Trying to help my son with his '88 740. Got a call from him at 10pm saying his car wouldn't stalled out after running really rough for a bit and it won't start again. Went to pick him up and will deal with it tomorrow.

He indicated it has been hard to start for the last few days and that he had to feather the gas to keep it running at least initially.

I realize I am not given enough info for a diagnosis but would really apreciate and ideas on where to start.

Will drive back to his car tomorrow morning to see if we can get it to run.

Ian

 

There is a good flow chart pdf on here click on it:

No Start on 740/940

 

Temperature warmed up some today (Still in single digits F) so I had a chance to look at the car.

Since I had resoldered the fuel pump relay connections a couple of months ago I thought I would try that again. Removed the relay, reinserted relay. After some cranking, the car started up rough and then stabilized. Left it run for a few minutes. Shut it off. left it for a few minutes. Refused to start again.

Working from the diagnostic guide (granted I jumped part way in because I wasn't going to be able to get underneath the vehicle today) removed fuse 11, disconnected fuel rail and tried to start. No fuel.

This would seem to indicate that it is the fuel Pump relay or the fuel pump.
Is the jumper test sufficient to confirm either the relay or the pump itself.

The relay is about $13. A fuel pump seems to vary from $15-$90.

 

the good Stribel relays are more like $40-50. the good bosch main fuel pump is more like $100 and the turbo version is $140 or so. the cheap stuff tends to be junk.

 

What he said^^^
With the cost so reasonable on these parts; no reason to go aftermarket. I've had some pretty wonky performance issues using aftermarket relays especially. Usually aftermarket pumps are not even close on liters per hour and pressure.

Yes, jumpering the relay socket is sufficient to diagnose pump. Keep in mind that for the relay fuel function there could be sensor, sending, corrosion, resistance or wiring issues, especially grounds.

 

Thanks guys for comments. I am going to try jumpering first to see if the fluel pump runs. I don't mind paying for the part. Just dob't want to repalce somehting I don't have to.

 

We none of us do.

My manual also suggests:
-fuel tank empty
-defective pump
-clogged fuel filter
-fuel tank filler vent blocked (suction when cap released)