No-Start True Story About Dead Fuel Pump

I recently bought a lovely 1988 Mercedes-Benz 560 SEC from a nice gentleman. The car had been sitting for years. The fuel distributor was so gunked-up that when I jabbed at the air flow plate with my finger, the plate stayed in whatever position I left it at. It’s supposed to bounce back.

Even with fuel in the tank, and a good battery, and compression, and spark, the car wouldn’t start, even though it cranked over nicely.

I wanted to rule out the fuel distributor. I could hypothetically remove it and soak it in solvent, but instead I got another unit from another 560 SEC, and I cleaned that and installed it.

IMAG8604

Still … no start. These cars are relatively simple as to fuel delivery. The “E” in Bosch KE Jetronic means that the fuel distributor optimizes the mix based on electrical input, but even without that, it basically works — mechanically, and for that it needs fuel pressure. To be precise: it needs A LOT of fuel pressure. It expects the fuel pumps to deliver more than 84 psi, and then a regulator right by the fuel distributor limits that to 84 psi. That’s very high, about double what a BMW of the same era would run, as fuel pressure.

To generate that much pressure, the car uses two fuel pumps, in series. When I cranked the motor, both fuel pumps got power, and there was the sort of hissing sound you’d expect from a functioning-fuel-pump situation. Still, no start. So, I suspected one of the fuel pumps was bad.

I removed the cradle in which the fuel pumps are housed, and tested each one individually. Sure enough … one was bad. That explains the no-start situation. We replaced the bad pump with a good used pump, and did a pressure test. Together, the pumps put out more than 84 psi. Yay! So, now they are ready to go back into the car — and now I suspect it will start.

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