No oil pressure and minor knock at idle.

So this jeep i have will start up and run fine when cold. After about 3 min or so oil pressure drops to zero ( 2psi on a mechanical gauge ) and will begin to make this minor knock from the bottom end. I just bought it and planned on replacing the oil pump but unfortunately after going through all the receipts that came with the jeep the pump was just replaced back in July 2016. Weird thing is its not a very loud knock and cant be heard against a reving engine. Does this sound like the rod bearings are on their way out?

Cam Bearings?
Oil Pump?
Clogged Screen?

I would get a sample of the oil analyzed. That will tell you a lot.

What brand of oil filter is on it?

Sounds like the PO had issues with low oil pressure.

cruiser54 said:

What brand of oil filter is on it?

Sounds like the PO had issues with low oil pressure.

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Thanks for reminding me. If the filter is a Fram, chuck it and get anything else. I recommend the Wix.

Ive responded to this question a few times because Ive had the same 'issue'. To start off, follow the Old Mans advice. Go with a Wix filter. I like the Napa Gold myself. Look at the fine print at the side/bottom of the box to see who makes it. Before using Wix, on used oil, Id have about 40psi highway and 5-10psi at idle, been that way since new (2001). Switched to Wix filters a couple years later and gained 10psi on both sides. With fresh oil it will give me another ten on top of that, around 55-60 highway and 20 at idle. The knocking itself is probably your piston slap, for lack of a better term. Mine has done it since about 35,000 miles, sounds like a soft deisel engine sound. It now has 210,000 and still does it. Runs great and strong, and I havent changed the oil pump yet either, so who knows. I live on the other side of the country from where I bought it, and ran into a guy who used to be a service manager at a Jeep dealer for many years. Hes had complaints about that sound many many times. He walked over and listened to mine and confirmed it was the same sound hes always heard, too. Apparently its a common thing.

I have had said Slap/knock. its worn rod bearings almost every time.

That 2 psi oil pressure makes me think of either the
oil pump or the drive. Since the pump is new, I'd pull
the distributor and check the oil pump drive shaft for
being stripped, broken or poor engagement where it
connects with the pump.

How many miles on it?

Mine always bottoms out on the gauge at idle, but everyone I know with xjs does it.
Always wondered if it was something bad or not, but as soon as I start rolling again it sits right in the middle

Ya I noticed mine actually dropped with new oil(5w-30 high mileage) and a mobil filter. Changed the oil pressure sensor and may swap the pump for a high volume pump.

kelso1 said:

How many miles on it?

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It has 141k on it. Sorry for the late reply.

To the person who asked what filter is on it i believe its a mopar but i could be wrong. Ill be at the jeep in about an hour so ill check. Ill change the oil with rotella and a wix filter as thats the cheapest/easiest route at this point and ill check the shaft for the pump.

Forgot to mention one thing. The jeep is a 01 and the head cracked at one point and had to be replaced. I had a jeep a while back that did that and I had no choice but to drive a few more miles home. Once i got it home it was knocking horribly. My guess is a result of excessive coolant making its way into the cylinder and not being able to compress so it blew out the rod bearings. I would say that this jeep is suffering the same fate but the fact that the knock is so minor and it only appears once the oil heats up and thins out makes me think otherwise.

Also I read somewhere a while ago that switching the filter to a smaller one will help build pressure. Does anyone know what filter that is?

BrOndon said:

It has 141k on it. Sorry for the late reply.

To the person who asked what filter is on it i believe its a mopar but i could be wrong. Ill be at the jeep in about an hour so ill check. Ill change the oil with rotella and a wix filter as thats the cheapest/easiest route at this point and ill check the shaft for the pump.

Forgot to mention one thing. The jeep is a 01 and the head cracked at one point and had to be replaced. I had a jeep a while back that did that and I had no choice but to drive a few more miles home. Once i got it home it was knocking horribly. My guess is a result of excessive coolant making its way into the cylinder and not being able to compress so it blew out the rod bearings. I would say that this jeep is suffering the same fate but the fact that the knock is so minor and it only appears once the oil heats up and thins out makes me think otherwise.

Also I read somewhere a while ago that switching the filter to a smaller one will help build pressure. Does anyone know what filter that is?

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Sorry to say your engine is most likely toast.

After I did an engine swap I just slapped on a cheap filter and some supertech (walmart) oil since the low miles engine I put in had been sitting for ten years. I used it just for a few hundred miles then put castrol edge oil and a mobil filter in it. I noticed on the crappy oil it had lower oil pressure. Did not bottom out but was lower. I have my doubts that this is whats going on with your xj but thought id throw in my 2 cents :cheers:

So I just checked the filter. Its a "prime guard". Never heard of it before. I doubt its going to fix it but its worth a shot. Gonna change the oil and filter this week and see what happens.

Your standard 0331 head crack is a crack between the rocker arms for cylinders 3-4. It can be seen by removing the oil filler cap and looking straight down. In other words, coolant escapes there, mixes with oil as it runs down the oil passages by the pushrods and falls to the oil pan. Coolant is very corrosive to engine bearings, so when the crack is ignored/unrepaired for long enough the bearings wear out and you get low oil pressure. This is a different than a crack between the water jacket and the combustion chamber. If changing the oil and filter does not resolve the issue, and the mechanical gauge still reads low, your bottom end is worn out.