Oil pressure drop with rpm?

So my jeep started acting up, which becomes a concern as my C4 vette wont start due to VATS and ECM reprogramming and I don't know what I'm doing shenanagans.

I actually need this thing to keep working.

It is a 4.0, 5 speed manual, a 1998 with 120 or so thousand miles.

Road trip (on highway, 3+ hours) I notice the oil pressure gauge sitting at about 5-10 psi, down from the normal 35-40. No biggie at idle but I was turning about 2k rpm or so and the lifters are clattering like crazy. Clutch in to neutral and at idle the pressure climbs back up.

If I rev'ed it, the pressure would drop back to 10 or so. Got pissed and revved it aggressively (for a jeep, that means like maybe 3500-4000 rpm) needle wobbled, pressure came back up, but slowly (like over 5 minuites) dropped back to 10 psi at a steady 2k rpm.

Stopped to use a rest room, let it cool off and had great oil pressure (35-40 psi) for the initial 30 mins or so until it got hot again.

I had noticed in the past the lifters would clatter like crazy after long drives that got it hot. So this isn't the first time it has done it, just the first time I took notice.

Any theories? Should I put in 20-50 wt oil? Dump in some magic lucas oil gunk everyone with jeeps run? Think the oil pump relief valve is sticking open? Oil filter bypasses continually goofing up? Jeeps are only made to idle? Time to sell?

It does have an electronic gauge, with a sender I put in maybe 2 years ago, I don't want to assume the gauge is off because A) I have already blown up a motor assuming my gauges couldn't be right when I had no oil pressure and B) the HLA's were clattering like crazy.

Anyways, Thanks for reading!

My first thought would be gunk blocking up the oil pump pick-up screen. I would flush it with some cheap oil and see what happens. If that didn't help, maybe move on to a solvent additive. If it truly does only have 10psi running at highway speeds, I'm not sure you have much to lose by trying radical flush solutions. I doubt it would last much longer, anyway.

Mine ('93 4.0 5spd) just turned over 250K yesterday:

You need to back flush your oil pick up. If you have a old remote filter adapter and compressed air it's easy to do. Spin off the filter, spin on the adapter and push high presure air in both holes. you may find you need to turn the motor over by hand aswell. Do a seach i've expanded on it more a few years back to someone with a Ranger pick up?

44dwarf

How hard is it to drop the pan?

Add a remote oil cooler?

Do you have an oil temp gauge

and excessive noise from the timing or cam sensor gears? no? ok back flush.... also dont rule out just the sending unit going bad.

If you want to save this engine you're going to have to stop driving it now. First go buy a mechanical oil pressure gauge and test the pressure with it. If it agrees with the electric one, do the flush as rec. above. If that doesen't help you're going to have to do what ever it takes to find out what's causing the problem.

Did you buy it new? If the oil has been changed regularly one or more componant in the oil flow path is failing. It could be the oil pump. It could be that the oil return passages that drain the valve cover are stopped up so that there is inadequate oil supply in the pan. When you replace the oil and filter make sure you use a top quality oil and OEM oil filter. An incorrect or poor quality filter can cause problems.

I worked on an engine with this problem once that had a beautifully painted engine and engine compartment. I dropped the oil pan to disscover that someone had painted the interior of the oil pan. The paint had come off in sheets and covered the pick up screen. I cleaned it, removed the rest of the paint and all was well.

Graefin10 wrote: First go buy a mechanical oil pump and test the pressure with it.

I'm guessing you meant mechanical gauge?

carzan wrote:

Graefin10 wrote: First go buy a mechanical oil pump and test the pressure with it.

I'm guessing you meant mechanical gauge?

Yep, LOL. I thought I was awake but I see I was wrong.

How many years has the oil been in it? Before the back flush (which I also suggest) switch over to 15w40 Rotella type oil (I actually used the generic form from Advance Auto Parts in my FILs K1500 4.3L with 350k). If it seems to even out a bit then back flush and use the 15w40.

I've had it for 2 years, it has had changes every 3k miles with cheap oil. The guy I got it from kept up with it, but not well. He probably changed the oil every 3 to 5k miles.

My plan was to put in a new pump and pickup. When I have it apart I will probably blast compressed air through the passages starting at the filter block (I'll look into it to see if it has a check valve I would need to pull), maybe with some solvent. Thanks for the idea.

This thing has no pressure problems unless I get it hot, and I don't get it hot that often.

Thanks for the replies.

