Funny sound and vibration at acceleration

[lylemiller]

91 300SEL. From time to time (not all the time) when I need to accelerate quickly from a stop I feel a vibration through the accelerator pedal and hear a clanking sound under the car. It sounds like something might be grinding or loose. The sound will stop when I left off the pedal and then resume acceleration. The sound seems to be comming from between the front axle and firewall areas under the car.

[Robert W. Roe]

I'm getting a similar thing with the 300TE. I get a grinding vibration around 45 mph then again steadily above 55 mph or so. Did you find out what was causing your vibration?

[richunters]

RWR, Recently had experienced symptoms similar to yours on my 89 300SE. Vibration very noticeable between 40-50mph; outside this range less noticeable. Turned out to be front flexdisk.

[Robert W. Roe]

Mine turned out to be a broken bolt on the rear flex disc.

[ILUVMILS]

103 powered 126's such as lylemiller's car were notorious for this complaint. On acceleration, the exhaust system comes in contact with the cross-member beneath the transmission. If you look at this from underneath the car you'll probably be able to see where it's hitting from the rub marks it leaves. Worn engine mounts are usually the problem, but re-positioning the exhaust system may help. Good luck

[lylemiller]

You know, that does make sense to me. I will take a look at that. I had motor mounts replaced this summer. Will look into the exhause system. I am about to run into a little extra cash and was going to have the seal on the timing chain replaced because of an oil leak. I was going to look into this issue as well a that time.

Thanks again!

[fz500sel]

I was having the same kinda' sound except mine seemed to be more noticeable when I was sitting at a light somewhere. One thing I found was causing a rattle was that the heat shield for one of my catalytic convertors had come loose and it was sitting against the convertor and would rattle from time to time.

Might be something to look at.

[S-Class Guru]

A quick check to determine if it's the exhaust pipe hitting the cross member is to see if it makes the noise more on an accelerating left turn, and less on a right turn. Turning left causes the engine to torque over more. Mine made sort of a clunk-clunk sound as the pipe banged the crossover - the harder the acceleration the harder the clunk.
To determine if the motor mount is shot on the passenger side, try to place your finger between the top heat shield of the mount and the base of the mount, the rubber should hold the mount sufficiently high to slip a finger in there easily. If the mount is good and the pipe is hitting there are two easy fixes to gain a bit of clerance; shim the crossover with three washers under the bolts on the passenger side, or loosen the exhaust manifold and the pipe mount at the tranny, jack up the exhasut pipe a bit, and retighten the manifold - but this might tear the manifold gasket, so proceed carefully if you try this approach. I found that the passenger mount sacks out in about 3 years, I guess due to the exhaust heat, even though it has a shield over it.
Those front flex discs are good for about 5 years too, but I have never had one make an audible sound, and I have trashed 3 so far on my '91 SE.

S-Class Guru

[lylemiller]

S-Class Guru, you just nailed it for me! Left turns make more of a sound that right, faster the acceleration more noticable. Bingo!

Thanks everybody!