Question I have a 1996 jeep grand cherokee 4.0 and when i'm driving normally rather it be on the highway or through town my oil pressure is fine but when i come to a stop at a stop light or a stop sign my oil pressure goes down to a 1/4 on the gauge then when i accelerate again it goes back to normal...anything to worry about?
Answer Hi Tim,
Not usually, but that depends on the accuracy of the gauge and the stability of the Oil Pressure Sending Unit. The minimum pressure at idle shouldn't be less than 13psi (hot), but you should really be concerned if it approaches 6psi. Those gauges are just a step above a 'dummy' light, and are often very misleading. Still, if memory serves me correctly, the gauge is in increments of 20, so at 1/4, you'd be at 20psi hot, which isn't bad. Realistically, though, you should be aware of what is 'normal' for your vehicle, and react when there is a large change. For example, if it normally sits at 35 at idle (again, hot), and it dropped to 20, I'd be concerned -- even if it's still within spec.
The Sending Units are almost always an issue with these things, and are a common replacement. Typically, the needle moves in a jerky way, which tips you to the fact that it's probably electrical, and most likely a faulty sender. They have been known to leak, and read lower, or just be skewed enough to deliver inaccurate readings (although somewhat rare).
If you're really concerned, have a mechanical test done (or DIY). Just unscrew the sender, and screw in an oil pressure gauge, and check the readings when cold and hot. See if they match up to the gauge, and if they don't, perform the proper diagnosis. At a reputable shop, this test should be relatively cheap (sub-$100, most likely around $50).
