Help!!!! Light Knock!

[The_Kack]

So, driving today, I stopped and could hear a light knock. I touch the throttle and it goes away, back at idle and it's back. I tracked it down to the rear of the engine (I think), pulled the rear cylinder's plug wire and it went away a bit. I'm seriously hoping that it's not going to need a rebuild, but I'm not 100% of what it could be. Any help would be greatly appreciated.

[gogmorgo]

What engine and tranny? Is it definitely coming from the back of the engine, or could it be coming from the bellhousing area?

If so, check your flywheel/flexplate. I've got a cracked flexplate, and it's causing an odd, intermittent knock. You might also want to try tightening down the flexplate to torque converter bolts.

[The_Kack]

It's the 4.0 with a 4spd auto. (haven't quite learned the codes). My best guess is rear of the engine. Like I said, when I pulled the sixth cylinder plug it died down a bit like there was less stress. I've been panic researching and found that it could be rod bearings, piston slap, or lifters if it's in the engine. With it not being constant I'm leaning toward piston slap or lifters. But I don't get why it's at idle, but not at other rpm. 

[The_Kack]

Also, it's an '89 with 183,500 miles. Was told it was rebuilt about five years ago and has only had 30k put on it since then.

[gogmorgo]

Yeah, I've got a 4.0/Auto as well. All autos on the Comanche are AW4's, with the exception of the 86 model year which got some Chrysler trannies, I believe. Depending on how they crack, flexplates can make a different variety of noises. Just search youtube for "jeep cracked flexplate" to hear a few.

The reason I jump to this is because you say it goes away with revs. Mine does strange things. Sometimes i hear it on startup while it's warming up. Usually if I'm driving in the city and doing lots of accel/deccel, I might hear it when I stop at a light, or I might not. Sometimes when I get home after about 10 miles of highway cruise at constant speed (roughly 1800 rpm) I hear it when I stop in my driveway, sometimes I don't.

But when it is making noise, it almost always goes away if I rev to above 2000 rpm, and may or may not come back when I let off the throttle. When I was trying to figure out what was making the sound, I saw a buddy in my parking lot, and rolled over a curb to get to him before he took off, and the knock started when I pushed against the curb, obviously increasing stress on the flexplatekL Then after idling for about ten minutes, it just quit making the sound. Revs didn't change, engine didn't get rougher or smoother, the sound just disappeared. But that said, I've also launched pretty hard (after a 1500 rpm brake-torque) followed by WOT, the autobox shifting almost at redline, then slamming on the brakes and dropping it down to second because OMG I FORGOT THAT LIGHT WAS RED AND THAT'S A FUEL TRUCK PULLING INTO THE INTERSECTION. And that didn't trigger it.

The reason I know in my case it's my flexplate is because one, that's where the sound's coming from, and two, I pulled off the inspection plate and saw the crack. Also, I started it with the plate off, and it was pretty obvious then where the sound was coming from.

 

This isn't to say that you definitely have a cracked flexplate. You won't know unless you look. While you've got the cover off, you should also check that the flexplate-torque converter bolts are tight, by putting a socket on them and cranking them down until you start spinning the engine.

 

I hope for your sake, it is just a cracked flex plate. There may be a lot of labour involved, but it's still better than having to disassemble and rebuild an engine.

[Minuit]

The first thing you should always check in the case of a knock is the flex plate. Make sure the bolts on it are tight also. I had 2 mechanics say mine was a rod about to come loose. It wasn't.

 

[cruiser54]

Here's 2 links for ya.

 

http://www.cherokeeforum.com/f51/how-fix-flexplate-knocking-125547/

 

http://www.cherokeeforum.com/f2/found-way-see-crack-my-flexplate-before-removal-171945/

[The_Kack]

Thanks guys. I'm going to climb under it later on today and hope that it's the TQ Converter or the Flex Plate. Another question I had was, I recently moved my exhaust around a bit trying to keep the driveshaft from smacking into the downpipe. I didn't hear any cracks or pops when I did this, but that doesn't mean much in cars, I know. Could the heating cycle have cause it to crack up by six and that's why the knock gets better with no spark? (i.e. no spark means no ignition which means reduced pressure coming out of the crack which means a lighter or less noticeable knock).

 

Also, if it isn't the TQ or FP, what would the next guess be from you guys? I really do appreciate all of the help. I want to keep the truck and need all the help I can get.

[Minuit]

If the flexplate isn't it my next guess would be a slapping piston. Not an engine killer but it does make a noise somewhat similar to a rod bearing. 'rod knock' should be the very last thing you consider.

[gogmorgo]

When looking at the flexplate, they seem to like cracking around the bolts holding it onto the crankshaft. I almost didn't see my crack at first, I just barely caught a glint off one of the edges right near the center and realized there was no way that shape should be there.

[The_Kack]

I couldn't see anything, but I'm getting a second opinion from my brother tomorrow to be sure. If it isn't the the FP or TqC, should I drive it until/if it worsens or just leave it down until rebuild?

[cruiser54]

Did you check the bolts for tightness or just gander up there?

[gogmorgo]

And when you check that they're tight, do so by putting a socket on them and cranking them down until the engine starts turning.

[carnuck]

Edit: (fixed)

Piston slap gets louder when you pull the plug wire off. Rod bearing knock goes almost totally away when you pull the wire off.

[cruiser54]

Rod bearings get louder when you pull the plug wire off. Piston slap goes away when you pull the wire off.

You're 100% sure of that? For about 40 years I've been pulling plug wires to check for rod knock. Combustion gone and knock is gone. Sounds like shooting from the hip and inaccurate bull$#!& to me......

[carnuck]

Could be backwards. I haven't bumped into it much in the past 20 years since I "retired" as a tech (car fell on me during the Duvall, WA quake of Jan '95) I started out pouring babbet bearings for Ford Model flatheads.

[HOrnbrod]

Could be backwards.

 

That's all that matters, the other stuff does not. Don't post a blanket statement unless you are absolutely certain that it is fact. Could cause other CC members wasted time and $$. Not good.

[carnuck]

What can I say? Once you get past 55, things get tougher to remember the right way around!

[cruiser54]

How about say nothing if you're not sure?

[carnuck]

I AM sure. Just the order was mixed up. :doh:

[cruiser54]

And that mixes up unknowing original posters.

[The_Kack]

Ok, so my brother couldn't see any issues with the flexplate either, all bolts tight. Should we assume that beings it fades when spark is pulled it's the rod bearings and due for a rebuild?

[cruiser54]

Rod bearings get bad fast. If it's staying consistent, and has been, it's probably some piston slap.

 

How's your oil pressure?

 

And, what brand of oil filter are you using now?

[The_Kack]

I'm using a Wix filter and I don't have a gauge to hook up to check oil pressure. I don't know if other Comanche's came with a pressure gauge, but mine's a base and just has basic lights, not even a temp gauge.

[cruiser54]

Yeah, they did.

 

What about the noise? Louder cold? Getting worse over time? Does it change when you accelerate?