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rod knock problems

xtremewlr

NAXJA Forum User
Location
San Diego
Seems the XJ I just bought has rod knock that I didnt notice on my initial test drive before buying it. :flamemad:

I have done some searching here on the forums and as suggested often, I checked the flexplate bolts. I pulled them out one at a time, cleaned and applied locktite to them and then reinstalled making sure they are nice and tight. Didnt fix the knock.

My next step is to replace the rod bearings. I will do all 6 of them just to be safe but I am wondering if I should even bother. What has been the success rate of doing this for others with rod knock problems? Am I just wasting my time with replacing the rod bearings? Should I just drive it till it blows up on me and then replace the engine? Just want some opinions.

The Jeep is an 89 with 190,900something in it. AW4. Plenty of power still, no leaks (nothing serious at least). I have not run a compression test on it yet. No knock at idle, it only becomes audible from about 1500rpm and up, progressivly getting louder. Any suggestions on what else I might try would be appreciated.

Forgot to add that the oil pressure does seem to be good although I am not sure what is normal pressure for the 4.0L going by the factory pressure guage. When cold it is around the 40 psi mark and then drops to about 35 psi at idle and warmed up. Normal driving takes it back up to around 40 to 45 psi.
 
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hope you dont have a problem, but if the knock is present when you increase the rpms, then you might. on my 89 i get a knock once in a while but goes away when i increase the rpms.

i would start with the basics first. oil change, cap, rotor, plug and wires. also clean the TB.

89 also has a knock sensor, maybe its bad.
 
89xj said:
hope you dont have a problem, but if the knock is present when you increase the rpms, then you might. on my 89 i get a knock once in a while but goes away when i increase the rpms.

i would start with the basics first. oil change, cap, rotor, plug and wires. also clean the TB.

89 also has a knock sensor, maybe its bad.

Did the basics already (oil change etc). The knock sensor is worth a shot tho. Hadnt thought of that. Although this doesnt really sound like detonation. Have to locate the kock sensor now and test it out.

I was also thinking that it could be the valve train but the knock is too deep of a sound for a loose rocker arm. I had to replace the PCV and CCV on the valve cover and checked the rockers for tightness and obvious wear while the cover was off. All looked good on the top end. No major sludge or anything up there.
 
I missed the part about no knock at idle. If the rod bearings were bad you would have a knock at idle. If the knock is more severe under load and goes away on level ground/light load, I would second the vote for knock sensor.
 
Well it doesnt go away under light load/level ground. Ive done the stethoscope test (long screw driver held to ear) at idle and did not hear any knock from the botom end. Deffinatly noticable off idle, progessivly getting louder as rpms increase.

Now where is the knock sensor located? I checked Haynes and Chiltons and all they say is that it is on the block somewhere.
 
xtremewlr, how loud is your knock? This sounds very similar to my problem (I replaced the rod bearings). The only difference is that my 4.0 (88 w/175K) makes a HUGE clatter upon start up, only to go away after a few seconds when the oil pressure builds up. The knock does not appear again until I hit 1500 rpm and, then increases with engine speed. I have not suspected detonation as my spark plugs looked good after 13K miles, when the knock appeared. However, my spark plugs were Autolites #3923 (vs. 3924) with a very wide gap (since replaced with stock Champions w/ no change). Could this be a bad knock sensor? I was getting ready to order a remanufactured engine.

Now for the novice wrench question: What is the flexplate?
 
desertred said:
xtremewlr, how loud is your knock? This sounds very similar to my problem (I replaced the rod bearings). The only difference is that my 4.0 (88 w/175K) makes a HUGE clatter upon start up, only to go away after a few seconds when the oil pressure builds up. The knock does not appear again until I hit 1500 rpm and, then increases with engine speed. I have not suspected detonation as my spark plugs looked good after 13K miles, when the knock appeared. However, my spark plugs were Autolites #3923 (vs. 3924) with a very wide gap (since replaced with stock Champions w/ no change). Could this be a bad knock sensor? I was getting ready to order a remanufactured engine.

Now for the novice wrench question: What is the flexplate?

What you just described is pretty much identicle to what I am experiencing. I havent been able to test out the knock sensor theory as yet since I am in the middle of working out a wiring issue (no taillights, brake lights work fine). I priced the knock sensor at Napa for $60. Might be worth a shot since you have tried everything else. Its located on the lower drivers side of the block, right behind the motor mount.

