knock/rattle and rpm issue
 

When I accelerate i hear a knock/rattle noise that sounds like it is coming from just underneath me but a little towards the front. It only does it when I accelerate and at first. I dont notice it once Ive gotten to maybe second gear. Anyone experience anything like this?
also when Im driving in the afternoon it seems like my idling is a little off. When I come to a complete stop it seems perfectly fine, but when i let off the gas it before I stop completly the rpms jump back and forth between 1000 and 1250. But when Im stop it sits at about 750 plus or minus a hair. Might this be a fuel pump issue? Im at 145482 and I heard that around 150000 you might have to replace it. Any help or experiences would be awesome. Im trying to drive this XJ forever and will do what it takes to keep her healthy.

I have a 99 4.0l automatic 4x4

Yea it could be your flexplate is either cracker or the bolts are loose,just need to remove the dust cover and check it for crack around the center and check the bolts to make sure they are tight.

could i google the flexplate and get instructions on how to do that? I dont even know where the flexplate is located. Im trying to teach myself how to be a mechanic and this forum is helpful but Im still learning.

Also I see you are a supporter of CF. How did you do that? Is there a link on here?

Supporting member link:

https://www.cherokeeforum.com/f9/cf-...mbership-7877/

yea you could google it,the flexplate is located between the motor and transmission,on manual transmission it is called a flywheel,flywheels do not have the same issues though because of there thickness.
The flexplate is what connects everythinf together and handles alot of torgue and abuse. It is not uncommon for them to have this problem. The idle/rpm problem can be from the flexplate moving around enough to cause the cps(crankshaft position sensor to get a bad signal.

so both problems could be caused by the same thing? would that be the easist thing to check for the rattle?

The best way to check is to remove the dust cover underneathe and look for any cracks around the center and check for loose bolts that hold it to the crankshaft. The sound is more like a knock than a ratle and it should be somewhat consistent with rpm.

mine goes down (or goes away) as my rpms (and speed) rise. It might be the fact that when I rev up the engine it gets louder than the knock. I can hear anything when I get to overdrive. But it does only do it when i am accelerating, cant hear it at all when I idle.

if it is cracked is it a hard fix?

That is where I would look first,if it is the problem then the transmission needs to be removed to repair it.
It is typical for them to make noise when exccelerating if they are cracked.

If the noise goes down as the rpms go up under acceleration then it is most likely a broken flexplate. When not under a load the flexplate can have enough movement for the broken parts to contact each other and cause the knock. When under load the pieces wedge together stopping or reducing the amount of noise heard. In order to replace a broken flexplate the transmission must be removed. This can be done in your driveway with general mechanical knowledge and tools. I strongly, STRONGLY recommend getting a proper transmission jack to do the job if you attempt it yourself.

This is what a broken flexplate looks like.

Attachment 12934

Tranny removed? #$%^
I dont know if Im gonna know how to do that hopefully I can find someone on the regional site that can help me out. I will try to check it this weekend and get it figured out. Thanks. I will repost and try to post pics of what mine looks like.

that whole pic is the flexplate?

Yes that is the center section that bolts to the crank.

If I get under there can the bolts be tightened without removing the tranny? assuming this is the problem