X1997J

Hey guys, I am having the typical fluctuating oil pressure readings. When the engine is warm at highway speeds (70mph) I get anywhere from 20-60psi and stoped at a red light 0-10ish psi. I have recently replaced the oil pressure switch with a Mopar unit and my next step is to install a mechanical gauge to compare the readings with factory gauge while driving around.

The 4.0 engine has 208,000 miles. If the two gauges are showing the same readings how likely is the problem to be the oil pump?

Thanx for the help.

third coast

Probably not the oil pump if the pressurre really is low at idle. When clearances open up on cam shaft and other sleeve bearings due to wear more oil flows out there and the pressure drops. Other areas get starved for oil. It may be OK for some time. Some have switched to higher capacity oil pumps to help minimize the problem. If it was me I would not be driving it long distances from home.

dave1123

If a mechanical gauge verifies the pressure is actually low, try switching to a heavier oil, like 10W-40 to see if you can keep the idle pressure above 10 psi. Engine temperature after heat soak seems to have a lot to do with XJ oil pressure. When I say "heat soak", I mean when every part in the engine has time to reach 210*. My XJ 4.0 seems to hold 30 or more psi at idle just as the coolant temp reaches normal, but drops off to 20 or so by the time it get fully heated. I'm running Rotella T6 5W-40 with only 145K miles on it, but with 10W-30 conventional oil, hot idle was only 5 psi! I've done experiments with things like eliminating the filter adapter that have shown no improvement and have contemplated adding an oil cooler, but never went that far, although I think that might help. The XJ gets really toasty under the hood, as we all know. I think venting will also help, like hood louvers or raising the rear edge of the hood on hinge spacers. The only problem with the spacers is any leaks wind up on the windshield! Maybe that's a good thing. LOL!

The reason I think it's all temperature related is my 2000 WJ uses the identical oil pump but never shows less that 40 psi at any time, even at 250* which it got to once.

The oil pumps used in this motor are a simple gear pump, which is critical for wear on the gears and body clearances. A "gerotor" pump used on some other makes is much better as they are a gear within a rotor and are much less prone to pressure loss due to internal wear. But now we're talking major design changes much better left to the racing guys.

00t444e

Try 15W40 diesel oil and a Motorcraft FL1A filter, it increased my oil pressure 10-15 psi.

Steve Hayes

I also run Rotella T6 5W-40 and it raised my oil pressures 10 to 15psi across the board.

X1997J

I shall try the different oil weights if the mechanical gauge still has the same readings.

2kuehl

My XJ has the same issue, Let us know what the outcome is X1997J. I'm needing an oil change very soon...

dave1123

I'm pretty sure it will. Jeep says 13 at hot idle is sufficient, but I get the ******* with less than 20. As long as it jumps right up at 1000 rpm or more, you're good.

country2

hell my 2000 with 165k miles or so has 9psi at idle , 65 at 65mph verified with a mechanical gauge. Been like this for 4 years doe not tick ant worse I'm just gonna drive it till there is a issue.

X1997J

I have not had time to get around to an oil change yet nor have confirmed with a mechanical gauge, but the last time I drove it I noticed the gauge moving rapidly from zero to what ever reading and not a gradual increase. I'm pretty sure its the gauge.

Lets say my oil pressure readings are in spec and I swap oil with t6 5-40, is that 10-15psi potential increase too much pressure?

2kuehl

if your gauge is jumping around, check your alternator output.

I was about to switch out my gauge cluster due to them jumping around but decided to check the alternator output and found it to be at 12.3v. After i switched it, it was at 13.4v and all the gauges were working properly.

dave1123

I think the pump's relief valve pressure is set to 95 psi. I think anything more than that might blow out the rear cam plug or rear seal. Probably pop the filter too.

2kuehl

My oil pressure was around 0-5 at idle before I switched the oil to Rotella 15w40. Now it's sitting at 30. Thanks for the heads up on that one guys!

X1997J

I did have problems with my gauges jumping around a few months ago then my batt died and changed the alternator. Now its just the oil pressure gauge. Thanx for the suggestion.

X1997J

I know its been a while since my post but yesterday I got the oil pressure reading problem fixed, finally. I had sludge behind the oil pressure switch after getting it out the readings are good. Thanks again for the help.