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, in 1950. 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 selected 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.
Next 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!
This was the type of membrane I used. It has a larger metal backing plate. There are different sized membranes between the different engines - 900cc, 1000cc and 780cc - so you need to check you're ordering the right one.
Update 4 November 2024 - Oh dear!!
Well, it seems I was wrong, wrong, wrong about the fuel pump membrane. After running well for several weeks, I took the car to a show - she drove magnificently, no problems at all - and then died on the drive home. I could not get the car to run at all more than about a minute before fuel starvation kicked in. As I have to drive over a long bridge to get home, I chose to have the car towed. Thank you RAC Roadside Assist!! https://www.productreview.com.au/reviews/99d09025-ed41-59bf-840f-7ea571f1b02d
At home I pulled the fuel pump apart again. I swapped out the membranes three times and each time the fuel flow was nil or a trickle. A trickle was the best I could do so I settled for that. At least I could get the started and moved in the garage where it wasn't blocking the Karmann Ghia. What was wrong?
When I consulted this diagram on DKW Hesse website I suddenly noticed that the fuel pump shows TWO springs, one on each side of the membrane. Two? My fuel pump has only ever had one. Clearly at some point in the past one of the springs was lost and never replaced. For whatever reason, the fuel pump continued to work but only just enough. Possibly the issue with hard starting was always the sole surviving spring stiffening up. I've been pointlessly changing the membranes which possibly freed up the spring a little, giving the impression of fixing the problem.
My friend Brendan O'Dell in New Zealand is sending me some new springs - THANKS BRENDAN. Brendan supplies Brazilian made parts to owners in New Zealand and further afield. I'll pop his website up shortly.
Arrived 21 November 2024. Third time is a charm!
And the problem is quite obvious when the pump was removed. The spring as it was in my pump was wrong to start with - the outer (small) spring was the mounted on the inside. It's quite obvious where they should go.
Like this - the spiral spring belongs on the engine side of the pump. The small spring fits on the outer side. The upper ends of both springs fit neatly into metal centre of the membrane. Another 30-40 minute job and it's done. The pump still only generates a weak flow, but its more consistent and is enough to get the engine running. I'll have to check in week or two whether I still have hard starting issues. In the meantime, a big thank you to Brendan O'Dell for his help. You're a champ!
