[T3] Leaking fuel pump

Jim Adney jadney at vwtype3.org
Thu Jan 19 11:26:47 PST 2012

On 19 Jan 2012 at 9:27, matthew n stanfield wrote:

> I finally got around to taking the leaking fuel pump off my 1970 FI
> Fastback. Most of the fuel seemed to be coming out around the plug. I
> have a spare pump body, so i switched the pump head (which jim rebuilt
> for me some time ago) to the spare body. I was careful to make sure i
> reassembled it correctly (with the notch in the plate by the -
> terminal on the plug) and reinstalled it on the car. Put fuel in the
> tank and it started pouring out of the pump again from around the
> plug. Removed the pump and disassembled. 

Sorry your pump with the partial rebuild started to leak around one 
of the O-rings that only gets replaced in the complete rebuild. It's 
unusual for that to happen unless the pump was allowed to dry out.

> In looking at it, i am thinking maybe the O-rings are not quite right.
> One seems to fit in the grooves nicely and is flattened on two sides.
> Seemingly indicated that it is sealing properly. The other one,
> however, seems much skinnier. It is the correct outside diameter, but
> does not fill the groove. Furthermore,it is still perfectly round in
> section. So i am thinking i somehow got the wrong O-ring in the pump
> somehow. I searched around my stuff, and do not seem to have another
> one of even close to that size. 

I use 2 kinds of O-rings for my rebuilds: Buna-N for cold weather 
climates and Viton for warmer climates where the extra solvent 
resistance may be useful. Unfortunately, I could not get the Viton 
ones in metric sizes, so they are a little thinner, but if you're in 
Ohio, I would have used the larger Buna-N O-rings. The thinner ones 
have sealed for me just fine, however.

I wonder if you got a bit of dirt in between those plates. Just a 
tiny speck of sand would hold the plates apart far enough to keep 
them from sealing.

Complicating all this is the fact that Bosch changed the diameter of 
the O-ring grove in one of those parts, so I have to stock 2 
different sizes (in 2 different materials) to cover all the bases. 

If it was actually leaking out around the electrical connection, the 
O-rings in the pump head will have no effect on that. Nevertheless, 
it's odd that the 2 O-rings did not show the same amount of 

> My next step is to try and find an the correct size O-ring for the
> pump. 

If you want another O-ring, email me directly and measure the OD of 
the groove. Tell me whether the 2 grooves (in the pump head and in 
the motor body) are the same diameter. Another O-ring would be cheap.

Note, however, that there's more to assembling the pump than just 
bolting the pump head back on. If you continue to have trouble, 
consider sending it back to me to reassemble. If that's all it needs, 
it would be cheap, too, but you'd still have to pay shipping 2 ways. 
If that didn't fix it, at least it would be here, where I could fix 

If you want, i could also "finish" the complete rebuild on your 
original pump for just the difference in price, $50, plus shipping.

Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA

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