace the filter with a Wix (Napa is Wix) and see if the pressure comes up some. You've already replaced the oil pump... If you're up for pulling the pan again, I'd suggest you Plastigage the rods and see if the rod bearing wear is excessive. If it is, you can replace the rod bearings with the engine in the car, but I'd go ahead and Plastigage the mains as well if the rod bearings are worn. If the bearings are ok and you still have low pressure, I'd worry about the cam bearings... that seems unlikely though. Usually the cam bearings will last through a couple of overhauls, way beyond the life of the valves and rings.

There could be something wrong with your replacement oil pump. What brand is it, a Melling? Could be that the relief valve is stuck open, or a manufacturing defect. I presume the pressure did not change much with the new pump?

hth!

 

My manual says the 258 is supposed to take 10W 30?

 

Yeah, but I don't think it'll make much difference in oil pressure. Multi-vis oil is supposed to keep the oil flowing at low temperatures, though (ideally) 10-30 should have the same operating temp viscosity as 5-30. I'd agree, you don't need 5-30 except in upstate Wisconsin in the winter, or such. I ran 20-50W Castrol for a few years in California, but it never got below freezing there. I didn't see a noticable increase in pressure compared to 10-30 Valvoline or whatever.

 

My 258 in a 1980 CJ has idle pressure of about 5-8 psi and cruising pressure of 20. No tics, lifter noise, etc. Took it to my mechanic and he said 258 was notorious for low pressure but it did not seem to adversely affect it.

Drive it and enjoy.....

 

Mmm, I doubt that story... the AMC V8s often have low OP problems, but that's an entirely diffferent animal.

My '77 258 pickup has about 40 psi when it's cruising at temperature, only slightly less at idle, more when cold. My 258 CJ-6 was similar, although that was a long time ago and I bought it new.

It's good that you don't hear any lifter tick or banging. Still, I wouldn't be entirely blase' about no oil pressure. These engines do have plenty of bearing surface and have way fewer bottom end problems than the AMC V8s, but they aren't immune from rod bearing failures.

Welcome from Boston, JohnnyReb!

 

Thanks for the response everyone. I think I'll order a new rod bearing set. I drove it today for a while once the engine is hot its got really low pressure. 10-12psi and barely 15 when running in 3rd gear at 40 miles per hour. It does pick up when the RPms are higher but its low enough to make me concerned.

Once I drop the oil pan again I'll see whats involved, but how much of a project are the bearings? I assume the timming chain and front of motor need to come off. Does the trans and rear of the motor get removed as well? If so is it easier to just lift the motor out?

thanks for your help.

 

No, to change the rod bearings, you need to remove the oil pan, rod cap nuts and rod cap. Do one cap at a time, and put the cap back the same as you found it. You'll also need a torque wrench and assembly lube (I've used STP before, but real assembly lube would probably be better).

You'll want to measure the clearance in the existing bearings. Use PlastiGage - it comes in a paper tube at the parts counter. Pretty sure instructions are printed on the tube... it's a little plastic rod that you squish between the journal and bearing, then measure its width after its squished. Look at the specs in the manual. If you don't have a factory manual (consider buying a reprint - www.thejeep.com and other places), there's a '72 manual at http://www.c104.net/manual/Section_01A.pdf - some details of the 258 change over the years, but the gist is the same.

Don't buy your bearings until after you measure and look at the inserts - if the engine has been rebuilt, it may have an undersized crank (and oversized bearings). If the journals are damaged (ie transfer of metal or scratching) don't bother with the inserts - yank the engine, buy a crank kit (reground crank and bearings) and replace the bearings and crank. The 258 is a common engine - your parts store should be able to get your bearings (or a crank kit) in a day. If you haven't changed the rear main seal, that's a really good "might as well while I'm in here" addition.

hth! and don't hesitate to ask questions!

 

Thanks for the welcome Timgr! Not sure what you meant when you said "doubt that story" assume you meant that you doubt my mechanic knew what he was talking about....

However:

I presently own 2 258 engines in a '80CJ7; '81 CJ7 and had one in the past in a '79 CJ5.
None of them show the oil pressure that you described but none of them have shown the least bit of engine problem in both on and off road use.

Sending unit? bad gauges?

I bought a new 1989 150 Ford pickup and the the oil pressure gauge showed the 302 operating at the bottom of the safe zone. Immediately returned it to the dealer and he said an advisory from the factory stated that a number of trucks for that year had received the wrong (gauge or sending unit?, can't remember). They offered to replace it under warranty. Never had it changed and that truck is still on the road.

 

Not doubting you ... I had a hard time wording that - instead thinking that your mechanic is mistaken. The AMC V8s are notorious for low hot idle oil pressure, and it seems to me (from other people's experiences) that rod failure is pretty common for them at high mileage. Not so for the very durable 258, again from my experience and what I've heard of others' experiences. It's true that 258s will run and run and run, but I don't think they have any special immunity to the effects of low oil pressure.

 

Thanks timgr...
Following this thread, I think I will take some good advice and have the oil pressure tested on a separate gauge for my 258's. Might save an expensive repair down the road (or maybe on the side of the roadR) )

 

Hey guys.

Tim Thanks again for the tech tips. Bearings are on order. Common parts seem to be sort of un-common here in upstate NY.

As for the oil and pressure arguments, I used 5w-30 because I use it in my DD and there was 1/2 case on the floor of the garrage. Going out to get 10w-30 would have been an extra trip out.

As for the guage. I assumed it was a bad guage before I bought the truck. Again it runs fine. After getting it home I did the "red neck guage check" Pulled the old sending unit and cranked the engine. 10 seconds later I had a large pool of oil. So I replaced the guage and the sender with a mechanical guage that is fed from a small tube. No electrical resistance on this one. Oil flows from the engine through a small plastic tube and pushes on the mechanical guage.

I put it in filled the engine with oil and read 80psi. Yea Job done. I drove down to the parts store to get some gaskets and lights. (Like all old cj's it had burned out lights and oil leaks) By the time I got to the store oil was at 20 psi and 10 by the time I got home. I figured I could not leak that much oil in 20 minutes, with out seeing a big puddle so I let the engine cool and debated another trip to buy a pump.

The engine was 1/4 quart low maybe less so I started it again and read 80psi on the guage cold. 20 minutes later it was back in the teens. The pump was replaced the next day. With the valve cover and pan gaskets as well same results on the guage. There was a coating of bearing material in the pan but again at the time everything seemed ok for an old engine.

The engine still runs great but 10-12 psi worries me personally enough to spend $70 for a bearing kit and another saturday afternoon covered in oil.
I hate doing things twice, but its worth a shot. I'll let you guys know if it works out.

If the bearings don't fix it I will run the motor for the summer or till the bearings are gone whatever comes first. It will settle the argument about bearings and oil pressure one way or the other.

 

My 258 runs and runs and runs. It has a tick, the oil pressure is probably 65, 70 on startup, 15 on hot idle.