Does anyone know what the expected resistance across the fuel pump should be?


This is the single pump for the 4.2L single ended fuel system, not the dual pumps in earlier Rs.

I have been having a very intermittent stall where the engine dies at low RPM. Seemed to only happen when the engine was cold. Never any codes and immediately restarts as if nothing happened.

Sunday it quit at a traffic light. Same signature, stumble and die. Didn't want to start this time but finally fired and ran fine the rest of the way home. Again, no codes or useful information.

On Monday I had it idling in the driveway checking some things, put it into reverse, stumble and die. Checked the fuel pressure with an external gauge and have zero.

There seems to be voltage to the pump so I disconnected the harness and checked the resistance across the pump leads... about 300 ohms. That seems way too high.

I have an old pump assembly that was replaced because of a bad check valve, and on that one the pump resistance is ~ 20 ohm. Clearly different than the one in the car, but still seems rather high. I was expecting something in the 1-4 ohm range.

Am I missing something? Can someone say what the measured resistance should be or if this is even a valid test?

I am looking to confirm that the pump itself is the problem before I go fishing it out of the tank.

 

Can't help with ohms but you might try a hard reset, this will allow the car to reset it's high an low idle settings. Much easier than digging the pump out.

 

I suppose that you are right that the resistance should be quite low, about 1-4 ohm. No way it can be 300 ohm. From my experience the single pump from earlier car draws about 8 amperes at 13v. This means that resistance of its motor is about 1.6 ohm. This pump flows about 100 liters per hour. In case of later single pump resistance can be even lower due to higher fuel pressure and fuel demand which means more powerful motor.

 

Finally had some time today to investigate the problem further.

Switched the key on and the pump primed just like it should. Voltages all check out, everything was normal. Resistance through the pump was low, about like you would expect. A few more key cycles the pump did the 1sec prime and then, it stopped. Resistance through the circuit was high again. Very odd.

The pump driver checked out and functioned normally even when the pump wouldn't run. In case anyone is curious, the driver applies the voltage to the pump all the time and switches the return side ground based on the signal from the ECM.

I then disconnected the wire harness from the pump and checked the resistance through the wiring... it was high. The harness is actually in two pieces, the one from the driver to the tank checked out fine. The part that runs along the tank to the pump had a high resistance.

Turns out the connector at the pump was packed with some kind of dielectric grease. I'm not sure if this is normal or was a consequence of having the pump module replaced a couple years ago for the bad check valve problem. Being that the connector is inside the car, I can't see an obvious reason that the grease would be applied from the factory, and I haven't run across any other connectors that had grease in them.

A good dose of contact cleaner to wash the junk out of the connectors, put it back together and now it works fine. Car started right up, and no amount of jiggling wires or connectors would cause any bad behavior, so I think it's fixed.

Strange, I never expected a simple contact problem to the be the root of this one. I can't complain about the cost of the fix but there is still a part of me that is skeptical that it is really so easy.

 

This is a long shot. But I had very similar stalling with my X300 for years. Discovered a lot of oil in spark plug wells. Fixed that and NO More stalling. That is until the next time I drive it after saying that.