high then low oil pressure

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Jeeplander

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Jan 30, 2010
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Newfoundland, Canada
Hey There, I'm new here. From Canada'a east coast, Newfoundland.
I drive a 97 jeep tj 2.5l. Last year I had the engine rebuild after I had a piston split. Everything was fine with the engine after the rebuild, until now. When Engine starts up, the oil pressure is fine. When I press the gas, oil pressure goes way to roughy 72psi until check valve lets the pressure off. After 20 mins or so of driving the pressure is normal. After driving a longer distance pressure drops down to 15psi.

I replaced oil, oil filter, oil pump. installed a mec. oil guage. The engine is not making any noises, no noticeable power loss.

Any idea whats going on here??
 
Oil thins and pressure drops as it gets hot, it is noticed more during the winter months. 15psi is at how many rpms when the engine is hot and idling for at least 20 minutes? At a traffic light idling in drive 15psi would be fine for a fully warmed up engine.
 
Welcome to the Forum!

If memory serves, the typical 2.5L should have 35-40 psi at 1600 rpm. Jeep oil pressure sending units aren't known for their longevity. I run a mechanical gauge "T'd" off with the factory unit. I never see pressure below 35-40 at idle (on hot days with extended use), but I run a high volume pump due to my Jeeps off road use and IMO, marginal upper oiling.
 
I have installed a new sender unit. also my mec. gauge is tee's off my sender unit. Is the high pressure ok, will this cause seals to blow..... If I put on a high volume pump, would there be extreme high presure, or would the check valve in the pump take care of that?
 
The regular pump is high volume for the 2.5
lol.gif
Your relief valve appears to be doing its job of limiting the pressure to 75lb ..which I'm pretty sure is to limit the stress on the distributor drive.


What is the pressure while you're driving down the road fully warmed up ..as in the lowest you have seen? While my 2.5 has a very hard time getting below 40psi even with very thin oil, that doesn't mean much. The 4.0's are all over the map for pressure output ..so I imagine the 2.5 would be too. In most parts catalogs they share the same stock pump. There is no HV offered for the 2.5 that I'm aware of..but I imagine that one would fit. I have one in my wife's 4.0. It has double the vane depth (pump body depth) of the OEM.
 
After the jeep is warmed up and running for awhile. When driving it runs 35 - 55 psi but drops hard to 15psi. In summer months I spend alot of time on the highway back and forth to cabins, fishing. Average temp here how is -5'Cel. When the temp warms up in summer I would expect psi would be lower..... This happened pretty much over night... not sure what else to do.....
 
If you've used a mechanical gauge to verify that the pressure REALLY is acting this way, then its time to start looking elsewhere. The low hanging fruit is to just replace the oil filter and see if the behavior changes (a filter with collapsed media can do some strange things to oil pressure, although it wouldn't really explain it going too high.

Next step- try replacing the oil pump/regulator valve.

The scariest scenario is that you have one or more bearing shells that have rotated so the oil passages no longer line up correctly and oil flow is partially blocked, causing the pressure to go sky-high. That would be Very Bad (tm). High pressure wont blow seals (no seals are directly exposed to pressurized oil anyway) but it can stress the oil pump drive gear, and it usually is a sign of other problems.
 
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Sorry guys... when I was saying "When driving it runs 35 - 55 psi but drops hard to 15psi" it drops hard when I come to a stop at lights etc. jeep idles at 15psi...... Is it possible that one of the bearinngs has spun..... I went from haveing 35 - 55psi to 15 at idle and 35-55psi driving over night.....
 
15psi at idle isn't bad. My 4.0 was around 12psi. If you were losing flow, then lifters would be rattling to beat the band.

I do understand your being disturbed by the apparent loss.

I'd change the oil to a synthetic to see if the engine sheared it. You may have nothing wrong at all.

If you want to control oil temps better, get a heat exchanger. You can sometimes find them on Ebay, but you can buy a new one at the MOPAR parts counter. Just have the guy look up the oil cooler for a 2005 Grand Caravan with HD towing package. I think it might be $140. Plumb it into your heater core (bypass) circuit and you will assure smooth oil temps.

It will fit fine on your 2.5 and allow you to retain the stock sized oil filter.
 
I don't mind driving around when psi is in the normal range. The big issue I think I have is the extreme high pressure when I First drive in the morning. When started jeep is roughly 55-60 psi. Touch the gas and psi goes up till check valve opens. after a few minutes of driving it settles down....... Will I damage the engine doing this....... I have been letting the jeep idle for about 15mins before I drive, until I get this issue figured out....
 
Originally Posted By: Jeeplander
I don't mind driving around when psi is in the normal range. The big issue I think I have is the extreme high pressure when I First drive in the morning. When started jeep is roughly 55-60 psi. Touch the gas and psi goes up till check valve opens. after a few minutes of driving it settles down....... Will I damage the engine doing this....... I have been letting the jeep idle for about 15mins before I drive, until I get this issue figured out....


The only thing its stressing is the oil pump drive, and that's pretty tough. Keep from winding it out very high when its cold, and it shouldn't hurt anything.

Its still odd, and I wouldn't be comfortable until I figured out *why* something changed. Its like your pressure regulator valve lost part of its range of motion, so now it doesn't regulate as precisely as before. Very weird. Since you have both high (as cold start) and then low (at hot idle) pressure, I doubt my speculation about a bearing rotating is true. For one thing, the engine is still running... for another, it would make the pressure high or low all the time.
 
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Thats the thing, I don't have any noise's for me to want to strip down the engine.......the only consistent thing is the high and the low.....I'm going to sit on it for and see what happens....
 
Just a thought. I don't know much about bearings within the engine. Do the cam bearings ware more than main bearings or vice versa??? does one ware easier then the other???? this pressure issue is driving me nuts. I got a very slow leak at the back of my oilpan which could me either the rear main seal or just the pan isn't tight. would the rear main seal be causing my low oil pressure????
 
What oil(s) have you ran, and what is in it now? What are your oil change intervals.

What oil filter, and how often is the filter changed?

When the oil pressure drops, pull over and quickly pull the dip stick, wipe it and re-insert it, and pull it to measure the level. It would be best to do this with the engine still running if you can. If it is real low it means the oil is not getting back down to the sump fast enough. This could happen when the engine warms up because the oil thins when hot. When thin it flows up through the engine with less restriction, and does not open the relief valve on the pump. If the upward flow rate exceeds the downward gravity return rate the sump will run out of oil, suck air, and the pressure will drop.

There are several things you can do if this is the problem. You could go with an oil that does not thin out as much when hot. And / or, you could increase the gravity return passage(s) ID.

BTW if your engine is thinning out the oil when warmed up, check the cooling system to be sure that it is not running too hot.
 
The high and then low pressure makes me think gummed up passages. If it was just rebuilt the passages should of been cleaned out. So what was the level of quality of the shop that rebuilt it? Some garage mechanics replace the parts but do not clean the block well. If the passages are clogged the oil pressure would be high and then when the oil thins out the pump pressure release would close and the flow would up through the engine would increase. Clogged return passages would then limit gravity return path and sump would get too low. Oil pump would ingest air and not be able to keep pressure up.

What is the oil change history of this engine?
 
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