Hey, guys! I'm back in need of Tech help....My brother's '89 Cherokee 4.0 (details below) has developed a strange knocking sound recently within the last few weeks. I believe it may be piston slap, but I'm not sure. Tried unplugging injectors to see if it goes away, but it didn't. No change in volume or anything either. Also checked TC bolts and they are tight.

I took a short video so you could hear it. It's loudest underneath at the oil pan as you can hear toward the end of the video. We'd really like some help figuring out what this is. Driving us nuts....It seems to fade away with throttle, but that may just be from the exhaust noise.

Oh, did a full tune up a little before it started doing this. Used Autolite Copper plugs, gapped at .035 and a Valucraft Cap and Rotor. Air filter was done at last oil change about five weeks ago.

EDIT: We just tried an idea we found on JeepForum; Take the engine to 3500, shut off the key and turn it back on before it dies out. Supposed to make some kind of clanking sound, but it didn't.

 

Check torque converter bolts first.

 

Oops, forgot to include they were checked and are tight. I'll add that to the original post now

 

Did you check for cracks in the flex plate? Or rounding of the bolt holes? I believe theres a writeup on here that explains a good way to check for cracks. When my flex plate was loose it sounded similar to that but louder. Get under there and check for anything loose that could be clanking and get a long handled screwdriver and hold the handle up to your ear while poking the end around wherever you think its coming from, cheap mechanics stethoscope. Also what happens when you increase the idle? The sound get faster or stop? Once I got the RPMs up a little the sound from my flexplate would go away

 

I'm starting to believe it may be a cracked flexplate as well. The sound either goes away with throttle or is just drowned out, but either way, you can't hear it over about 1500 RPM. Used a mechanic stethascope and it seems loudest at the back of the oil pan. You wouldn't happen to have the link for that writeup would you?

 

https://www.cherokeeforum.com/f51/ch...xplate-172081/
Yeah sounds like it might be a cracked flex plate. Does it run okay? No loss of power or anything? If it runs fine then my biggest suspect would be the flexplate because I'd think if something in the bottom end was making that kind of racket that you'd have adverse engine problems.

 

It does run good, just a bit of a rough idle and reduced gas mileage. Think flexplate problems would contribute to that? Then again, it is an '89 with 325,000 on the clock....Not really all the surprising.

 

Did the rough idle start at the same time as the noise, or has it been there for a while?

 

It's been there a while

 

Alright, so I think we finally figured it out. Got a nice day today, so we were out looking at the flexplate, turning the engine and lightly prying on it. Boom, two big azz cracks on either side and almost all the way around the center flange. So, that's a definite replacement soon....New questions time!

Where do you guys recommend to get a flexplate? Remember, workin' on the '89 down below here....We found them at Autozone and on RockAuto for about $60. Are there any other parts we should check/replace while the transmission is out? And would a cracked FP cause low fuel mileage and a rough idle?

 

Check with O'Reilly's. I believe they have better parts than Autozone. You don't want to have to do this again.

Cracked flexplate causing vibration? Maybe. Bad fuel mileage? I don't see why it should. Probably some other cause.

Vibration + bad mileage = misfire?

 

Not really much of a vibration, that I know of. There is a vibe at around 40-45 that seems like a rythmic vibration. I don't know if it'd be connected, but might be. I'll definitely check out O'Reilly's since I have had problems before with AZ

 

If you can spend the money, buy a flex plate from the dealership, that way you know its good quality and properly weighted and balanced. I've heard of people having issues with those kinda things when using aftermarket flex plates. And I'd think misfire too, I have bad gas mileage and a little vibration which is from one of my injectors not responding right, this also has caused a rough idle

 

The problem I've seen is the flexplate is assembled wrong and makes the engine out of time. The holes that trigger the CPS are off.

 

I actually hadn't heard of that before, is it pretty common with cheap flexplates?