So that you know how desperate my situation is... i found a 2000 jeep grand cherokee for my girlfriend to buy her son. The exterior and interior are in fantastic shape....but, long story short is i think i (we) have been taken for a ride. I've bought used vehicles before. I check under the hood, the fluids even under the chassis for wetness (leaks and such). Didn't see anything that caused me concern. In fact the oil had recently been changed.(yeah...). On the way home the oil pressure drops and the check gauges light comes on. So we get it home. Check the oil and it is over full. Doesn't appear to have water in it. Faint smell of fuel. So i change the oil, drive it and same thing. I pulled the plugs, none are wet like if the injectors were leaking. Pressure is great when first started, warmer the motor gets the lower the pressure goes, until the check gauges light comes back on. Check the oil.... thin. Could the main bearing and seals be gone? Holding pressure until it gets warm? Help? Please? Looks like there is a lot of knowledge here. Really disappointed that I've let my girlfriend and her son down with this purchase.

 

Hello. Do you know what codes are stored which cause the check engine light to come on? You should be able to get the stored codes by using the key on-off-on-off-on method. Turn the key to the on position (Not to "start" but just the first click to turn the power on) then turn it off then back on then off then on again and leave it there. The check engine light will start flashing. Count the flashes. It should start with code "12" One flash then a short break then 2 flashes. And it should end with code "55" 5 flashes then a short break then 5 more flashes. Anything between those 2 are stored "trouble" codes. For example code 21 (O2 sensor) would be 2 flashes then a short break then one flash.

 

I just went and checked. There is a check engine light and a check gauges light. The check engine light has never come on. But anyway, i tried the key switch thing...the check engine light doesn't flash at all. The odometer does say "done" though?

 

I see. I mis-read and thought you had said the check engine light came on but now I see it was the check gauges light. So once this happens, the pressure drops, what happens next? Does the Jeep still drive normally or does it start running badly or cut off? Does the oil level stay the same or does it disappear?

 

This is a moot point, but 99 and newer will display the actual code in the odometer. "done" is the end of the test.

AND you never said if this was a 4.0 I6 or 4.7 V8.

The oil pressure should be checked with a mechanical gauge to be sure there IS a problem.

 

Yes that was the direction I was heading with my questions. It's possible it is merely a sensor or gauge problem and the oil pressure may in fact be fine.

 

i'd say follow whatever dave123 says if you can, he will not lead you wrong.

he's helped me a few times with things.

ryan

 

My 2000 WJ acts the same way. Very low oil pressure at hot idle (5 psi). Verified by a mechanical gauge permanently mounted in engine bay. Using 15-40 conventional oil to help combat the issue. Have been driving it like this for over 7,000 miles on and off-road with no changes. I suspect main bearings.

 

It could be a worn pump. My 2000 WJ 4.0 has 232K miles on it and I use 10W-30 oil in it and she holds 40 psi hot at idle and 55 psi at 65 mph. That was verified using a mechanical gauge.

Again...if this is a 4.7 it could be worn cam bearings. Unfortunately, the cams run directly on the aluminum head casting. One if the reasons I hate that motor. It is a good engine unless neglected, then anything goes.

 

Thanks for the replies guys. To answer all of the questions...
It is the 4.0
the pressure is fine until it warms up really good. Then at red lights and at slower constant speeds the pressure drops. If I shift to neutral and rev the motor the pressure comes up but doesn't stay up.
The engine runs fine, it doesn't skip or sputter, runs "strong" I guess.
I have notice a slight vibration in my feet, which kinda leans toward a main bearing issue?

 

The 4.0 has a history of worn oil pumps. It's not an easy job to do, but could save a complete teardown. You could check the bearing clearances while you have it apart.

 

Doesn't sound to bad then. My one XJ the pressure drops very low at red lights and the rpm's drop a lot. It's been like that for 10 years.. A vibration at your feet could be anything even a u-joint or just a steering/suspension issue. As long as it is running strong and you aren't physically losing oil I wouldn't be overly concerned you bought a vehicle with a problem. Could even just be a bad oil pressure sensor

 

I guess I'll just have to pay someone to check it out and go from there. Just can't afford to throw parts at it and eventually fix it, lol.

 

Test for fuel in oil, breakdown of main bearings and other issues with an EOA, engine oil analysis from Blackstone.
Test for exhaust gasses in coolant may also a good idea (second link).
And perhaps a compression test on each cylinder.
https://www.blackstone-labs.com/