First time posting so let me know if any of my formatting is wrong or if this should be posted elsewhere. Sorry for the big post but Im trying to give the most thorough information to help your insights!
Car: 1994 E320 Sedan - 104K Miles 1 owner prior - serviced by one mechanic - from whom I have all records 50K onwards.
Location: Pasadena, CA
Wire Harness, Throttle Body, Thermostat, All Coils, Spark Plugs, Spark Plug Wires, Harmonic Balancer (a misplaced bolt fell into the engine bay and literally tore apart the balancer, that was bad...), Motor & Trans Mounts, Tensioner, Belt, Fan Shroud
Flush (by local mechanic I trust a lot), Filter Change, New Shifter Bushings
Suspension I haven't touched yet besides an alignment and new shocks - but I dont think suspension matters for the problem Im chasing!
To the problem:
When the car is put into reverse or drive the engine RPM drops about 50-100rpm and a vibration begins. The vibration seems to oscillate in a cycle maybe 2-3 seconds - wom~WOM~wom~WOM - the whole steering wheel begins to shake - I can feel the vibration in the floorboards as well. Its not awful but it feels much much rougher than when you take the car above ~900 RPM. I understand this is an old car but I feel like its not idling correctly.
I first noticed this when at times a metal on metal vibration SOUND seemed to be coming from under the driver's side floorboard. So I got it on a lift and had it put in drive - the vibration SOUND seemed to come from the catalytic converter that has a mounting bracket attaching it to the transmission (Im assuming this is the first cat?). I checked the heat shielding and nothing was loose. In the same time on the lift I also had replaced the engine and trans mounts, which if anything made the engine feel no in idle smoother and the vibration SOUND more prominent.
The exhaust was shaking quiet a lot when I was looking at it on the lift but maybe thats normal. The hangers looked like they had never been replaced and I haven't serviced the exhaust system at all.
I'm not sure what could be the problem - I feel if I can correct the engine vibration it will also cure the vibration SOUND. Please someone send help this is driving me insane!
No vibration in park or neutral but vibration in any drive gear is almost always
engine/ transmission mounts. I'd look at those again. Maybe just loosen them and retighten them. I'm not saying that is necessarily the case. It just almost always is. I'd even look at the diff mounts, although those tend to manifest as a clunk when going into gear.
As for the sound, it could be coming from anywhere but seeming to be coming from the point you observed. It's part of the problem with having a well insulated car. In wagons for instance, a loose hatch strut, manifests as a rattling noise from the B pillar of the same side. Follow the entire exhaust all the way to the back and see if there is any contact anywhere.
I don't think you need to worry about the exhaust hangers; as long as they hold up the exhaust without excessive sagging they're good. I've seen an exhaust hanger that was almost completely perished still doing the job. Although, thinking about it now lots of lateral movement in the exhaust could be the source of the noise, so replacing the hangers might not be a bad thing.
Fuel economy is not great on these cars. Four speed, non lockup, non overdrive transmission in a heavy car. I have averaged ~22- 24mpg in my 3 w124s and the one that was averaging nearly 24 was the one I drove to Phoenix once a month from Los Angeles. Highway mileage isn't too bad, because of excellent aerodynamics, expect about 24- 25 if you drive smoothly and have long highway drives. I always employ hypermiling techniques and they make a giant difference. (coast to stops, maintain momentum, try to get to top gear as quickly as possible without accelerating harshly, etc) Even using the AC makes a noticeable dent in fuel efficiency. If i'm driving less than 20 minutes I rarely use it; may cars black interior soaks up heat and it takes a while to dissipate.
Try going to a good Mercedes mechanic
vs a "local mechanic you trust." It'll make a world of difference. I can personally vouch for Walt at Burbank Mercedes. He's a 2nd gen, Mercedes mechanic. And his dad was a MB mechanic in Germany.
Please keep us informed of your progress diagnosing this issue. And welcome.