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New XJ..Rod knock?

9

91Red4.0

NAXJA Forum User
Location
Malden, MA
91 XJ 4.0 AW4 auto/231 t-case

Bought a new XJ today.. Drove home and getting near my house I noticed it had started to knock. Has about 20psi hot oil pressure, 45+cold.

Sounds like rod knock to me, except it's not loudest at the oil pan. It's loudest around where the engine/transmission meet. Possibly flywheel? PO states he replaced it though and I'm having a bit of trouble identifying the inspection cover.

Above 1700 RPM it is not present accelerating. Idle to about 1500 it will knock, both accelerating and decelerating. It is painfully annoying at idle. Anyway to positively identify wether this is the flywheel that's cracked again or if this an internal engine issue..and if it is the flywheel, can I keep driving it for a bit? I'm pretty good with my hands but I'm not sure I'm up to pulling the transmission.
 
20psi at idle, or at 2000+ RPM? I have 20psi at idle and 40psi give or take a bit at normal operating driving speed.

You have an auto, so you have a flexplate not a flywheel. Could be causing your knock, you'd have to pull it apart to see... or use a mechanic's stethoscope, that may help you narrow it down. I have a pretty mean knock on my 91 at startup, hot idle, deceleration, and acceleration that sounds like a piston is trying to get out of the engine, but I'm hoping it's the flexplate.
 
20 psi at idle. I haven't looked at what it's saying when I'm driving around to be honest. This is probably the original sending unit so I'll swap that out a bit later and see what it says.
 
Changed the oil and put in 10w40 and a new OP sensor. No change in engine noise at all, and no shavings in the old oil. Pressure is slightly under 20 now at hot idle between 20-30 driving hot. I don't know why on earth the inspection cover bolts are so large but I'm missing half my sockets so that's put off until I get to sears tomorrow
 
I just used the ring gear teeth and spun it around with a screwdriver and that worked fine albeit very slowly.

Pulled the inspection plate and got about 3/4 of a turn on one of them. Another one wasn't even finger tight-had backed out quite a bit. The rest were okay. I see a ridge that seems to go around the diameter of where the crank bolts are.. Looks like it's cracked. Plus tightening the bolts didn't really help. It made the knock at idle intermittent but that's it.

This sucks. Previous owner had said he recently replaced this too. What would a shop charge to pull the transmission and do this? I'm seriously lacking in tools and patience so I might go that way. Not sure yet
 
There is a ring that is sandwiched between the flexplate and the bolts where it attaches to the crank. Perhaps that is the ridge you saw. The flexplate could very well be cracked, too. I don't know what a shop would charge you to drop the transmission and replace it, but if i had to venture a guess, I would say in the $500 range.
 
TerraWombat said:
There is a ring that is sandwiched between the flexplate and the bolts where it attaches to the crank. Perhaps that is the ridge you saw.
Click to expand...

You wouldn't see that unless you pulled the transmission back. The ring for the CPS blocks the view.
 
I have read on here that you need to loosen, loctite, and retighten all 4 bolts on the flexplate to cure the knock- from what I read, simply tightening down the loose one doesn't properly realign the parts or whatever physically causes the racket. I found one that while not loose, wasn't as tight as the others, and tightening it down changed the noise, but did not eliminate it. I'll have to go after it again.

On the subject, anyone have the torque specs for the 4 bolts? Anyone run into the issue of the downpipe preventing removal of the lower passenger side bolt? It was a PITA getting the cover out, then back in, with the lack of clearance and extra obstacle right there.
 
hubs97xj said:
I have read on here that you need to loosen, loctite, and retighten all 4 bolts on the flexplate to cure the knock- from what I read, simply tightening down the loose one doesn't properly realign the parts or whatever physically causes the racket. I found one that while not loose, wasn't as tight as the others, and tightening it down changed the noise, but did not eliminate it. I'll have to go after it again.

On the subject, anyone have the torque specs for the 4 bolts? Anyone run into the issue of the downpipe preventing removal of the lower passenger side bolt? It was a PITA getting the cover out, then back in, with the lack of clearance and extra obstacle right there.
Click to expand...

Don't know if it will help for your 97, but the 90 FSM says 10mm bolts set to 25 ft/lbs and 12mm set to 42 ft/lbs.
 
Yeah just for laughs I removed all four (one at a time) and used loctite on them and it didn't make a difference except noise at idle. Wether that ridge I see is a crack or not I can't be sure until I get it out. I do get the same clank noise when I move the flexplate around though, so I'd bet money thats it on mine.
 
Any particular brand I should look for when buying a replacement flexplate by the way? PO replaced it with used before and well..it cracked again and I don't to have to deal with this again
 
91Red4.0 said:
Any particular brand I should look for when buying a replacement flexplate by the way? PO replaced it with used before and well..it cracked again and I don't to have to deal with this again
Click to expand...

Bugger. This is one of those parts that you have to decide how much you would or wouldn't mind changing again.

When I do a clutch or internal slave--best quality all the way, because my time is worth more than the few bucks I would save on a cheaper clutch/slave setup.

As far as safety issues, NEVER cheap out on them.

Things to consider when considering parts:

If I cheap out, worst case scenario how much damage to my vehicle and/or how much extra work for me. Again, never cheap out on safety.

OEM from Mopar online source or dealer would be the best quality, salvage yard you could be buying almost anything, chain parts stores it will probably be foreign made.
 
Haven't had the time/money/tools to replace it yet but I've had a replacement flexplate sitting in my basement for a week or so now. I made another thread about it not restarting when hot (now it won't start period without starting fluid, runs ok after) plus it cuts out at 3/4 throttle occasionally

Took a look at the bellhousing and I can see the flexplate now with both inspection covers on :O Looks like I'm missing atleast three bolts where it goes to the crank and since I can see this clearly the bellhousing bolts are probably missing/broken/backed out.

Previous owner replaced the plate in early 2009.. Lovely.
 
I have a spare-parts AW-4 (really gross fluid, clutches are probably toast, so I'm just gutting it) you can have the bellhousing off of if you need it. It's about an hour drive, I'm usually around evenings, drop me a line about a day beforehand so I can make sure I'm around.

Sounds like the PO didn't know what the hell they were doing!
 
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