Jeep CJ 4.0 HO Low Oil Pressure

[60Bubba]

I recently posted this on a different thread:

"When I got off the highway, I noticed the oil pressure was low.  This is not unusual for this Jeep.  It has the Cherokee 4.0 HO engine (1995) but an original CJ oil pressure gauge, and I've been told they aren't calibrated the same.  This time, however, the pressure was down around zero and as I waited to go through the guard house to get on the base, I heard a continuous "tock-tock-tock" sound that varied with engine speed and no indicated oil pressure. 

I shut it down at work and checked the oil.  It was a little low, so I topped it off.  I restarted the engine about 4 hours later, and had 20-23 PSI indicated with no strange noise. Within about 1-2 minutes, however, the gauge started bouncing between 15-20 PSI erratically.  It seems to only be when the engine is hot and the oil is thin that the pressure drops to zero. 

One of my coworkers is going to help me tow it home.  I'm not sure what to do next, though I'm planning to get a mechanical oil pressure tester and/or a mechanical oil gauge and see if the in-dash gauge is accurate.  I'd like to believe it's just a bad gauge, but the newly developed "tock" sound doesn't make me optimistic."

Moses responded with the following:

"Sounds like we need to discuss this engine situation.  The pressure should be tested with a mechanical oil pressure gauge and the engine completely warmed up.  Don't run the engine to that point if you have reason to suspect that the bearings and lubricated parts are not getting oil!

The aim is first to confirm whether there is oil pressure to the crankshaft bearings and valve train.  Try the mechanical gauge at startup with the engine cold, just the immediate startup test.  Realistically, a 4.0L Jeep inline six should produce at least 30-40 psi at an idle cold, although the "factory" allowance is much lower than this figure:  13 psi at 600 rpm (presumably with the engine warm).  Warm, a 1995 4.0L should produce 37-75 psi at 1600 rpm.  The 37 psi would be minimal and raise concerns.  The AMC inline sixes run higher oil pressure than most engine designs.  The oil pressure relief point is 75 psi."

Here is what I found today after installing a mechanical oil pressure dash gauge:

Cold Start:                 60 PSI                       WARM:                                             HOT:

:30                             62 PSI                       850 RPM         32 PSI                       800 RPM          22 PSI

1:00                           62 PSI                     1500 RPM         48 PSI                     1500 RPM          42 PSI

1:30                           62 PSI                     2000 RPM         50 PSI                     2000 RPM          46 PSI

2:00                           60 PSI                     2500 RPM         52 PSI                     2500 RPM          50 PSI

3:00                           60 PSI

 

I didn't hear any unusual noises on start up.  After driving about 2.5 miles at 35-40 MPH max, I took the "warm" measurements above and again, didn't hear any unusual noises.  I drove back to the house and took another round of measurements labeled "hot" above. At this point, I could hear some very faint knocking noises.  I imagine most people not familiar with the Jeep wouldn't even have noticed it. 

These pressures seem reasonable to me based on what you said above.  This was done with an inexpensive dash gauge, vice a dedicated oil pressure test kit.  It was what I could get without having to order something and wait on it. 

When I replaced the clutch and rebuilt the transmission, I also changed the rear main seal on the engine.  I was using Mobil 1 High Mileage 10W-30 oil in the engine, but I switched to Valvoline Max Life High Mileage synthetic blend 10W-30 as it was on sale (and they didn't have the Mobil 1).  Do you suppose changing oils would have any impact on potential noises made by the engine?

 

[60Bubba]

So I've been thinking more about the engine oil pressure, and I wanted to add some info and thoughts to the previous post.  When I bolted the oil pan back up after replacing the rear main seal, I had to add almost 6.5 qts of oil to get the level to the full mark.  I wasn't too worried about this, as I changed the filter and I know that may account for a little excess.

When I got to work after the low pressure incident, I checked the oil after I got to work and it was barely on the dipstick.  I ended up adding a quart to get the level back to the upper middle of the add/full range.  There is absolutely no leakage or evidence of burning oil.  Also, the pan appears to be a stock 4.0 pan, yet I have by my measure 7.5 quarts of oil in the engine right now.

Questions:

1. Is it possible I moved the dipstick tube in the oil pan, which is changing the position of the end of the dipstick with relation to the oil in the pan, thereby making it read lower than it actually is? The dipstick tube is still attached to the stud on the engine block.

2. I know too much oil isn't good.  Am I in danger with about 7.5 quarts, remembering that this was a totally dry engine and oil pan before I refilled it?

3. I'm considering replacing the oil pump and pan as well as possibly using plastiguage on some of the bearings to see what my clearance is.  After that I may just keep running it since it seems like my oil pressure is in the safe range. Does this seem like a logical course of action?  Time, money, and the wife's patience for Jeep projects are in short supply after ring and pinion swaps, new suspension, and a transmission rebuild...

[Moses Ludel]

60Bubba...Confirming your conclusions.  You have "normal" oil pressure for a 4.0L.  I am suspect of 5W-30 oil in general.  The 5W pour factor is very thin.  A multi-viscosity oil at 30W in hot weather performs marginally, too.

Contrary to wishful thinking, multi-vis oil does not "thicken" when hot, it simply has the ability to lubricate like the advertised weight of a similar, straight weight oil.  This means that your 30W oil is no better than a 30W oil subjected to considerable heat, which of course means "thin" oil.  Imagine a 30W oil running across 250-degree F metal.

There is nothing intrinsically wrong with Valvoline.  I, in fact, am happy with Iso-Syn formulation Chevron Supreme oil that behaves like a synthetic and sells for a fraction of synthetic pricing.  Available at Costco and elsewhere, the Iso-Syn oils work well and protect our engines.  (I use Delo400 Iso-Syn in the Ram-Cummins engine.)  My choice for the XJ 4.0L engine is 10W-30 (winter) or 10W-40 (summer).  I use a block heater in the winter, so the 5W factor is unnecessary and unwanted.

Moses

[60Bubba]

Moses,

I did some more troubleshooting, i.e., driving, watching the oil pressure, and listening.  I was able to isolate the noise I was hearing to times when the clutch is in.  It's not loud, but I can hear it.  I'm thinking it's that new throwout bearing.  I guess I'll just keep monitoring things.

[Moses Ludel]

60Bubba...Clutch disengaged noise can be the disk, T.O. bearing, release arm rattle or the crankshaft pilot.  You installed the new pilot at the crankshaft and lubed it, right?  If a bushing, that can take time to burnish, a caged needle bearing simple needs lube.  New front bearing retainer on the transmission?  Could take time to match up with the T.O. bearing collar.  The T.O. bearing collar did have grease in its recess?

If the T.O. bearing is moving too far into disk when you depress the clutch pedal, the clutch cover fingers could be touching the clutch hub.  Make sure you have adequate free play between the T.O. bearing and cover fingers when the clutch pedal is released...The pedal height is adjustable.  If the pedal moves the T.O. bearing too far, adjust the pedal height.  Test this by not depressing the pedal to the floorboard; see if this eliminates the noise.

Moses