TrollHammer

I am completely stumped. I have a 91 XJ that has had a 97 or 98 engine installed. The frame has about 200k on it and the engine has about 100k, so far as I have been told. I've been driving it about 5 years and haven't had any issues until now.

Last week, before labor day, I parked it after work and went on vacation. Tuesday, I started it up and headed to work. Other than rough starting (needed to clean the plugs was my thought up to this point. Replaced cap and rotor, but plugs looked OK and I was lazy).

Anyway, about 2 minutes after starting the engine, a loud clanking sound started in time with the engine RPM. Fearing the worst I turned it off and coasted to a stop. Realizing that if it was a rod it was shot anyway, I checked fluids and after finding no issues, restarted the engine. No abnormal noises.

I shrugged and drove another half mile and the noise started coming back. I played with gear selection and found that it made the noise under power (at the time, forward and reverse). I met up with my co workers and got a ride the rest of the way in.

Got back at the end of the day and started it up, no noise. Went ahead and started driving the 2 miles home and it started clanking about half way there.

It seems that it takes about 1-2 minutes to warm up to cause the clanking and wont clank on restart when turned off for half a minute. This lead me to believe it had something to do with the exhaust as it heats and cools rapidly and could make that tone of clank, and flexes with the engine torque. I have removed everything in pieces from the manifold back, however, so this seems le likely.

Thinking it might be a cracked flex plate I removed the inspection cover. So far, other than a few scattered metal shavings (assumed to be from the starter), I haven't seen any issues with the flex plate.

Currently, the issue only manifests when engaged in reverse after a warmup. Occasionally, it will continue in neutral or drive lightly, but is loudest in reverse.

I'm stumped. I'd think if it was an issue with a rod or other thing in the engine it wouldn't vary with temperature and engine torque.

Any ideas? Will be checking compression this weekend.

JeepingDan

The flex plate usually cracks around the crank shaft bolts and is not visible until its removed.

You can check it though.. Remove the cover and put a wrench on the harmonic balancer. Rock the crank back and forth while watching and listening to the flex plate it should give it away if it is broken.
Other thing to check the exhaust after the cat. Sometimes the pipe will make a racket on the transmission cross member.

TrollHammer

Great info. Thanks a lot!

I figured the cat heat shield or something at first, but I have the exhaust from the crossmember back disconnected, and the downpipe from the manifold to the crossmember disconnected, but still had the noise.

I'll try the proceedure you posted. I take it that it's not horribly rare for them to crack, then?

JeepingDan

No, I've gone through 3 of them on my 89 XJ.

I tried to get them from the dealer but discontinued.. So I've been using aftermarket.
Mine had pretty much same noise like the engine was screwed.

TrollHammer

Wow, I'm going to have to look over my used cars better when I buy them.

Found the problem: Loose engine.

It appears that the mechanic that installed the engine before I bought it didn't torque the bolts down. I had one main bolt all the way out, the other main (the big ones on the sides) was about 3/4 of the way out, and one or two of the top two or three small ones were missing, the one that stayed in didn't have enough room to fall out and is holding the breather on as well.

So, basically the flex plate was all that was holding the engine to the transmission, and it was rubbing on the inspection plate and probably various other things inside there making the racket. Considering I had just pulled a moderately loaded trailer over the pass with this vehicle a week or two before this, I think I got by pretty blessed.

Anyway, in my great wisdom, I decided to replace the straight pipe exhaust (yes, it had a cat, but no muffler) with a cherrybomb I had been given. Sounded great, but it turned out to be too close to the floorboard and tried to ignite the carpet padding after about three miles of driving.

Question is now (though the thread is solved and I don't expect a reply) is how far away is the muffler supposed to be, and do the stock mufflers have heat shields built in or am I missing something?