AstonishingXJ

Hello everyone, this is my first post on here although I've read a lot and learned a whole lot more!

I have a problem... My XJ has developed a knocking noise and I cannot tell if it's the infamous flex plate or if it's an engine knock. This is a link to a video I posted on YouTube of the sound: any help would be awesome! I just bought the jeep about 3 weeks ago and It has 223,000 miles on it and it's a 1996 4.0

Also the CEL is on and I took it in to autozone and it said O2 sensor and 4th cyl misfire. so I went ahead and replaced both upstream and downstream sensors and I reset the light and it came back on. What causes one cylinder to misfire?

Thank y'all in advance!

freegdr

Try tighting the flywheel bolts yet ?

AstonishingXJ

I haven't had a chance to get under there yet, going to tonight, or tomorrow night.

freegdr

Deff worth a shot then go from there best bet and labors free.

cruiser54

Here's a good read:

Straightshooterjeeps cracked flexplate diagnosis

I removed 3 of the 4 bolts.

Then loosened the last bolt about half way.... I left this one in to maintain connection between the tq and the plate. Just so I wouln't have to fiddle with putting bolts back in too much.

Now the flexplate is pretty much seperate from the torque converter.

Got my flashlight pointed up in there

Grabbed a big flathead screwdriver and started prying on the flexplate

Don't see anything yet

Turn the flexplate a half rotation

Pry again

and there it is! A big fat crack up near the center of the plate.

When the flexplate is loose from the torque converter you can get it to "flex" real good and you just might be able to spot your crack.

Just to be sure you do have one before dissasembly.

It was very obvious at this point, but before pulling those bolts I looked and looked and could never see a crack.

Hope this helps save someone a little time.

AstonishingXJ

Thanks guys, tonight after class I'll check out the flex plate and tell y'all what I find!

AstonishingXJ

I pulled that cover to the flex plate and all the bolts were tight so then I took out 3 of the bolts and got it to "flex" never saw a crack in it. So I put it back together and torqued it all down and it's still making that noise.

What should I try next?

cruiser54

See if your trans to engine bolts are loose.

red4dr

your in the same boat im in. I even pulled the head and replaced the lifters/cam. fired it up, noise still there. facepalm! I spent about an hour under the jeep with a mechanics stethoscope and seem to have zeroed the noise to the front bearing or cylinder so I will be dropping the pan soon to see whats going on in there.
If you do what I did and play with the stethoscope, just remember that the fan never stops spinning....... got so into listening to the noise that I put the rod right into the fan! that one hurt my ears and almost dirtied my shorts.......

AstonishingXJ

I just replaced my oil pan and gasket cause it had a small leak, when I was under there everything looked good. Didn't have any metal shavings or anything down there. But then again I don't really know what to look for.

AstonishingXJ

I just checked them and they are all good!

cruiser54

I hate it when that happens....

DFlintstone

Well here's my knock. Can't be a rod since it waz doing it more then 100K before this vid. I'm wondering about crank end-play, and checking that at the crank pulley. Waz doing this at 145K, 279K now.

AstonishingXJ

Sounds the exact same! I'm thinking of doing a compression test... I'm just stumped on how to diagnose it.

red4dr

This is one of those things with the 4.0 that pisses me off lol. I love the motor, don't mind the noise, but a lot of the noises sound like the "don't worry about it" noise, but the way my luck usually goes i will not worry about it and then proceed to put a piston through the block...... Did you do the whole, pull a plug wire or fuel injector plug off to see if the noise goes away in one of the disconnected cylinders? You can also get a timing light to see if your noise is per a single rotation, one knock per light, or in the head with 2 knocks per a light. I tried the spark plug wires thing, and besides shocking myself good a couple of times, didn't find anything out. The timing light thing helped me reassure that its in the bottom end. too bad i didnt know about that trick before i did the cam and lifters lol