engine diag help
 

Ok so i have a 2000 XJ, 147,000 miles on it. When i start up the Jeep there is a rattle in the lower end. The Jeep basically sounds like a diesel for about ten minutes till it warms up. After it warms up the motor sounds normal. I doesnt seem too have any power loss. The oil gauge reads 50 on start up then when it warms up it goes between 30-40.

If i stop at a stop light the oil will go down to about 20, I've been told the oil fluctuation is normal for XJs.

Anyways does any one have any idea as to what that rattle could be? You guys are the experts and i trust you more than a local shop. Thanks in advance.

First, check to make sure your torque converter bolts are tight. (There's a cover plate on the lower bellhousing to do this) If its not that then knocking is NOT good. Remember in engines there are two diagnostic noises. 1) Valves n valvetrains "tick" and 2) Rods n pistons "knock".

Use not the torque converter bolts,i already checked. It doesn't sound like a lifter tick and the nose is coming from the bottom not the top.

At one point in time i put 15w-40 in it and it wasn't as loud. The last oil change i went back to 10w-30 Valvoline next gen.

Don't use the Next Gen, its recycled oil. Use regular oil. I use 20w-50 Max Life in mine. But also you li e in a colder climate so thick oil will cause noise till it warms up and flows to the parts. Thinner oils like 0w or 5w are more for colder climates.

Anyone else have anything they can add to my dilemma?

Absolutely nothing wrong with NextGen. Do you know what crude oil looks like? It takes alot less refining to start with used motor oil than to start with crude, and the finished product is identical.

Piston slap has been known to have the characteristics you describe. If you pull spark plug wires off the distributor cap one at a time and listen for the 'knock' to stop, or diminish in intensity, that will be the cylinder that has the problem. If the knock never goes away when pulling plug wires then it could be a cracked flex plate. Flex plates do crack under the crankshaft flange where the plate bolts up and you can't see the crack(s) until the plate is removed.

So if i pull all the plugs and the sound is still there then it could be the flex plate? I've already tried tightening the flex plate bolts and after doing so i got a new sound. I then loosened them back up and that new sound went away.

If it is piston slap, what's the cure?

What brand of oil filter did you put on when you did your oil change. I know when I first bought my current XJ it had a fram filter and it had a slight knock when cold. Changed it out with a mopar brand and the knock went away. Fram are garbage on the 4.0. You should use wix, mopar. These are the only ones I use on my Jeeps and never had a problem with them.

Right now i think its a Bosch, before i bought it it was a fram tho.

If you pull all the plug wires (one at a time) and the 'sound' stays it's an indication the problem may not be in the reciprocating parts of the engine, i.e., pistions, pins, rods.

What new sound?

It sounds like you've entered the twilight-flexplate-zone. I've never heard of what you are saying but it sounds like you've discovered a new planet. I know it's nearly impossible to torque those bolts to spec with the oil pan in place; most anyone could hope for is getting them as snug as possible (with Loctite Blue thread locker on the threads). I hope you got the bolts evenly torqued (tightened). The torque spec on those bolts is 28 ft./lbs. This is a WAG on my part, but it could be that the plate is cracked around the crankshaft flange and when you tightened/loosened the bolts it affected the geomometry of the plate at the crankshaft attach point.

Q: Did you see any radial cracks on the flexplate when you were tightening the four flexplate bolts? Radial meaning, cracks radiating out from the crankshaft like spokes of a bicycle wheel. Circumferential cracks will be around the crank flange.

The cure for piston slap is to replace the piston. Many XJ operators don't let it bother them and just keep driving it. It's called "Run-To-Destruction". Eventually the piston skirt will break off and end up in the oil pan. Damage to the cylinder surface will result from the broken piston.