Zero Oil Pressure

Recently my 98 Grand Cherokee 4.0L began displaying zero oil pressure AFTER reaching full operating temperature, UNLESS RPMs are held over 2,000. Oil sender and oil/filter have been changed without result. Ideas?

Check the oil pressure using a reliable mechanical oil pressure gauge. If it still shows no oil pressure your only alternative is to rebuild or replace the engine. It will not survive at zero oil pressure.

…or see if it just needs a new oil pump.

While working at a Chevrolet dealer in Tucson in 1998the factory rep told us that on any of their engines 7psi oil pressure at idle was in spec. I know this data does not help your situation,Good Luck

A worn oil pump is a technical possibility, but I’ve never seen one that was bad enough to cause zero oil pressure on an otherwise good engine. From what I’ve seen, it’s the other way around: excessively worn bearings bleeding pressure generated by an otherwise good oil pump.

That’s not to see it can’t be the pump. But if the pump is worn I’d expect the rest of the engine to be even more so.

I had an oil pump fail on an engine without any other problems. It had about 375K miles and was in good shape, I shut it off immediately and had the oil pump replaced. The engine continued to be fine, the pump had just failed.

Should have mentioned that this is a fairly low-mileage vehicle for its age—30k. Also, should have been more clear that normal pressure is held until engine heats up, then normal oil pressure is only held if RPMs exceed 2,000.
How about the bypass valve being stuck open? And, where is it, if there is such a thing?

Possible that there is a pressure relief valve or something similar stuck. The most important thing is to determine if this is a real loss of pressure or an instrument issue. If you really have low oil pressure, you don’t want to drive it until it’s corrected.

Are you saying that your oil pump failed and provided no oil pressure at any time, or did it hold pressure until the engine was warm? Any part, including oil pumps, can fail catastrophically, But for the OP’s engine to have oil pressure until warm indicates something other than a worn oil pump to me.

Bypass valve is on the body of the oil pump itself. The fact that the engine has pressure until warm indicates the valve is not stuck.

What appears to be happening is that as the engine gets warmer the metal parts expand, leaving slightly more space between the rotating parts and the bearings they ride in. This space allows the oil to bleed out, resulting in a loss of pressure. The space would be smaller, but the bearing surfaces are presumably worn, and thus the excessive clearance. Couple this with the fact that the oil gets thinner as it gets hotter, and you can see why the pressure drops off as the engine warms up.

You may be able to mask the problem temporarily by using a heavier oil, such as 20-w50, but be assured that it would be a temporary fix that wouldn’t last long. Your only gaurantee of knowing exactly what’s wrong in this engine is to drop the oil pan and inspect the bearings and oil pump. I think you’ll find the crank bearings are badly worn and need replaced. If they do, the cam bearings will also need to be replaced.

In my case, the oil pump failed partially such that it had normal pressure on the highway (over 3 bars), but the pressure fell to zero when the engine went to idle (instead of it’s normal 1.5 bars at idle). I pulled off a highway exit (after 2-300 miles of steady 70-80 mph driving) and saw the pressure gauge drop (no idiot light in this car). I shut off the engine and had it towed, the engine was fine with a new oil pump. I assume it would have failed catastrophically if I had driven long enough. I did not disassemble the old pump to determine what failed, so it could have been an internal pressure relief valve. This may be an unusual failure, but it happened at least once.

Possible, but that seems unlikely on a 30K mile engine. I agree that the solution is to drop the pan and take a look, after verifying that there is an actual loss of pressure.

As I read your kind replies from beginning to end, for a second time, I just realized that I can probably read the oil pressure data in the ECM stream. I’ll hook my Nemisys code reader to the car this evening and report what I see. Seems this procedure would verify that the gauge is correctly displaying the data stream.
Also, I note for the record that the top-end of the motor doesn’t go “tick-tick-tick” on zero pressure, as I’d expect it to. Seems the valves and lifters would dry out first???

I would use a good old fashion mechanical gauge and determine the actual pressure. I suspect the actual pressure is OK.