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Old 01-13-2019, 09:19 PM
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Join Date: Jan 2019
Posts: 1
Diagnosing 240D Engine Noise

Hi All. I was hoping to find some help identifying two different problem noises coming from the engine compartment of a 1983 240D. I have posted youtube clips of each one, so I'll put the links below with some remarks about each one.

Noise #1

A pulsing noise. This sound doesn't appear right away, but shows up after the engine has been running for 3-5 minutes on a cold morning. It goes away again after the engine gets all the way up to operating temperature. It seems to come from the front of the engine compartment. Alternator, vacuum pump, or fan, perhaps? Not too concerned about this one, but I'm guessing this may be a readily identifiable issue, and so someone might hear it and render a clear verdict right away.

Noise #2

A rapid vibration noise, like an exhaust leak. This is the more disconcerting of the two. I'm fortunate enough to have two W123 experts in my region, and both have misdiagnosed this one. Both mechanics checked for exhaust leaks and found none. This video is noisy, so you have to listen carefully, but you I tried to draw attention to the sound with sudden depressions of the throttle. It is especially prominent at 29 seconds and 50 seconds.

The sound is only audible when the engine is revving, under heavy load, but at low RPMs. Once the engine revs reach a certain threshold, the sound seems to disappear. So it is most prominent at the low end of each gear. In the video, I'm mostly in fourth, but let the speed drop down and then rev up again to trigger the noise. You can hear it go away once I get my speed up.

Mechanic #1 noticed that there was a lot of vibration coming from the transmission hanger, and he put a clamp on it (?). That did nothing. Mechanic #2 thought it was the air filter housing mounts, which I know can cause extra noise on a 300, but the mounting brackets that are the culprit on the 300 are not present on a 240. He re-machined the mount points and installed new mounts, and the housing did indeed calm down, but the noise persists.

Other possibly relevant information: the car has new engine and transmission mounts. I don't know the state of the engine shocks. Mechanic #1 performed a valve adjustment about 7-10 days before I started hearing the sound. That was was my first valve adjustment on this car (I have another 240 I've had for 9 years; this one I've had for about 9 months). The car has an unusual amount of vibration generally, none of which was reduced by the new mounts. It idles rough, and shows clear signs of needing injector replacement/rebuilding, so some of the vibration and extra clatter is just injectors (clearly audible in the first clip). But there is a lot of extra vibration independent of the rough idle (I plan to make separate post on this, so I won't explain further here). I have no idea whether the general extra vibration and this particular sound are connected, because the sound is new but the general vibrations are not.

Any input appreciated.
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Old Yesterday, 12:19 PM
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Join Date: Oct 2012
Posts: 4,486
Sounds like a contact point of metal to metal in the second video to me. As you load the engine down it tends to tilt sideways a little. Are you certain you have decent clearance between the exhaust components and the car body? At the same time I think the mechanic you used should have checked for that. Plus probably did. Did you have the noise before you changed the engine mounts? I also wondered about a loose baffle issue inside the muffler. Although I am out of my league on this one.

On the first video issue. I am trying to remember if anyone has even posted a sound like that engine has on a 240d. To me it sounds like the engine is hunting. Take it as the humor intended. The engine sounds powerful in that second video under load. This is not typical I think. I own a couple of them.

At least neither issue seems to be really engine life threatening. I am interested in what other members post on this thread. The vast majority of problems have been experienced by one or some of them at one time or another.
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Old Yesterday, 09:14 PM
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Join Date: Jul 2014
Posts: 169
Video number one, I am willing to bet is the alternator. I have noticed sometimes at idle my engine makes this exact noise and that the alternator actually vibrates a tiny bit in its brakes (even tho all the bolts a tight).

Reach down and give the alternator a good shake (engine off) and see if it has ANY play. I don't think if it does its much cause for concern unless its REALLY wobbling about.

I can literally put some pressure on my alternator when the engine is running and the noise will go away.
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