Grinding noise while turning and under high throttle
 

The car is a 1986 300 SDL.

Today my car developed a 'grinding' noise while turning and when the accelerator is pushed down to the floor. I also tried putting it in neutral and revving the engine, and the same noise starts at about 4,000 rpm.

Think it could be a bad transmission mount? Or could it be two or more seperate problems(unlikely, considering all of this started at the same time)?

Check the crank damper. Does it wobble with the engine running? Remove the belt and check all the pulleys.

Sixto
87 300D

first off, NEVER rev in neutral to 4K!!! especially if you have a strange GRINDING noise! get a chunk of garden hose, and start listening to the various parts of the motor at idle.
if nothing is found this way, shut it down, pull the belt, and check the tensioner, and the vacuum pump. with the belt off, grab each pully and feel for any movement/grinding feel.
safety first!

Thanks for the advice. Today, I'm going to follow the steps both of you have listed and see if I can locate the problem. I have not driven the car since the noise started. I hope it's nothing expensive. It does have more than 200k on it, and I imagine that the timing chain can go at any time.

I did not know that the engine was the source of the noise; it sounded as if it were coming from the rear of the car. Given how hard I drive the car, I suspected CV joints or a transmission mount, but the grinding noise appearing when revving it in neutral at 4k+ caused me to consider other possibilities. I only tried it once after the noise started.

I do need new CV joint boots. They are torn. The car has spent a lot of its life under my ownership in excess of 100 mph.

I took it for a drive today. It makes the noise while turning; I tried revving it again once more and it didn't make the noise then, but it still makes the noise while turning. It sounds like it is coming from the middle or rear of the car.

I did try jacking it up and taking the rear wheels off. When I try to move the rotors by hand, I hear a very light "clunck" noise that, under very gentle spinning, prevents the rotors from spinning further. also, the noise is much louder when making left turns than right turns.

I am thinking I could have a bad transmission mount(s), bad CV joints, bad bearings, or any combination thereof.

It does not wobble. This damper was also replaced last year. The entire engine itself wobbles slightly at idle(< 2 mm), but the motor mounts were also replaced last year.

Tried that and they are good.

Just a thought, are the rear wheel bearings OK?

Wheel bearing is a possibility as you have the problem with side load.
Does the noise occur with your foot on the throttle, off the throttle or just maintaining speed?
Does it occur turning when in neutral?

I don't know; I suspect they may not be. The car has probably never had them changed in its entire life. The good news is they aren't expensive to fix, unless they weld themselves in place post failure.

The noise occurs only under heavy throttle when in a straight line(> 70% with kick down engaged). I can coast in a straight line in neutral(a common practice to save fuel when not 'playing' with the car) without the noise being noticed. When cornering, the noise occurs even at no throttle with drive engaged. I have not tried turning in neutral and will do so tonight.

While turning in neutral, coasting with the engine off, the noise is still present.

My money is on a rear wheel bearing.

I'm going to have to fix it then. I think it would be a good idea to replace both of them, given the age and mileage of the car.

Is $300 in parts plus tools a good estimate? There's a DIY guide here on this site for the W126 and I'm intent on doing this job myself.

When the cv joints begin to fail they will sometimes make a rumble when loaded a bit. This could be with a load in the back or on turning. Did you check for play in the half shafts? To do this put the car on a level place, in neutral with the parking brake set or wheels chocked and grab the half shaft and see if it will wiggle up and down.

Good luck!