Knocking problem

Ok, I', new to the list, so bear with me. Got a knocking sound with a '97 cherokee with a 96 engine. body has 150k, motor has 100k, and 1000 miles since the motor swap. It only knocks at idle. doesn't START knocking until its been running for about 5 minutes, Knock does speed up with engine, but comepletely dissapears after about 1300rpm. It is rhythmic with the engine but gets louder and quieter intermittently, sometimes stops for 2 or 3 "repeats" but then comes back. Put in Marvel Mystery oil, no change. Checked flexplate, no change. Ran motor without serpentine to see if it was any accessories, no change. No metal in oil. That rules out lifters, any pumps or pulleys, or the tranny. Hopefully the engine bearings. Tightened anything and everything that could hit or tap the engine while its running. I've listened all over the engine with a dowel, and can't pinpoint the sound to save my life, but when you just listen normally from the side of vehicle it seems to come from the back of the motor where the engine meets the tranny. I have good oil pressure (40-45 underway and 20 - 25 on idle) and it runs kind of cold (about 180 degrees when up to temp). What in the hell is going on? Piston Slap? crank or rod bearing? HELP!!!!!! Its driving me CRRAAAZZZYYYYY!!!!!!!!!

Piston slap is a good possibility.
You said you checked the tightness of the screws for the torque convert at the flex plate right?

Let us know what you find. Try Castrol 20-50 weight oil.

yeah, i checked the flex plate bolts under the access cover.. They were tight, and the plate had NO cracks.

i know the xj's call for 10w30 - why put in 20-50 - and is there any danger using a heavier oil like that in the winter??

I like where this thread is going...I was actually thinking of putting some lucas oil in to thicken er' up a bit. Keep in mind I live in michigan,l so it gets pretty cold here. It is october, so its gonna freeze pretty soon. Anyone in the northland run lucas or heavier than 10w/40 in the winter?

the 4 bolts for the flexplate on mine were tight but its possible that the other bolts are loose. I'm having the same issue

other bolts? Where are the other bolts? you mean inside the bell housing on the tranny side of the flex plate?

so no one knows anything more about this, Or is this topic just beaten to death?

mines a '91 and does the exact same thing. to be a bit more specific, try parking it facing down a decent slope as it seems to soften the knocking sound on mine. if it does, then we've got the same issue...but i have no idea what it is lol. ive lived with it for over a year with some high RPM mudding so I wouldn't worry about it.

Just for giggles, try putting your dowel on the body of the distributor (not the cap - though that should work too).

Facing a prblem similar in my xj as well..... knocks for first 1.5 seconds on startup.... and doesnt give me noise again til its been sitting at 210 for a while...

Ive got perfect compression yet i have blow by?! which most people tell me thats impossible but come see it yourself!! So i had to rerun the lines to a bottle to collect my blowly... I had previously ran 20w50 in it, although as i filter out the blowby oil im adding straight 50w... Im in florida, so the cold isnt that great of an issue...

I have NOT personally checked the flex plate, nor do i know where it is,...time to do some research!!

Be sure to keep us posted if you can diagnose your problem, b/c im sure it could help us all!

slykedaddy said:

I like where this thread is going...I was actually thinking of putting some lucas oil in to thicken er' up a bit. Keep in mind I live in michigan,l so it gets pretty cold here. It is october, so its gonna freeze pretty soon. Anyone in the northland run lucas or heavier than 10w/40 in the winter?

Click to expand...

I have had my 97 since new and it has 133K on it...piston slap is a known issue on some motors. The 4.0L is a noisy engine, but the flex plate problem is on accel and decel usually.

Castrol synthec now comes in 5w-50 which is a huge range.

Slyke...one more thing...can you hear it from inside while driving or stopped at a light?

Yeah, i can hear it stopped at a light inside the vehicle, but only if the radio, heater, etc... are turned off. Cant hear it while in motion. Why do you ask?