1996 XJ 4.0 Knocknig Noise (video) + Compression Test Results. HELP!

Just got this XJ last week. I'd like some help identifying the cause of my tick/knock.

Did a compression test (kept 5 of 6 plugs in while checking the 6th- turned engine over for about 5 seconds).

Results (psi):

Cyl 1: 145
Cyl 2: 145
Cyl 3: 125
Cyl 4: 145
Cyl 5: 130
Cyl 6: 135


Here's a video of the noise- it's hard to hear it from outside of the car, so I took video right next to the engine.

I'm also noticing a trickle of oil from the ccv tube going into the air filter. And by "trickle," I do mean "trickle." A drop or two over 200 miles.

171k miles. Oil pressure is 20psi when engine is hot (not warm), and goes up to 70 when revving.

Piston slap?

Doesn't sound like a rod, doesn't sound like a flex plate, and doesn't sound like a lifter.

If piston slap, I would just run it. You'll have a noisy motor, but the cylinder is probably scored to hell already anyway.

What happens when you disconnect the injectors 1 by 1, does the noise go away on a particular cylinder?

Your compression is good, and oil pressure is normal, although your oil pressure is on the lower end of the spectrum (13 is the bare minimum), so your bearings might be worth replacing soon in the future.

thatxjguy said:

Piston slap?

Doesn't sound like a rod, doesn't sound like a flex plate, and doesn't sound like a lifter.

If piston slap, I would just run it. You'll have a noisy motor, but the cylinder is probably scored to hell already anyway.

What happens when you disconnect the injectors 1 by 1, does the noise go away on a particular cylinder?

Click to expand...

It's a 5 speed, so that rules out flex plate (plus, my other XJ had the flex plate issue- sounds different).

I'll disconnect the injectors and see. I tried disconnecting the plug wire to cylinder 3 (that's where the noise seems to be coming from), and the noise is still there.

It seems like a pretty high frequency knock...

That doesn't sound like piston slap. Youtube it, you'll hear the difference. Sounds like it could be an exhaust leak to me. Does it vary with engine speed?

An exhaust leak? That clearly sounds like metal-to-metal contact, no?

Sorry, didn't have headphones on. Yes, it does sound like metal. I don't think it's piston slap though. Listen to this at the 1:45 mark for the noise that almost sounds like marbles in a can (https://www.youtube.com/watch?v=jpbqRCys1Co). Piston slap generally goes away over 1500 rpms and also gets quieter as the engine warms up.

I've got a three foot length of plastic tubing about an inch in diameter. It works really well localizing sounds like that. I can usually zero in to a few inches of where the problem is.

My WAG (wild arsed guess) is a connecting rod bolt (or bolts) came loose.

First zero in on the sound as near as you can. Make sure it is coming from ether the top or bottom of the motor and it isn't the distributor. Then take the pan off and double check the torque on your connecting rod bolts.

I've heard something similar that turned out to be the rotor hitting the poles inside the distributor cap. Yours sounds loader and more metallic, but that may just be the audio. Pop the distributor cap off and push sideways on the shaft and see how much play you have. Up and down play on the shaft doesn't really mean much in my experience. It is not unheard of to loose a couple of teeth on the distributor shaft drive gears and/or the cam distributor drive gears.

I've had the distributor noise before, and that's more of a chirp than a knock. It's much slower than that.

I also just replaced my rod bearings last week to fix a knock, and the knocking you'd hear from a rod doesn't sound like that. It's much also slower.

I guess it doesn't hurt to double check though. Seeing as how your noise is in such fast tempo, it could be that you have multiple cylinders affected. But it's also possible that you have poor audio in your video and diagnosing the sound is next to impossible.

What RPM is your Jeep running at in the video?

Turned out to be the distributor. Lots of play. Knocked pretty bad.

Thank you for posting the result to your issue. It will help someone in the future if they are searching for solution to similar issue.