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mechanical oil pressure gauge

Old 01-17-2019, 06:00 PM
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Default mechanical oil pressure gauge

2000 XJ 4.0 Total Newbie trying to learn...

I am getting random low pressure readings and occasionally no pressure readings on the dash gauge. I haven't noticed any unusual engine noises, but this is a recently installed motor so i am not 100% familiar with it's sounds yet. I searched the forums and read I should install a mechanical pressure gauge to see whether it is a bad "oil pressure sending unit" or some other really an oil pressure problem. I have a test kit, do I install that right into the hole where the "oil pressure sending unit" goes? What sort of pressure should I see at start up, Idle, under load? Or, should I just replace the "oil sending unit". I use quotes because I am not sure that is what it is called.

Any help would be appreciated.

Last edited by 98 Cherokee Classic; 01-17-2019 at 06:05 PM. Reason: add info
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Old 01-17-2019, 06:17 PM
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As you already have the mechanical gauge, I would recommend testing. That way you know for SURE what your oil pressure is. Yes, it goes right where the oil pressure sending unit resides by the oil filter. Takes only a few minutes.

The oil pressure sending unit is a very common failure on the 4.0 engine, so it could be the cause of your low oil pressure readings you are seeing on the dash gauge. And testing is always the best way to proceed here. Is this a rebuilt engine or salvage yard engine? Do you know how many miles are on it?

For reference, here are the oil pressure spec readings for the 4.0: If you are in this range, you should be okay.

*Not less than 13 psi at a HOT idle
*Between 37-75 psi over 1600 rpms


If you do end up replacing the sending unit, I recommend buying a Mopar unit directly from Jeep for best reliability/accuracy/mating with your factory gauge.

Good luck and keep us updated on what you find

Last edited by tjwalker; 01-17-2019 at 06:20 PM.
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Old 01-17-2019, 09:54 PM
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AND clean out the fitting it screws into.
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Old 02-04-2019, 04:59 PM
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Originally Posted by tjwalker View Post
As you already have the mechanical gauge, I would recommend testing. That way you know for SURE what your oil pressure is. Yes, it goes right where the oil pressure sending unit resides by the oil filter. Takes only a few minutes.

The oil pressure sending unit is a very common failure on the 4.0 engine, so it could be the cause of your low oil pressure readings you are seeing on the dash gauge. And testing is always the best way to proceed here. Is this a rebuilt engine or salvage yard engine? Do you know how many miles are on it?

For reference, here are the oil pressure spec readings for the 4.0: If you are in this range, you should be okay.

*Not less than 13 psi at a HOT idle
*Between 37-75 psi over 1600 rpms


If you do end up replacing the sending unit, I recommend buying a Mopar unit directly from Jeep for best reliability/accuracy/mating with your factory gauge.

Good luck and keep us updated on what you find
It took me a while to get back to this due to the polar vortex. The Weather warmed up this past weekend so i could get back to it. It is junkyard engine with supposedly 99k. The 4.0 came out of a 2001 Wrangler. The oil sending unit is a little further back on the block but i was able to locate it and install a mechanical pressure gauge. Pressure is around 20 PSI on a cold start. after a trip or two around the block, it idles at 13 PSI. I changed the oil and filter. Oil was clean, no coolant, no debris. I cleaned out the hole the sending unit screws into as best I could. Even though there is no knocking or engine noise, there is something going on with the oil pressure. What would make the pressure DECREASE with more RPMs? Idles at 13 PSI and drops to 10 PSI when I rev the throttle. Very confusing. Would a bad oil pump cause this?



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Old 02-04-2019, 05:00 PM
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Originally Posted by dave1123 View Post
AND clean out the fitting it screws into.
It took me a while to get back to this due to the polar vortex. The Weather warmed up this past weekend so i could get back to it. It is junkyard engine with supposedly 99k. The 4.0 came out of a 2001 Wrangler. The oil sending unit is a little further back on the block but i was able to locate it and install a mechanical pressure gauge. Pressure is around 20 PSI on a cold start. after a trip or two around the block, it idles at 13 PSI. I changed the oil and filter. Oil was clean, no coolant, no debris. I cleaned out the hole the sending unit screws into as best I could. Even though there is no knocking or engine noise, there is something going on with the oil pressure. What would make the pressure DECREASE with more RPMs? Idles at 13 PSI and drops to 10 PSI when I rev the throttle. Very confusing. Would a bad oil pump cause this?
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Old 02-11-2019, 08:56 PM
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I replaced my sending unit and it still reads way high. The needle at times is beyond the max, just seems broke. Is there a way to get the gauge fixed or would I be best off picking up a couple from the junk yard and hope one works?
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Old 02-12-2019, 03:17 AM
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Originally Posted by 98 Cherokee Classic View Post
It took me a while to get back to this due to the polar vortex. The Weather warmed up this past weekend so i could get back to it. It is junkyard engine with supposedly 99k. The 4.0 came out of a 2001 Wrangler. The oil sending unit is a little further back on the block but i was able to locate it and install a mechanical pressure gauge. Pressure is around 20 PSI on a cold start. after a trip or two around the block, it idles at 13 PSI. I changed the oil and filter. Oil was clean, no coolant, no debris. I cleaned out the hole the sending unit screws into as best I could. Even though there is no knocking or engine noise, there is something going on with the oil pressure. What would make the pressure DECREASE with more RPMs? Idles at 13 PSI and drops to 10 PSI when I rev the throttle. Very confusing. Would a bad oil pump cause this?
Sounds to me like the oil pump isn't putting out very much volume. At higher rpms the lifters are venting more oil and the pump can't keep up. Just my opinion.

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Old 02-12-2019, 11:52 AM
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Originally Posted by rgallardo View Post
I replaced my sending unit and it still reads way high. The needle at times is beyond the max, just seems broke. Is there a way to get the gauge fixed or would I be best off picking up a couple from the junk yard and hope one works?
Verify your pressure with a mechanical gauge.

If it varies between the 2, start by cleaning up and checking the connector the sending unit plugs into, clean up the port where the sending unit plugs into and if all that fails, replace the sending unit. If you *really* believe the gauge is bad or you've looked at everything and swapped out the sending unit, you can replace it.
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