hey XJ enthusiasts - I now also have the infamous engine knock: 4.0-1998-190k. Located at the oil pan, specifically when engine is hot. Great oil pressure, 50/60 psi cold, 30 psi when hot. How to diagnose piston slap vs. rod bearing culprit ? Thanks !

 

The bearing woulda let go by now.

Auto trans? If so, it could be flexplate bolts or cracked flexplate. Simple to check.

 

Get it running and knocking then pull a plug wire one at a time and see if the knock changes that way you can find what cylinder is making the noise .

 

That's not very high tech....

 

No it is not high tech but it works .

 

Absolutely!! Lots of good old tricks like that.

 

These engines do not have bearing failures/lubrication issues, unless they ingest a ton of water or some other physical contaminants.

Just like any internal engine issue, you start diagnosis with a compression test. Extremely low compression in one cylinder would point to poor ring tension or a cracked piston.

After the compression test, if you need to go further, begin a visual inspection. Valve cover removal to make sure the valve train is tight and nothing is broken or bent. Bellhousing inspection cover removal to check for a cracked flexplate or loose converter bolts.

 

my last 4.0 had pretty bad piston slap but that seemed to be worse cold and got better as it warmed up.

x2 for removing the bellhousing cover & inspecting the flex plate, it's pretty quick to check and sounds like the same general area where i'm currently having some knocking & a confirmed cracked flexplate.

 

And 99 times out of 100 you luck out and it's loose bolts.

 

Thanks - already checked - no loose bolts and no cracks

 

wil do- thanks

 

Thanks - will do a compression test

 

don't forget post #3.