amorgan93

so, my weekend warrior 92 XJ 4.0 AW4 that i recently got has a knock, has had it since i got it. Im becoming curious as to what the knock is, so in an attempt to further my knowledge (not so worried about the jeep) ide like to attempt to figure out what the knock is. 178k on the odometer.

here we go:

1) knock is not present when first cranking
2) knock starts very faint then gets louder and louder as the engine warms up
3) knock isnt a very pronounced "knock" like a rod bearing would be, it seems to be a continuous, very low low pitched grinding/groaning/shuddering sound, with a little "knock" thrown somewhere in the mix, but is still noisy in between the audible "knocks"
4) loudest from drivers side, seems to come from the middle-rear of the engine when laying underneath it
5) can be heard up to 2200rpm, where i suppose the engine noise drowns it out
6) after driving the jeep very hard, keeping my foot in the accelerator, when i come to a stop and let it idle the knock is much much quieter, but slowly gets louder as it idles and then reaches a certain noise level, where it stays for the most part.

torque converter bolts/flexplate?
...well it doesnt change pitch at all when in drive/under load
im stumped but i believe the ol' 4.0 is just worn out, considering all the blowby oil in the air filter, blowing exhaust out of oil cap hole when oil cap removed, burning however much oil is between "full" and "add" on the dipstick, every 50 miles or so. as well as blue smoke out the tailpipe sometimes, also white smoke out the tailpipes sometimes like the headgasket is half blown or something

anyway, any advice is appreciated!
thanks!

rideordie66

with proper maintenance the 4.0 is not broken in till 150k. If it was treated right before you got it then its probly an exhaust leak. Manifold almost always cracks.

cruiser54

I would check the flexplate to torque converter bolts first.

Also take a peek at your harmonic balancer on the front of the crankshaft. Has it come apart?

8my$

Got the same problem. Only 61,000 on a 91. Just bought the rig. I hope it's an exhaust leak, but not sure how to check it??? 5 speed, no flexplate.

94cherokeeman

Same problem on my 94 ho and 230k

bythehour1

Def they tend to waller out the flywheel

94cherokeeman

Mine are good

92xjcher

Same noise, torque converter bolts tight. Doesn't make
noise when cold. ??

beefmobile

i know you said it sounds like it's coming from the middle/rear of the motor.....but it could possibly be your timing chain. I had that problem in my YJ with the 4.2; it wasn't really there when i started up, but halfway warm to operating temperature, it was like short scraping/knock noise. Replaced it and now it's gone. It started wearing a groove in my timing cover it got so bad......

just an idea.....

amorgan93

didnt think about that but it just seems a little lower pitched than a timing chain would be. And something i dont understand is why does it go away immediately after i drive the **** out of it, and then slowly come back as it idles?

DFlintstone

Maybe check the TC bolts since it's a 10 minute job. Verify oil pressure with a mechanical gauge. Then a fresh Wix or Napa filter and some Rotilla 15-40? See if that helps.

morganbearden

I have a 2000 Cherokee XJ, 144,000 miles in very good condition, uncut and unmodified. For the past year there has been a knocking noise that seems to be coming from deep in the lower portion of the engine. The noise isn't so noticeable when the engine is idling, but becomes very apparent when I give the engine the gas. Also, it is more apparent when the engine is cold. I’ve been told that it is probably a bearing – either a crankshaft or cam. Any ideas?

cruiser54

Rod or main bearings with enough clearance to cause a knock would not last a year. You possibly have a broken piston skirt?

morganbearden

THANKS for your thoughts. I've had many folks speculate what this might be, both over the internet and in person - but you are the first person to actually offer a REASON why the knock may or may not be caused by a particular malfunction. Thanks!

The difficult situation that I find myself in is that no one seems to be able to give me a confident diagnosis. I've taken the Jeep to the local Jeep dealer, the Ford dealer (service manager worked at Jeep for years), and to the Aamco franchise. No one can say definitely what the problem is and I would rather just pay for a rebuilt engine than to have someone go on a fishing expedition trying to diagnose the problem: main bearing, cam bearing, valve problem, piston slap, etc.

Thanks again!

stockli

This.