I have a 1994 E34 525td which has stumped everyone I have taken it to.
The injection pump can be adjusted to make it either idle smoothly, but with no throttle, or to have throttle but run very rough - hunting, I believe is the correct term. This a fly-by-wire system - no accelerator cable.
It has a remanufactured injection pump, reconditioned injectors, a new in-tank fuel pump, and the fuel filter has been replaced.
I guess part of the problem is I'm in New Zealand and I'm only aware of 2 of these in the country, which would be why nobody knows much about them. I believe the other one is still running OK.
Before the new injection pump went in it didn't hunt as bad as it does now.
It runs well for about 4 minutes - until the engine starts to warm up - then the hunting quickly gets worse.
With the top of the pump removed (I think) the mechanic has watched the throttle device moving quickly back and forth causing the irregular idle etc.
I really like the car, but it's driving me crazy!
I will be very grateful for any help/sugestions.