This is my daughter's friend's Jeep (haven't seen it in person) and I'm told it's a '98 Cherokee with the straight six and 169,000 miles. He's in college and I'm trying to help over the phone. The dash guage drops to zero and the oil light comes on at idle once the engine warms up. There are no knocks or bangs from the engine. The guage reads about 40 PSI while driving.

It sounds to me like either the oil pump is shot or the sensor is no good. The first thing I'd do is add an external guage to the motor and see if it drops to zero when the dash guage does. Do these engines have a history of bad oil pumps? Is there anything else for me to suggest he have checked? Will he be OK to drive home from school (about 400 miles) and then his family's local shop can check it out? Thanks for any ideas.

loose connection or bad sending unit, low oil level, weak pump, clogged pickup screen, bad motor..

I'm betting if the oil pressure gauge is accurate, the 400 mile trip home will solve the issue, if he makes it back home. Think I'd start out by checking the oil to see if there is any, and if it needs to be changed. For most college kids I've known, its low on oil, and hasn't been changed since they got to school. Your results may vary. Fresh oil in the proper amount may solve the issue as well. Gene

I forgot to mention that I asked him about the oil. He bought the Jeep from a lot and they changed the oil before he picked it up. The light started flickering after he had his first oil change. He's 1000 miles into this oil change and the oil level and color is good. I'm concerned that the lot used heavy weight oil to hide this issue and the oil change was with the correct, lighter oil.

I don't know if this pertains to Jeeps, but I know some of the Minivans and the Intrepids had this same problem. One these other vehicles, the wiring harness that goes to the oil pressure sending unit must be replaced. I can't recall the exact model years affected by this harness, but it was in the late 1990's to early 2000's. There was a TSB on this problem.

Thanks for the heads-up. I'll tell him to look for that as well.

Like you said, add an external gauge to check against the dash gauge. If oil pressure was zero, the valves should be clattering.

oil pumps on some needed changed around the 150000 mark

If the gauge is reading 40 psi running then it seems to be working. agree a parts store mechanical gauge is a good diagnostic but it sounds like the oil pump is worn or main bearing clearance getting big. To get it home, some heavier oil may be in order.

Oil change places should have 15W40 diesel oil. That would likely help things... or some 20W50. Either should work in winter if its not crazy cold.

Thanks for all the replies. He drove home a couple of days ago with no issues. I'll find out what their mechanic says and let you know.

My '97 Grand Cherokee (5.2) did this exact same thing after the quick lube did an oil change for me. At idle when warm the gauge dropped to nothing but just off idle I had plenty of pressure. Turns out they used 5w30 but the owner's manual says 10w30. I made them change it out and it's been fine ever since. You wouldn't think that minimal difference would matter but it did on mine. Worth a try since you mentioned a recent oil change.

This just happened to my 4.0 Cherokee about a month ago. The gauge used to run at about 40 psi at idle and 60 or more while driving. All of a sudden one day it dropped to 40 while driving and went to zero at idle. It was the sending unit. Cost about $50 and was pretty easy to replace.

His mechanic replaced the oil sender and that fixed it.