Jeep problems

dear car talk: I have a problem with my jeep grand Cherokee Laredo 5.2v8 losing oil pressure when the engine reaches normal operating temperature. the jeep sat for 2 years but we got it to run. when normal engine temperature occurs, the engine drops oil pressure and stalls. I have changed fuel pump,oil pump and oil pan gasket. also ran a cleaner through the engine. it doesn’t matter if its winter or summer. the only way I can keep it running is to keep my foot on the gas pedal. if I don’t do this the jeep stalls. a mechanic said it was the bearings. I don’t believe this because there is no knocking noise when the engine is running. it has 103663 miles on it. it has just city and highway miles on it. never has been off road. hope you can help me to solve this problem-terry

What he means by “it’s the bearings” is that the main and rod bearings are worn out.
The oil pump creates pressure by forcing the oil through the small spaces between the bearings and their corresponding wear surfaces. This pressurized barrier keeps the wear surfaces separate. However, if wear causes the spaces to be too big the oil passes through too easily and the oil pump cannot maintain pressure when the oil is warm (at its thinnest). Failure to maintain oil pressure when at temp and at idle is probably a far more common manifestation of worn out bearings than knocking.

I guess first I’d ask if you’ve actually confirmed loss of oil pressure with a “T” fitting and a calibrated gage. If you (or your mechanic) have, than your bearings are worn out. If you haven’t verified the pressure, you should before doing anything else.

If you have, and it’s definitely an oil pressure loss problem, than you might try a heavier base weight oil. It won’t fix anything, but it might elevate the pressure just enough to keep the motor running until you decide what you want to do from here.

Excessive cam bearing wear can result in a significant drop in oil pressure without causing a knock.

And you did replace the oil filter?

I don’t believe low oil pressure is causing the engine to stall. If you can keep the engine running by holding the accelerator I would suspect the idle air control motor is failing. Also the intake manifold belly pan gasket is a common failure on the 5.2/5.9L engines causing a large vacuum leak and should be checked.

changed oil and filter

If you press on the gas enough to keep the idle steady, at about what should be the normal idle rpm, what does the dash’s oil pressure gauge read?

Nevada, I wondered about the low oil pressure as a cause of the stalling too, but some vehicles automatically shut the fuel pump down if the oil pressure fails. I had one. I don’t know about Jeeps, and don’t have access to the necessary specs.

Chrysler products don’t turn off the fuel pump when there is no oil pressure, the oil pressure sensors are unreliable. There don’t want another reason for the car to stall. People do sometimes notice the oil pressure is low after the engine stalls but that is a misunderstanding.

Good enough for me. Thanks for the clarification.

If it’s idling really low before it stalls, that will cause the oil pressure to read low too. (at least right before it stalls) But it does sound like you have worn bearings. You got the vehicle used, correct? So basically you have no idea if the previous owner ran it out of oil, rarely changed the oil, started it up on a cold winter day and immediately floored it, etc. Incidentally, is the ‘check engine’ light on, and if so, have you read the codes?

As others have mentioned, I would verify the hot oil pressure with a mechanical gauge. On the plus side, if you need an engine, this is a very common engine and you should have no trouble finding a good used one. Maybe even go to a 5.9L if you want an upgrade…

the normal idle rpm is 1000 when i keep my foot on the gas, i keep the rpm’s at 1000. when i start the vehicle up the oil pressure reads 40-45.when I take my foot off the gas,the oil pressure drops to zero and stalls out. I bought the jeep new. gave it to my brother to drive in 2012.i found out later after he gave it back to me that he didn’t change the oil that often. he also ran the vehicle on low oil. the check engine light is not on.

Does the oil pressure drop to zero and then the engine stalls out
Does the the engine stall out and then the oil pressure drops to zero ??

oil pressure drops to zero and then engine stalls out

On the 1996-1998 Grand Cherokee the oil pressure display value is sent to the instrument cluster from the PCM over the BUS. If something is shutting down the PCM the oil gauge will drop at the same time.

How long is the oil pressure at zero before the engine stops?

instantly. on a 1995 is it the same

1995 Grand Cherokee the oil pressure sending unit is wired directly to the cluster. It is normal to have no oil pressure when the engine stops turning. Work on the idle problem, sounds like the idle air control motor is sticking/failing to open.

What is the oil pressure when cruising at say, 2,000 RPM?

its 40-45 then when engine start to warm up it drops to zero.

If it gradually drops as the engine warms, it sounds like a classic case of worn engine bearings. You need to verify by having the oil pressure checked with a mechanical gauge. But it sounds like you may need a new engine or a rebuild.