Got a 740 1990 B200 engine. On an intermittent basis, although the frequency is increasing, the car starts o.k., but then runs rough with the RPM varying significantly. The throttle has little effect, and there is no power. I have replaced the fuel pump relay, recommended by the breakdown man, and both pumps seem to working.
Previously, when the car was running, at about 60mph, there were a couple of "coughs" or "hick-ups". When this happened I noticed the RPM dropped to around 500 to 1000 RPm, but the engine was still in gear, and so was turning over at the same speed as the road. Not sure if this is significant ?
A suggestion from another breakdown man, was that the fuel pipe in the tank may have split and so the fule pump would be sucking air into the line along with the fuel. Does this make sense ? The tank is less than 1/4 full, but I put another gallon in from a petrol can with the engine running and this made no difference. ( I thought if there was a split pipe more fuel could cover the split and stop air getting into the pipe )

After the two times I have needed a tow home, the car has started and run fine.


Update on symptons.

Had diagnostics on the car, just as we proved that the problem was not the fuel pump relay, or the fuel pumps, the car started running fine !

However we did discover that the spark on lead from the coil to the distributor, was erratic in time with the engine rough running and the fuel pump relay clicking in and out.

So our suspicions were with the red/white signal wire that connects the ignition apmilifier with the coil / fuel pump relay / tacho.

After this stage the engine was running fine, so we could not try and track the fault further. We ran the car for about an hour with no further failure.

Today the car is running rough again. As a precuation this morning I replaced the coil and the ignition amplifier. Neither had any effect.

Two things I have noticed
1 the wire that comes from the "Hall" sensor on the distributor was showing a "stepped" siganl from 5v to between 1.5 and 0.7V, this was a variable voltage, but never went down to earth, 0V.
2 when I start the car, without using any throttle, it idles fine at about 600rpm, but the dash board lights do not go out. As I very slow increase the throttle until the dashboard lights go out (~1000rpm), the car runs rough, and does not return to normal if I reduce the throttle. When I turn off and start again the symptons repeat them selves. How are the dashboard lights controlled to go out after first starting the engine ?