Random knock at idle

Hi everyone. I have a '96, 4.0l auto 96k miles and for many months i've been noticing a random "knock" coming from the engine. Now that is winter I've noticed upon start-up the noise is horrible. It only occurs at idle, if you give it gas is goes away. If it's in gear and you do a little "brake torque" there's no noise( I would think this rules out broken motor mount or rod knock?). When the engine warms up it seems to get louder-like something is either vibrating/shaking,(exhaust manifold?)- goes away with/higher rpm. I've read the posts that narrow it down to flexplate bolts/cracks, and knock sensor. If the knock sensor went bad would the engine make that loud of an audible noise(at random times?) and if so, why? I've peeked under the hood to see if I can see something shaking around -with no luck. I did notice the random knock noise did seem to occur under the rear of the intake- around the driver side rear area of the block (knock sensor? cracked exhaust manifold?) I'm starting to drive my other car until I can isolate the cause. It may be awhile before I can tear anything apart though-weather, $$$, etc. Do you think I can drive it as is with any of the above mentioned conditions? Thanks in advance.

XJGUY1-George

George, It's not the knock sensor the 96 does not have one. Jim

Well i wouldn,t describe a cracked header as a knock,but most factory headers crack at the junction of the collector,look for a hairline crack where the top tube joins.
Crawl under and check the tightness of the TC/flex plate bolts.Run it too long with them loose and it will be an expensive fix.
Wayne

Thanks

Thanks for the responses. That's good to learn that there's no knock sensor-I would have been looking forever... I could understand how lose bolts would eventually screw things up so I'll try not to drive it. We should be getting some warmer days around here to melt all the snow, so I'll try to get under there and check 'em out- I hope that's all it is. I'll post an answer if I get one- thanks again.

George

read this thread.. does it sound familiar??

http://www.naxja.org/forum/showthread.php?t=9048

sounds like you may have a simialr issue.. never fun to bring bad news, hope it's something alot more simple

mike

My 96 had a similar noise. Just turned out to be a sticking lifter. All I did was run a quart of ATF in the engine oil for about 250 miles. Problem solved. I was able to isolate the noise to the valvetrain before I ran the ATF.

You said the noise goes away when in gear? This might be an indication of a bad rod bearing. More investigation is needed before making a decision on how to repair it.

Bryan

Hello,

Sorry it took this long to reply w/answer but couldn't get around to it. I took the inspection cover off and was expecting to see t-conv. bolt & nut visible like on my BB chevelle. When I only saw the bolt head visible, I thought i was going to have to disassemble everything. Then I tried for s@!t's and giggles turning the exposed bolt head and to my joy it turned very easily- letting me know it was loose. I went around all the bolts tightening them as tight as I could. I ran to the seat to start it and upon start-up...NO NOISE! It was great, for once,the easy/cheap fix prevailed. I have to go in and check them again-maybe take one at a time out- put some lock-tight on them and re-tighten them. Either way, to the person who suggested those bolts-thanks a million. I now have one less worry on my mind. And to the others, thanks-you taught me some new things about my xj and gave me food for thought. Hope this last reply helps anyone in the same shoes I was in a few months ago.

Thanks, George/XJGUY1