Replacing the DKW fuel pump membrane

DKW's are quite light on for maintenance; there's no engine tuning, no changing of oil, but there are several jobs that do need to be regularly attended to. One of those things is replacing the fuel pump membrane.

DKW introduced a fuel pump with their first postwar passenger car, the F89P. Prewar cars and the Schnellaster van had their petrol tanks mounted over the engine so simply used gravity feed, like in a motorcycle. The petrol pump used by DKW used vacuum pressure from the crankcase to pull petrol from the petrol tank in the boot. It was a very simple and economical solution, but also a weak point as the membrane in the pump wears out or stiffens over time, reducing the pump's effectiveness. A perforated or leaking membrane can actually lead to catastrophic engine failure by letting petrol leak into the crankcase, flushing the grease from the bearings.

My DKW F94 began to exhibit hard starting about three months ago. The car simply wouldn't start without a dose of starting fluid if it hadn't been driven in the past fortnight; then in the past week; then it wouldn't start without it at all. I had fiddled around with the carburetor as that is often a leading cause of hard starting, but it did not help. This weekend I decide it was time to look at the fuel pump.

It's a very simple process to remove the false radiator grill - undo the two clips holding the grill in place and it lifts straight out. This provides easy access to the ignition timing and the fuel pump. First the fuel filter is removed, then the pipe to the carburetor. Two 17mm bolts hold the pump to the engine. Undo the bolts carefully so as not to damage the gasket.

Eight 6mm bolts hold the two pieces of the fuel pump together. After removing the bolts you can carefully separate the two sides of the fuel pump. There is a spring underneath the membrane and you do not want that flying across the garage as you need to put the spring back in correctly. If you place it on the wrong side or upside down, it will not work.

The membrane on my pump seemed okay. It flexed well and the spring was tight, but I replaced it anyway. You must take care when reassembling the membrane with the spring. You do not want to the spring to penetrate the membrane and of course the spring wants to fly. Aligning the membrane with the bolt holes also takes care. Re-installation is simple enough, reversing the process.

Although the old membrane seemed okay, a quick test run showed that that was, in fact, the problem. After only a few seconds of cranking the engine spluttered and caught and then was away. I was very happy! How long do the membranes last? In my experience they only last (reliably) for about three or four years before they begin to deteriorate. Fortunately, it's an easy repair, even for an amateur like me!