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Re: FI pressure drop

> I had that problem shortly before the thing refused to start
> altogether.  What it turned out to be was the check/overpressure valve
> on the side of the fuel pump head(the third hose connection).  The
> rubber tip had hardened up and broken off, letting the high-pressure
> gas go right back into the tank. Replacing the fuel pump fixed it.  I
> no longer have to flick the key a number of times to let the pressure
> build up, even after letting it sit a week.  Usually once is enough,
> pausing just long enough for the pump relay to time out.

The third hose should carry fuel flow only if there is a fault in the 
fuel line that prevents the pressure regulator from doing its job.  I 
don't know what pressure this should release at, but it is well above 
the 28-31 psi of normal operation.  I think the same plunger serves 
both functions, but this really shouldn't be a problem as long as the 
pump really has output.

In my case the fuel loop maintains its pressure at the nominal 20 psi 
after shutoff, so this part is functioning corrctly.

Mercedes had a similar problem in the 70s with their 4.5 SELs.  I was 
at a Bosch seminar somewhat later where the instructor told us that 
Mercedes and Bosch recommended drilling a very small hole in a 
particular spot in the pressure regulator that would serve as a 
bypass for the fuel and would allow zero pressure on shutoff.  This 
was supposed to prevent the system from sucking air back in as the 
fuel lines cooled back down.  I think their reasoning is sound, but I 
have never tried this.

I think in order to do this I would have to make a special drill 
bushing/guide that would pilot the drill.  It needs to fit inside the 
inlet tube and position the drill bit against the inner portion 
of the side of the outlet tube.  I keep meaning to try this.

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

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