I changed the oil a few weeks ago. Right after the change I noticed the check gauges light turned on and the oil pressure dropped to zero. This problem had not happened before this. The engine runs perfectly and does not sound like there is low oil pressure, and it only drops to zero at idle. Can someone explain?

 

Check the connector for the oil pressure sending unit, located directly above the oil filter.

You may have bumped it when you were changing the oil filter. Very common issue actually on the 4.0

Be sure the connector is firmly attached to the sending unit and that there is no damage to the sending unit itself or associated wiring.

 

Thanks, I thought the sender unit or connector could be at fault but wanted to know if there were any other possibilities.

 

Try removing the oil pressure sensor and cleaning out the inlet hole with a pipe cleaner. Worked for me with the same issue a while back.

 

Well, a spun bearing or a bad/weak oil pump come to mind but that's a little extreme at this point.

Bad oil filter could cause it too. I hope you're not using a Fram.

 

Not a thread hijack, promise, but Firestorm you once similarly advised me re same problem. To develop your answer a tad, are there other symptoms that go with either of these conditions independent of tearing things apart to check? Thanks for any further input

 

True low oil pressure will result in a bunch of noise as parts aren't lubricated correctly.

 

ive done this myself , kinda spooked me at first till i realized it was unplugged some

 

I guess the easiest way to check for at least some oil flow is to start it, then shine a light down the oil filler hole in the valve cover and see if oil is moving around in there.

 

Nowhere do I see what year Cherokee you're referring to. If it's '90 or older, a Renix, then it's not a problem with the connector to the sending unit. I don't know about the '91 or newer, but the Renix gauge will read HIGHER with a bad connection. The more resistance at the connection, the higher the oil pressure reading. If you disconnect the wire completely from the sender, the gauge needle will peg all the way to the right, well above the 80 psi mark.

 

Sorry, it's an '01

 

Damaged and contaminated connector can create either a low or high resistance fault. Not just one type.

 

Makes no sense, salad. What's a low resistance fault? A no resistance "fault" on a connector is no fault at all. Point is, the gauge will read high due to a corroded terminal (resistance greater than zero) at the sender end, and I guarantee you, the needle wil peg to the right on a Renix oil pressure gauge at infinite resistance (disconnnect). A sender wire with damaged or missing insulation, if grounded, will result in a gauge reading zero, but a "low resistance fault" (any resistance above zero) at the connector won't do anything except make the pressure reading just a little higher than actual, and the higher the resistance in either the connector or the wire, the higher the pressure reading. I am battling that very problem on my Comanche right now. It appears that I need to repair some wire or connector, because my oil pressure gauge is reading 80 psi at 2000 RPM, and 40-50 at idle. Unplug the sender wire (gauge pegs to the right with key on), remove the sender, and connect a mechanical gauge to confirm, and the actual pressure is 45-50 at 2000 and 17-18 at idle. The oil pressure gauge is nothing more than an inverse reading voltmeter (0 volts to right, 12 volts nominal to the left), with power to one terminal, and grounded through the sending unit at the other terminal. It's the variable resistance in the sending unit that varies the reading on the gauge to indicate oil pressure.

 

As others mentioned. Check the connections to the sending unit. Also, since you said that the low pressure didn't start until you did the oil change. What brand of oil filter did you put on it? Yes a crappy brand of filter can give you low or no oil pressure. Especially on our 4.0's.
Keep us updated.

 

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