I recently purchased this 83, 123 chassis Turbodiesel about 3 months ago. It has 160K miles and has been very well maintained. I recently replaced the injectors, glow plugs, adjusted the valves twice, with three weeks between adjustments, and the compression currently checks out at 340,380,380,380,380. I got quite a bit of benefit from doing a diesel purge, banjo cleanout, and ALDA adjustment. All of my efforts are aimed at bringing this car as close to optimal operations as possible.
The problem I am chasing after right now is that I notice that there's a pronounced vibration, which I feel both through the driver's seat as well as through the center arm rest, most strongly, when the engine is going through the range of from 2000-2150/2200 RPM during load/acceleration. Then it smooths out for a bit and kicks back in somewhere about 3500-4000 RPM. I originally thought the issue was with the flex joints/center support bearing because I had originally only noticed the vibration at about 40 MPH and then again at around 60 MPH. So my first effort as to replace both flex disk and the center support bearing, but the vibration remained. So this evening I took some time to see if it was speed related or engine RPM related and I found that no matter what gear I am in, the shaking occurs at around this just stated RPM range. From a brief visual inspection, by an inexperienced person (me) the motor mounts seem to be OK but this is where my attention is drawn at this point. My concern is, however, that there may be something internal to the engine or transmission that may also be a potential culprit. Any tips for where to look, and in what order, would be appreciated. NOTE: The vibration does seem to be some sort of a harmonic series i.e. at around 2K and then, again, at around 3500, if that's any help.
I am adding this detail after my initial posting: I had not simply tried moving the engine through the same RPM ranges while the car is in park. I did that this AM. The result is that the engine vibration at those speeds corresponds with the stronger vibration that I feel while the engine is under load. So just as the engine goes to 2000 RPM and a little beyond, the vibration is significantly more pronounced. And I also noticed that there's also a pronounced vibration at around 1000 RPM as well. I have checked the crankshaft pulley to make sure that the center bolt and all 6 of the hex bolts are tight.
Finally, the transmission is definitely showing signs of internal wear - it shifts rough from 1st to 2nd and there's a pronounced flare from 2nd to 3rd. These are better than they were after doing the valve adjust/ALDA adjustment, but still show that the transmission is going to need some repairs. Any possible relationship to the vibration I am seeing?
A final detail. Spent some more time today under the hood, varying the throttle to see/hear what I could while the engine was at the different RPM that I mentioned earlier. I found that there was an exact correspondence between a sharp, metallic noise localized under the valve cover, somewhere above the fourth or fifth cylinder, and that got the loudest at exactly the same time as the vibration got the strongest. I am a bit concerned that perhaps something that I did during the valve adjustment could be in play here.