Low oil pressure/knock at idle

Once the engine in my 90xj 4.0l has gotten up to temp my oil pressure ussually is sitting at about 17 on the regular gauge and maybe 5-10 on the mechanical gauge I have. Also once it has gotten to temp it has a noticable knock at idle but deffinately goes away when I accelerate or pretty much over 1000rpms. The location of the sound has proved to be extremely hard to locate no matter where we listen it seems we can't identify the location of the knock(using a long ratchet extension with our ear to it to listen for it). We think it is in the very top of the engine but still not completely sure.

As far as history on the engine previously this year the L arm that holds the oil filter up had to be replaced when the seals in it broke(??), The rearmain seal was also done at that time. Then after that I had extremely high oil pressures(70-off the gauge). So I had to replace the blow by system because it was so clogged and had never been cleaned. My jeep had also been running at about 210+degrees at that time but everyone here said that was normal. Well a couple of weeks ago I flush my radiator system and checked my thermastat using the boiling water technique, it never openned even when the water it was in was boiling. So I replaced the thermastat and am now running 180*. The low oilpressure and knocking had appeared around the time after redoing the blowbye system. I was actually getting 0 psi at idle when the jeep had been driven a lot, on both gauges until I switch from 10w30 to 20w50.

Low oil press. could be many things....as far as the knock and not being able to find it....I had /have the same prioblem, checked the motor in various spots....couldn't find it.....told motor was junk 'cause it sounded like that by a few mechs.......I wasn't buying it 'cause it sounded great driving down the road......broke down, bought a cheap mechanics stethocsope....still couldn't pinpoint where in eng....well after screwin' around for awhile...it turns out it is coming from my TC and radiating to sound like an eng. rap. Good luck

Inisadow said:

Once the engine in my 90xj 4.0l has gotten up to temp my oil pressure ussually is sitting at about 17 on the regular gauge and maybe 5-10 on the mechanical gauge I have. Also once it has gotten to temp it has a noticable knock at idle but deffinately goes away when I accelerate or pretty much over 1000rpms. ........................
The low oilpressure and knocking had appeared around the time after redoing the blowbye system. I was actually getting 0 psi at idle when the jeep had been driven a lot, on both gauges until I switch from 10w30 to 20w50.

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That sounds to me like the main bearings are worn and there's too much bearing clearance, hence the low oil pressure. Rod bearings could also be worn but the knock from those is usually worse under load.

Heres my two cents worth. I jave an '01 which, yes can be a beast of a different color, but I also have that sound under the hood. Mine was more of a clicking or tapping sound at idle, almost like a LOUD sticky tappet. It turned out to be coming from a 'dashpot' type gizmo (forgive the jargon as I have not looked into what this item is for) with vacuum lines running to it. It is attached to the firewall on the passenger side near the top of the compartment. I also used a stethoscope confirming this source. Sound goes away upon acceleration. It may or may not be the same thing.

As an old shade tree mechanic, if the mains are loose and you are gettng low oil pressure that improves with thicker oil and the noise goes away, we used to pull the pan and replace the bearings without turning the crank. We simply went with the same size bearings as the original. It takes up a good percentage of the clearance. It isn't the best way to do it and you need to verify the journals are not scored or galled, but you can do it in a couple of hours for a few bucks. It either helps or it doesn't. You can replace the rod bearings at the same time as well. The key is to replace them before the crank gets worn down due to the low oil pressure and excess clearances.

Yes you can plastigauge the bearings and have the crank turned and rods reconditioned, but sometimes the $$$ just aren't there. The job isn't that much harder than replacing the rear main seal.

If you get to that point, contact me and I can give you a few pointers.

I agree with Old_Man -- replace the bearings. On the 4.0L the rod bearings usually go before the mains, but once the pan is off you might as well replace both. You'll probably find that your bearings are worn right down to the copper shell.

Some suppliers sell bearings that are oversized by a thousandth or two to make up for worn cranks. You might look into it. I have used them with good results in engines with worn rod bearings.

Damn, I', glad to see that I'm not the only one who has done this. It sure beats pulling the motor. I've done it for several people when we pulled the pan to do the rear main seal. I pulled the bearing caps and took a look to see how the wear was doing and found it could benefit from the swap. Grinding the crank is an overused solution. Not that there aren't times when it is needed. The main thing is to also do a compression check. If the rings are worn, you are wasting your time.

Sparkman said:

Some suppliers sell bearings that are oversized by a thousandth or two to make up for worn cranks. You might look into it. I have used them with good results in engines with worn rod bearings.

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Just put rod bearings in my 96 last weekend. If you read the FSM you'll see they actually mixed and matched bearings shells on the same rods from the factory!! You can have STD and .001 on the same rod! As I recall the STD needs to be on top though. I had 3 with both STD and three with one of each. There's paint on the rod cap indicating what you have. The only problem is it's faded. My bearings were bad and I replaced the pump with one 20% more volume and it did quiet it down. However I still have a noise that starts after thirty seconds and lasts untill it's completly warm. When we first put it back together with the new pump primed with vasoline it was quiet all the way. Then I changed my oil.. and the knock knock was back. That's how it started the first time.

JoBo