I would appreciate any diagnostic help offered on this problem. I have read all the FAQs and they point me to the “usual suspects”, but I am unsure which of these to attack first, or if I am even on track.

1989 740GL / B230F with automatic / 214,000 miles

Problem: Sluggish acceleration with the engine feeling as if it is occasionally fuel starved. The more pronounced problem however is engine vibration... low frequency and most noticeable when accelerating, but present all the time. Feels like teeth chattering! This is definitely engine related (not drive-train or wheels). The engine starts well hot or cold but the RPMs may fluctuate after a while when idling (usually dropping then recovering). No engine stalling. The only other un-noticeable thing is some whine from the external fuel pump (which is not new).

Here’s what’s been done on the car in the last 1-6 months: New motor and trans mounts; new trans filter and fluid, new exhaust, new intake manifold gasket, cleaned throttle body and re-installed with new gasket, reset throttle body switch, checked all vacuum hoses for cracks, new Bosch plugs and wires, new Bosch distributor cap and rotor, oil and filter change, new air filter, new Bosch fuel regulator, checked timing and advance as OK, new timing belt, new fuel filter, new temp sensor (one for the dash gauge), cleaned oil breather box and replaced flame trap., cleaned cold start injector, checked functioning (electrically) of all fuel injectors and they seem fine (although I did not clean them).

Valves are a bit noisy, but not unusually so. I checked the connection for the knock sensor and it is OK (no bent pins, etc.). I can disconnect either the RPM sensor or the ECU temp sensor at idle and the engine quits. Wires on all of these sensors seem OK. If I pull the connector off from the knock sensor at idle the engine continues to run as it was (I suspect this is normal as this should work only when a knock is detected). I do not believe that the radio suppression relay or the fuel pump relay are contributing to this, but welcome any ideas on these items.

I am considering replacing one or more of the above three sensors, but am unsure if this makes sense, and how to diagnose each further. I am also considering adjusting the valves but would appreciate any inputs on this being the problem. Last, where else should I look?

I appreciate the help! I’d like to get my “best” car back in top form.
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Chas Mike, former '85 760 Diesel (loved it!); now '89 740GL 230k and counting