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Re: Output specification for the primary (in the fuel tank) fuel pump[700/1989] posted by Dave Stevens on
Friday, 23 March 2001, at 2:09 p.m.
Can't help you with those pump specs. Your symptoms sound pretty classic for a failed in-tank pre-pump. The one in my '89 740 went not that long ago. Now rather than measuring all those pressure and flow rates, why not simply use a process of elimination? Mind you, if I'd coughed up for a nice fuel pressure tester I guess I'd want to justify its existence.
You should be able to easily hear the pre-pump running when you listen down the filler neck. If there's a lot of background noise then listen through a piece of hose dropped down the neck.
First, disable the pre-pump with the engine running. The simplest way to do this is by pulling the fuse (#11 in your case). If the noise at the main pump doesn't change then the pre-pump is likely at fault. Again, try listening down the filler neck to see if a buzzing noise stops.
Second, try disabling the main (under chassis) fuel pump and see if the engine will run. Just find and disconnect the electrical connector for the pump. If you don't find a connector at the pump itself then you'll have to use the connector with the pink and black wires up by the relay tray (deep on the left side of the tunnel as I recall). A working pre-pump should be able to at least keep the engine idling. While you're under the car, inspect for damaged gas lines that might be restricting fuel flow.
There's always the possibility of the rubber sleeve between the pre-pump and the pickup tube rotting, although this is a problem more commonly associated with older 240's and/or the use of ethanol enriched gas. In that case, the engine will start to cough and then stall only when you've got less than 1/2 tank of gas. You'll need to pull the sending unit from the tank to inspect for this. If you do find the rubber connector has gone then don't discount a bad pre-pump as well. Also, use a flashlight to check the bottom of the tank. If there's a fair bit of sediment then you might suspect that this has contributed to a blockage or prematurely failed pump. Try to suction out any sediment if this is a problem.
Now if all that doesn't isolate the problem then I'd say it's time to put your fuel pressure tester to good use. You should be able to do all the necessary testing by attaching the gauge up by the fuel rail. I don't have the specs handy, but I'm sure one of us can supply them if needed.