I've been having some problems with my 99 cherokee sport, lately.

The oil pressure has been dropping to around zero when stopped, after its been running for a little bit. While i'm driving it goes back up and stays around 30-40. I just tried changing the oil and oil filter but it is still dropping when idling.

It also has died on me twice recently just after i started it and tried driving. Both times it was in the cold and just wouldn't idle. *The first time was after i left a light on and the battery died, it started but it chugged really hard before i could give it any gas (not sure if thats relevant).

Any thoughts?

 

Best option is to put a mechanical gauge in place of the pressure sender temporarily and see what you've really got. Might not hurt to flush out the port that feeds the sender too.
Any ticking or valvetrain noises when the pressure goes to zero?

If your battery is marginal, the later XJ's get funky. Testing it might be useful.

 

This is typically the way the oil pressure gauge behaves when the oil pressure sender is failing. It's also the way the gauge behaves when you have a real oil pressure problem. As Radi said, you need to be certain by putting a mechanical gauge on it. They're $25 at Harbor Freight, or you can get a mechanic to diagnose it for you. It's very, very easy to install the mechanical gauge and check it yourself.

If the pressure is good with the mechanical gauge, the problem is most likely the sender. If you have to replace it, you'll be better off in the long run getting a Mopar sender from a dealership, because the aftermarket senders are questionable.

If the mechanical gauge shows low oil pressure also, you have bigger problems. This is why you won't want to assume it's a bad sender. You definitely want to check it with the mechanical gauge to be sure.

 

A rough running engine that won't idle at the correct rpm will cause oil pressure to be low.

 

What should it be idling at when in neutral? Could it be the air idle control valve? Would the idle affect the oil pressure?

 

NOT LESS THAN 650 RPM. Yes rpm and oil pressure are linear. The oil pump turns at half engine speed and 300 rpm at pump does not create enough GPM to overcome internal leakage. Oil pressure at idle is not all that important....it only takes 1/2 of one psi to LIFT oil to top of engine. Oil pressure does not pump up lifters. As long as there is free flowing oil in lifter galley the lifters suck in the oil and pump themselves up. Lifters are actually like a hydraulic jack.

 

From what I've seen, low voltage causes issues. Check ALL cables for clean and tight. Check belt for tight, know how old it is??

 

A quality set of mechanical gauges are a very good investment I have a spare set I have owned for at least the last 10 yrs that replaced an older set. At least an oil pressure, temp, voltage, fuel pressure are ones most likely to be used with an XJ and I have used all of them on mine just to check on my factory ones. I didn't buy them for the XJ but for my race cars to check for any bad ones of if I thought one may not be reading right.


As for my own low oil pressure readings at idle when I first bought it (220,000 mi.) of 5 psi. and 25-38+ @ highway speeds. When I pulled the sender to check it with a manual external gauge I found a lot of crud in it so I checked it anyway and found idle to be 20 psi and at 2500 rpm's 42+ psi. I decided to clean the old sender and try it, when installed it was within 5 psi of the external gauges good enough for me use it instead of a new one and it was free.


To the OP on this issue he was right on in my case anyway.


YMMV.