Clunking Noise While Waiting For Traffic Light To Change.

lsrxlex · March 9, 2012

My 1990 LS400 (with 372k) sometimes makes loud clunking noise, it started after I stopped at traffic light for about 2 seconds. It's like holding in one hand a brake rotor and strike it with another rotor, every second. The sound got louder and louder, and was so loud that I was embarrassed because other drivers looked at me and they might think my car was falling apart at the stop light. Sometimes it stopped if I switched from "D" to "N". If so, then when traffic light turned green, and I switched to "D", I got a clunk, just like I got rear-ended by another car. After that everything seemed okay.

This intermittent problem happens only at low speed (always below 40mph) or at traffic light stops (started after stopping for about 2 seconds). The clunking noise seems to be from "under" the engine, or between engine and transmision. It happened a couple times a day, not all the times.

I removed a small cover ( ~ 6x4 inch ) at the transmission, and tried to tighten the 14mm bolts ( which are almost at the perimeter of the flywheel ), while turning it, to get to all of the bolts. And all bolts seemed tight and okay. (But I noticed some oil leak, maybe the rear main seal needs to be replaced ?)

And in the process of trying to fix this problem, I recently added silicone to fan clutch, changed air filter and fan bracket, replaced the bearings of: AC compressor pulley, tensioner pulley, idler pulley. (I haven't got to the bearing of the alternator yet).

But all of these works are of no help.

I checked all fluids ( oil, T-IV ATF, PS, coolant, differential ), they looked okay. No warning lights of low levels.

Any help would be appreciated.

jaswood · March 9, 2012

At those kind of miles you need to take a hard look at all of the driveline component mounts/cushions, motor mount, rear differential, outer axle suspension points. I wouldn't be surprised if you need an entire set of u-joint bearing also.

billydpowell · March 9, 2012

also did you address the rear main leak?? if so did you retighten the bolts that attach the flexplate to the crankshaft, that also bolts up to the converter??

lsrxlex · March 23, 2012

also did you address the rear main leak?? if so did you retighten the bolts that attach the flexplate to the crankshaft, that also bolts up to the converter??

At those kind of miles you need to take a hard look at all of the driveline component mounts/cushions, motor mount, rear differential, outer axle suspension points. I wouldn't be surprised if you need an entire set of u-joint bearing also.

I'll check those areas and let you know.

Thanks.

lsrxlex · March 23, 2012

also did you address the rear main leak?? if so did you retighten the bolts that attach the flexplate to the crankshaft, that also bolts up to the converter??

The leak is somewhat small. After cleaning of the leak, the oil won't reappear until 2000 miles later, approximately. So I just leave it that way for now.

Regarding the clunking noise: How can I tighten the bolts of the drive plate at the crankshaft? I think there are 8 bolts around the center of the drive plate. Do we need to separate the transmission for access? If so, can we do it at home with just jack stands and a floor jack? I checked Iexls.com for rear main seal replacement, and the job was beyond my ability.

Thanks.

landar · March 23, 2012

You would have to separate the engine from the transmission to tighten the flexplate(flywheel) bolts. If the rear main seal fix is beyond your abilities, then so is tightening the bolts. But good news...your issue is, most likely, not the flexplate bolts. As a first step, check the transmission mount and replace that if it is rotted. Check all of the driveline components as jaswood suggested in post #2.

jaswood · March 24, 2012

You would have to separate the engine from the transmission to tighten the flexplate(flywheel) bolts. If the rear main seal fix is beyond your abilities, then so is tightening the bolts. But good news...your issue is, most likely, not the flexplate bolts. As a first step, check the transmission mount and replace that if it is rotted. Check all of the driveline components as jaswood suggested in post #2.

Many years ago(***) I had a flywheel/flexplate crack al the way around through the center of the crankshaft mounting bolt holes. The crack was jagged enough that it would still "convey" full engine torque and under drive torque there was no "clunking" noise.

*** Kansas City, June, 1963, brand new heritage burgandy '63 T-bird, returning to MT from Honeymoon. Ford repaired it under warranty, they would actually do that back in those days.

billydpowell · March 25, 2012

in my 30 years of owning my shop, we have had a half a dozen cracked flywheel (never on a lexus) flexplates, but many loose converter to flexplates bolts, and you can get to them from under the car. and they will make a clunking sound when drive pressure is released while stopped, not trying to convince anyone, just trying to help find a mysterious sound.

VBdenny · March 25, 2012

Rather than guessing, I'd take the thing to a Toyota dealership and have them determine what's wrong. For $75 you'll know if it's big or small. The only other suggestion is if the clunk is loud, make a video of it, post it on YouTube and maybe you'll luck out. Good luck with the car.

lsrxlex · May 4, 2012

I finally got time to take care of my car. My work schedule makes it difficult for me to make time for this. Sorry for the late reply.

Anyway, I bought 2 engine mounts and trans mount, oem. And my mechanic installed them, and they showed some cracks but not very badly separated. It's about time for replacement. I could feel the difference after installation.

But unfortunately, that didn't resolve the rattling noise problem.

The mechanic then checked and found the noise comes from the LH catalytic converter. He let me bang at it and it rattled loudly. When I even banged at the RH cat, the LH one still rattled too. Only the LH cat is making noise.

Other than the rattle noise, I got no CEL or codes. All O2 sensors and catalytic converters are original oem and work just fine since 1990.

What should I do just to fix the noise?

Thanks for all the help.

steve2006 · May 9, 2012

Correct solution is to replace the cat obviously but an unconventional way is to remove the old cat and shake or break out all the broken pieces but this may then lead to another problem, the Engine Management Light may come on!