dogholes

Hi , all the parts you mentioned where changed 3 times with used parts - no success yet. the jettronic III is supposed to work without impuls on this differential pressure regulator as the range of its impuls goes from -10mA until +10mA when unplugged (0mA) it stays open in the models from 1989-1991.
Maybe all my exchange parts are also broken........

latchkeykid

That sucks. Okay, few more things you can do...

Are you able to pull "blink" codes from the ECU? Usually the fuel pump relay has a socket for a fuse on the top, insert fuse, turn the engine over 5 or 6 times, leave key on and count the number of times the check engine light blinks...the full procedure is outlined up on the SJM auto website.

Are you absolutely positive every electrical connector is ok and in it's proper place?
Can you check if the connector TO the differential pressure regulator is sending power? ( 5 volts-ish)
Is there another connector for the fuel distributor potentiometer tucked in just under the upper lip of the fender? Have a good look at all those wires.
Any chance the valve to the carbon canister is stuck open? Try blocking off the hose and see what happens.

Try shorting the fuel pump relay so the fuel pump runs for a couple of minutes, then "crack" open one or two of the injector lines at the fuel distributor...if there is no flow you've got to work your way back from there.

Worst case I can think of is either one or both of the following: the fuel injection control unit or the electronic knock sensor ignition control unit....

That's about if for me in terms of shots in the dark, hope you find something definitive.