The flex plate is between the crank and the torque converter, ataching the engine to the tranny. I read posts saying that the bolts can come loose and easily be mistaken for rod knock.
 
The flex plate can also crack around the reinforcing ring where it bolts to the end of the crankshaft. This also causes a knock that sounds a lot like a rod. However, in that case you hear the knock at idle in neutral, but when it's in gear and has resistance the knock goes away.
 
What about water pump bearings, or timing chain? It doesn't sound like a rod knock to me, especially with such good oil pressure, but you should be able to eliminate that possibility in a jiffy just by pulling one plug wire at a time as it runs (big rubber-insulated linemen's gloves or plug-wire pliers highly recommended!). You can pull the wires at the distributor end if easier to reach there. If the noise goes away when you pull one plug, you've got it.
 
What are the chances that a shot harmonic balancer rubbing the timing cover are the cause? I just pulled the balancer off to replace it and its been chewing away at the cover pretty good. I took some good pics of it too which I will upload later. Looks like I got this thing just in time. Im replacing the belt and idler pulley while Im at it in an attempt to eliminate the gawd awful squeel Ive been getting.
 
Okie Terry said:
Any votes for piston slap? That's what I chalked mine up to. Been knocking for 50,000+ miles.

Possible but not likely. Piston slap was one of Chrysler's "improvments." It wasn't a problem on the '89s.
 
Well this sucks. While installing the new balancer, the washer that comes with the installer decided that it likes the balancer so much that it doesnt want to come off. Im going to have to back the balancer off some, hopefully persuading the washer to let go. The washer is also holding the threaded insert for the tool hostage in the crank snout. :(
 
Finally done installing the new balancer. Had to "modify" some tools to do it :D It did not, however, seem to make a difference with the knocking that I am getting. Still smooth and quiet as can be at idle. All I hear are the injectors and just barely some of the rockers. Knocks like crazy at start up tho. I was revving it to about 3k rpm without getting knock but if I bring the rpms up slowly to that level, the knock starts up. Need to try the knock sensor still, see if that changes anything.
 
Ok, just back from a good drive. Still knocking but it doesnt seem as bad as it was before. I am starting to suspect that it isnt rod knock now. I can only hope. She deffinatly drives alot better now that the balancer has been changed :) The begining of knock seems to have moved up about 1k rpm to 2500 rpm when it starts. So, looks like I will do the knock sensor next and see what happens from there.
 
Is it really "knock" that you're hearing, or is it "ping"?

Rod knock is usually rather loud, and it sounds like there's a little guy inside the engine beating on the side of the block with a large hammer.

"Ping" (pre-ignition, which is what the knock sensor controls) is a much lighter, "tinnier" sound, more like a rattle than a BANG-BANG-BANG rod knock. It's hard to describe, but (naturally) everyone who has ever heard both sounds knows exactly what I'm trying to say. :huh:
 
Eagle said:
Is it really "knock" that you're hearing, or is it "ping"?

Rod knock is usually rather loud, and it sounds like there's a little guy inside the engine beating on the side of the block with a large hammer.

"Ping" (pre-ignition, which is what the knock sensor controls) is a much lighter, "tinnier" sound, more like a rattle than a BANG-BANG-BANG rod knock. It's hard to describe, but (naturally) everyone who has ever heard both sounds knows exactly what I'm trying to say. :huh:

Not ping, this is a knock. I know my description sounds like ping tho, sorry.

I just need to try some more things to know what its doing for sure.
 
Like mentioned, pulling one plug cable at a time, is how I always tested for rod knock. If it changes and/or goes away, it´s a good indicator of rod knock.
Had a loose timing chain that would slap the inside of the cover, made a pretty good racket.
Plasti-gauge, check of the rod and main bearings, would probably be a last resort. But would tell you, approximately how much wear there is in the bottom end. Pretty good time to clean out the bottom of the pan, the oil pump pick-up and replace the main bearing seal.
Have a little piston slap on mine at idle. Goes away off idle.

A long shot, but a too hot plug and regular gas, can cause a pretty good, ignition knock, on some motors.
Might want to put a timing light on the motor, just to check how the advance is operating. No timing to check, but you can see the computer advance the timing with the RPM´s. A few different sensors, will mess with advance.
 
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