The last time I saw something like this, the oil return holes in the head were full of gunk, not letting the oil drain back into the pan like normal. Let it idle and the oil has time to drain back down. Might be something else, but I bet there's some garbage somewhere messing things up. As far as "hot" goes, your engine temp should always be between 180 to 220 degrees when driving, normally right around 200 degrees.

Get a mechanical gauge. Wrangler's of this vintage are notorious for bad readings on the electronic gauge. The sending unit goes to the ECU and sends an idiot signal to the dash gauge that isn't accurate. A new electronic sender can also be bad out of the box.

The sending unit is easy to get to and put a T into it on the pass side of the engine near the filter. Put a mechanical gauge in there and be sure. If it does loose pressure once warm, then I'd suggest going thru the pump, etc.

That tractor motor is basically brand new

Little update: pump, screen and pan gasket is more than $100, so gonna try other stuff first.

Ran some flush through it hot, then blasted some more solvent through the oil passages and hopefully the pickup with the filter off.

Spent some time with a mechanic stethoscope as the noise returned while idling with the flush solvent. Although it sounds like noisy lifters, it is coming from the distributor. No play I can detect so might be a bearing surface inside.

went to take the sending unit off to re-seal the threads and it disintegrated in my hands. was gonna put on a mech gauge but the one I had in my drawer is broken. Will put on a new one later, glued the sending unit back together.

All buttoned up with fresh 10-30 (what I had laying around) and the indicated pressure was 40 cold at idle. I will pick up a mech gauge and keep an eye on the distributor. Hopefully it was a combo of a bad sending unit and a noisy distributor. Keeping my fingers crossed.

Thanks again for the help.

gonna guess your disty gear was coated in plastic and the plastic has broken down clogging the pickup screen.

Cut the oil filter open w/ snips, separate the pleats from the tube w/ a knife, open the pleats up, see what's in there... might tell a story there as to what's floating around.

Is it possible the pump is throwing too much oil upstairs? Some engines have drain back issues. That's why many old school V-8 builders stick with standard oil pumps vs. high volume pumps.

I’ve had the same issue with my Cherokee. Started getting weird oil pressure readings and decided to check the sending unit. Turns out oil was actually leaking out of the sender and it broke apart went I removed it. Replaced the sender and it was fine after that.

The lifters clattering is a good indication of low oil pressure. If you have good pressure cold, it indicates that everything is working as it should. I seems that the oil is overheating or thinning out for some reason. As was mentioned above, perhaps you do have a drain back problem

The oil pick up is cloged you need to drain the oil and back flush the pump from the oil filter location that should help clear out the crud / plastic from timming gears / distrib ect. before you put the drain plug in dump a quart of cheap oil in to flush out any crud pushed out of the pick up

It sounds as if however your oil pump is driven, it's slipping--if that's possible. I don't know the internal pump drive in this engine. But, think of the oil pump like a drill with a lose bit. Everything is ok at slow speed, but, speed up the drill, and the bit starts to spin in the chuck as it goes through what you're drilling. In this case you're "drilling" oil. And the fuel pump is the drill.

If the pickup were clogged, why would the pressure go down, not stay the same? With the exception, of course of a problem of oil not draining back. But, doesn't hot oil flow faster than cold oil, so would it not "self-correct" --to a degree--if there was a problem with the oil staying in the top of the engine? ie, it would flow faster back into the sump since it's hot.

IIf the oil pump/drive is fine, it sounds as if the engine warms up, gets hot, oil thins out, and due to excessive bearing clearance, the pressure goes down even though the rpm is up.

EDIT: By any chance did you recently change oil filter brands? Some engines do NOT like Fram filters at all. Or the filter is larger than the old one?

44Dwarf wrote: The oil pick up is cloged you need to drain the oil and back flush the pump from the oil filter location that should help clear out the crud / plastic from timming gears / distrib ect. before you put the drain plug in dump a quart of cheap oil in to flush out any crud pushed out of the pick up

Did this exact thing, thanks. Cut up the oil filter, nothing I could see.

Now have 45 psi hot at 2k rpm, 25-30 psi idle hot too.

Not sure if it was clogged, bad sending unit or a combo of the 2. Picking up a mech gauge tomorow.

Thanks again.

Helped a buddy figure out the same issue in a MKII Golf. It had a trick oil pan with hinged baffles. Hinges broke, baffles would get pulled up and block the pick up. Actually discover it when dropping the oil pan to replace the oil pump.

Potentially bad pressure bypass spring. Could be either gunked up or broken.