Another "knocking sound" thread...

98XJ 4.0 AW4 74kmiles

Did some searching and couldn't find anyone with the exact same symptoms but, the last couple days I've been getting a knocking sound from underneath. It doesn't matter if the jeep is cold or warm but it does seem to only do it when the jeep is in gear. The inspection plate on the transmission was already removed so I checked the bolts to the Torque Converter and all were tight. While inspecting the flex-plate I did notice, closer to where it bolts to the crank, there was what looks like small cracks, although I couldn't say 100%. When in gear the knock gets faster with rpms, but you can rev motor while in park and only get the knock sometimes, but not like while in gear. What my question is, would the flex-plate just crack all of a sudden and then cause the knocking? My oil pressure, with a warm engine, is around 18psi with 10w-40 oil. So I really wasn't thinking rod knock, but I am definetly open to opinions. There isn't any smoke and the jeep continues to run perfect other than the horrible knock..

Sure sounds like a cracked flex plate. Yes they can suddenly crack.

Ok, i just didn't know if they cracked suddenly or if it was something that happened over a period of time... Most of all other cracked flex plate symptoms I have read, the noise seems to remains regardless if the trans be in park or in gear. That's what had me thinking possibly something different..

It will get worse as the crack spreads.

Any particular brand to buy? Will the Pioneer brand from Autozone work?

Check rockauto.com online

Can you make a video w/ audio of the engine compartment from the front, back, both sides of the engine, and from underneath? It'd definitely help with diagnosis.

It does sound like a flexplate crack however.

I could possibly make a video.. Like I say by visually inspecting the flex-plate it does seem to have a couple 1"-2" Long cracks.. By taking a small screw driver, and running across what I think is the cracks, the screwdriver stops when hitting the possible cracks.. If that makes sense lol

1-2" long cracks? Yeah, that needs replacing for sure even if you have another issue as well.

I'd order a new flexplate, pull the trans off, leave the flexplate on, bolt the bellhousing back onto the trans, bolt the CPS and starter back on, fire it up and see if the noise still persists. If it does, you have something going on in the engine too, if it doesn't, it's just the flexplate and/or trans stuff... that kind of noise I'd never expect out of a trans that still moves the vehicle so at that point either swap/repair the engine and flexplate or just the flexplate depending on what you find.

When you remove/reinstall the trans, make sure to remove the torque converter bolts so the torque converter stays in the trans during removal, and make sure it's fully seated in the trans before reinstallation, THEN bolt it back to the flexplate. A very common mistake is to leave the converter bolted to the flexplate during removal or bolt it to the flexplate before reinstalling the trans, both of these make it needlessly difficult to remove/reinstall and can cause transmission/torque converter damage that will require removing it again.

Your saying when I remove the trans, make sure to un-bolt the 4 bolts to the torque converter first before removing the transmission? Then when re-installing, bolt the 4 bolts back to the torque converter after the transmission has been reinstalled back into the vehicle?

Correct. Make sure the converter is fully seated before installing the trans, too... the snout that goes into the crank should be below the plane of the bellhousing face by a little bit. Its easiest to get it seated if you stand the trans on end in a clean bucket (since fluid will come out.)

Sharpshooter23 said:

Your saying when I remove the trans, make sure to un-bolt the 4 bolts to the torque converter first before removing the transmission? Then when re-installing, bolt the 4 bolts back to the torque converter after the transmission has been reinstalled back into the vehicle?

Click to expand...

If you bolt the torque converter to the flexplate (adapter plate), then try and "stuff the transmission into the torque converter, there is a 99.9999 percent chance you will damage or destroy one or more of the following: the torque converter, the front pump in the transmission, the transmission itself.

As all of those items are costly, and your chance of success is 0.0001 percent, you should definitely listen to the good advice you have been given. :yelclap: