M30 Crank, No-Start Fix: Fuel Supply … In-Line, Main Fuel Pump

After isolating fuel supply as the cause of the no-start condition on my M30-engined 1984 BMW 633 CSi, we removed the cradle that holds the main, inline fuel pump and the filter, both.  These are located on the passenger side near the rear wheel, underneath the car.  Getting the cradle out is a little tricky, even after the 10mm nuts have been removed. Some wiggling and jiggling helps.

We removed the fuel pump from the cradle and we bench-tested the pump.  Sure enough, it was bad.

We removed a unit from another BMW, and we bench-tested that too. It worked. That pump went into the cradle, and the hose to the fuel filter was attached next. The cradle went back into the car, and the fuel lines and electrical connections were hooked up.

Even so, the car didn’t start. So, we split the problem into fuel pump and supply vs. wiring to the fuel pump.  We hot-wired the fuel pump with a separate, stand-alone battery, with positive hooked up to the green wire, and negative hooked up to the black wire. The pump started buzzing, and the hose that we’d removed from the fuel rail started spurting fuel, into a cup that we’d optimistically pointed it to.

We disconnected the stand-alone battery, and inspected the hose to the fuel rail. It had started deteriorating but not terribly. So, we cut off 1/2″ or so, and the hose could still reach without being overextended.

The next challenge was to get the hose all the way onto the fuel rail.  It’s a tight fit, and it’s hard to shove the hose all the way on.  Yet, neglecting this can make the hose come off, which will cause the car to not run, and it might well cause an engine compartment fire too.  So, I heated up a cup of water, and stuck a plastic bag over the end of the hose, and stuck the hose and bag into the hot water, to make the hose more pliable. This helped. I removed the hose from the water and the bag, and slipped a new hose clamp around the hose. I shoved the hose onto the fuel rail as hard as I could. When it didn’t go far enough, I poured some more hot water directly onto the hose, and tried again.  Finally the hose was pushed up against the main fuel rail as far as it could go. I tightened the hose clamp. The system had integrity again, yay!

I reattached the separate battery, and I could hear the pump going, and fuel being pumped through the fuel rail and back into the tank. I liked being able to do this without the added noise of the engine being cranked over, and I liked being able to walk around and inspect the various fittings for leaks.  I found none.

Finally, after convincing myself that things were in order as to the fuel supply, I hooked up the car’s main battery and I cranked the engine. The car started. 🙂  Yay!

Next, I get to figure out why the fuel pump isn’t getting power … but meanwhile, even if I have to hot-wire the fuel pump, I at least have a drivable car again.  Better …


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