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Fuel Pump Issues

6K views 13 replies 4 participants last post by  robertobaggio20 
#1 ·
Hello, I have a problem.

I have a 1991 535i. The fuel pump stopped working so naturally I replaced it only to discover that it wasn't the problem. After the fact i hooked up a 12v directly to the old pump and it runs just as strong as the new one. Next I hooked up a multimeter to the plug that goes to the pump/fuel level gauge, showed 11.92v. Then went to the contacts at the pump in itself. NO POWER!!:mad: The car does turn over and there is spark, it will run, but barely, off of the injector pump. Please help me!!! :cry:
 
#2 ·
Jump terminals 30 and 87 at the fuel pump relay socket. See if that makes the pump run continuously (it should) and the car should run this way, but it may throw a code for a fuel pump relay (1261). If jumping the terminals causes the pump to run and i.e., the car runs also, then you need to replace the relay.

If you jump the terminals and have 12v to the electrical connector, but the pump won't run, then the new pump is probably faulty. If that is the case, I would try the old pump again. If you have 12v at the switch with the terminals jumped, then it has to be the pump.

I don’t know if you have it, but there is a searchable version of the Bentley manual here on the fest. Testing of the fuel pump is spelled out very well.

Good luck.
 
#4 ·
PM sent.

Electrical stuff is not my forte, so bear with me, but I can't figure how you can have 12v at the connector and the pump not get that voltage :dunno: Can you be a little more specific as to when you have voltage at the connector? I.e. is it continuously, with the key in position 2 or position 3 (cranking)?

If you have 12v at the connector, then the relay should be fine, as should the fuel pump fuse.
 
#5 · (Edited)
I have 12v continuously at the connecter with key in position 2 "running" fuse is fine, i changed it to be sure. There does look to be a scorch mark on one of the solder points on the bottom side of the plate. When the connecter is plugged in there is no voltage to the solder points or to the leads that go to the pump

One note on that, Its possible that the 12v im seeing is for the Fuel level sensor which runs off the same plug.
 
#6 ·
Key in position 2 should not energize the pump continuously. In fact, from what my very dependable and competent independent mechanic told me (along with my experiment on this very issue on my E36), the only time the fuel pump is energized is upon turning the key to position 3 to crank the engine.

I suspect a bad fuel pump relay. Again, jumping terminals 30 and 87 at the fuel pump relay socket will supply continuous voltage to the pump. If your pump is good, and assuming that there is not a break in the wiring somewhere, then the pump should run and so should the car. If jumping the terminals fixes the no-start, then you need a new fuel pump relay. This is a common fail item.
 
#7 ·
I haven't worked on an e34 but had similar problem with e36. On my car the pump and sender have separate wires. There are four wires in the plug - two go to the sender and two to the pump. I suggest you double check the pump connection since I agree with what BMR LVR said. If you have power to the connector and you know the pump works then it should run. From what you describe I would suspect the relay. If it works with the terminals jumped as BMR LVR suggested then the relay is bad. I would use a fuse on the jumper for the relay just in case you connect the wrong terminals or if there is a short somewhere. Good luck!
 
#13 · (Edited)
Thanks for the writeup Steve. I think you and your indy is right.. Just read through the manual and it says this :

Residual pressure, checking
For quick restarts and to avoid vapor lock when the engine
is hot, the system is designed to retain fuel pressure after the
engine has been turned off. This residual pressure is primarily
maintained by a check valve at the fuel pump outlet and the
fuel pressure regulator. The fuel pump check valve is not serviceable
as an individual part.

So there is a check valve at the fuel pump. I believe that when fuel return pressure disappears, as when a hose somewhere bursts or is disconnected, the check valve notices this and stops the fuel pump from functioning. This is probably a safety measure. However i should think that the current to the pump remains live. And I am a little surprised that this happens when we are at key2. Perhaps because the fuel pressure regulator does not function because the engine is not running (the fpregulator needs engine vacuum to function), there may be some version of pressure detection that kicks in at the pump and causes it to go into standby mode or stop functioning altogether..



rgds,
Roberto
 
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