98 xj w/ engine knock coming from rear upper.

ok so i been searching the boards and various others and best i could come up with is a stuck lifter or flexplate.

let me explain what happened. I was driving normally and all was fine. i needed to get over to make a turn and had to floor it to get around a slow poke. when it downshifted and the rpms went up it instantly started knocking. it is a good healthy metal on metal knock. doesn't sound like the low end but its hard to tell. i checked and tightened the flexplate bolts. i started the xj while i was under it and you can clearly hear the knock is above. sounds like the back of the head. I took the VC off and all pushrods were tight and everything, although quite sludgy but typical 180,000mi, looked and felt fine. even had a mechanic friend come over while i had it off and he said he thought it was the flex plate. i don't know what to do next and it suck cuz this is my only car. i took the last couple days off work to try to fix it and nothing is working.

any insight or anything would be greatly appreciated

Pull the spark plugs. Crawl underneath, remove the inspection plate, use a pry bar on the teeth of the flexplate to rotate the engine. At multiple points using a bright spotlight and the pry bar move the flexplate back and forward, observing and listening for anything unusual.

will do. thanks for the info.

didn't see or hear anything abnormal. slight flexing of the plate itself but no noises. this xj is seriously getting on my nerves!!

I would pull the valve cover and inspect the rockers and push rods.

If it is just a collapsed lifter you could try draining out 1 quart of oil from the crankcase, replace with 1 quart ATF, then run the engine for 30 minutes--DO NOT REV OR DRIVE, just let it idle. Drain, replace filter, fill with oil (I like Shell Rotella, any good diesel engine oil will do--highly detergent) and try it out.