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Doug In North Carolina
Doug In North Carolina, ASE Certified Technician
Category: Jeep
Satisfied Customers: 302
Experience:  ASE Master Mechanic, 12 Years Experience Specializing in HVAC, Electrical and Driveability Problems.
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1999 Jeep Grand Cherokee Laredo Oil Pressure Issue

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I have a 1999 Jepp Grand Cherokee Laredo with 192,000 miles. Recently when it warms up, the oil pressure gauge drops to 0 and the check gauge light comes on. It only drops to 0 when my foot is off the gas and I am idling. The second I put my foot back on the gas the pressure goes back up. This happened about a month ago too, but as soon as I got an oil change it worked fine, but it has only been 100 miles since I got the oil change. What could be causing this?
There's a couple of issues that could cause this. The first is going to be low oil pressure. You would need to hook up an oil pressure gauge in place of the sending unit and check the actual pressure to see if you really have low oil pressure or if you instead have an electrical problem.

In order to assist you in checking this you would need an oil pressure gauge, a special socket (available at most parts stores) and a multimeter to check the electrical side of the system.

Are you looking for just a list of posibilities or are you able to do these tests and looking for further assistance with those tests?
Customer: replied 7 years ago.
About a week before the issue happened last month, my car had been in the shop, they had replaced the starter, fuel pump, and the fuel pump housing. Could any of this have affected the oil pressure?
It's possible that they bumped the oil sending unit, it's in the area near the starter but you would need to at least start with a visual inspection. You would be looking for a damaged connector, frayed or severed wiring, or a damaged sending unit itself. It should look like this:
Customer: replied 7 years ago.
How likely do you think it is the sending unit if originally this issue happened a month ago, then we got an oil change, and it worked fine until today?
Sensors have a lifespan as well as every other component on the vehicle. At the end of their life they may be less accurate at both the high and low ends of their range. Or they could simply fail to report accurately at all.

However, with the mileage that you have on the vehicle there may be a bigger problem in that the engine truly is suffering from low oil pressure at idle. As the engine RPM increases so does the oil pressure. New oil will be thicker than older and cold oil thicker than hot so pressure may be higher after an oil change or when the engine is still cold.

If you brought the Jeep to me that's basically how I would explain it. I would give you the option to perform an oil pressure test which should cost about 0.5 hours ($50.00 or less) and based on that I would tell you which direction we would go. If the engine always reported low after being warmed up and in my tests I found low oil pressure then I would present the option for a newer/remanufactured engine given your mileage. If the oil pressure stayed about say 25psi or more the whole time then I'd suggest either electrical diagnostics or just put in a sending unit and see if things change.

From the timeline you've presented, the work that's been done and mileage on the engine I would still want to look for possible damage from their work, perform my testing but prepare you up front for the possibility of it needing an engine.
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