Re: Easy Staring with Fuel Pump Prep Run

[ti@amb.org (Ti Kan)]

Robert Dalton writes:
> The fuel pump on my 1990 Audi 100 normally runs
> for about 3 seconds when the ignition swith is
> turned to run (not start).  Although this seems
> like a great idea, to build up fuel pressure before
> starting, my books say the only time the pump is
> supposed to run is when cranking, and then afterwards
> when the engine is running (sucking air).

Many 80s non-turbo Audis work the way you describe.
The fuel pump relay is supposed to supply power to the fuel
pump only run when it gets ignition pulse signals from
the ignition coil, signifying that the engine is running.
This is so that in the event the car stalls the fuel pump will
automatically stop too.  When turning the ignition key to
the on position, the fuel pump relay receives power and it
is designed to briefly run the pump to pressurize the system.
However, I have also noted on my 4000 that this doesn't
always happen.  Probably a flakey FP relay.  It is an aftermarket
replacement.  The original relay always reliably ran the pump
after turning the key to on, but I replaced the relay thinking
it was defective, only to find that it was only a loose connection
at the relay pin.  I threw away the good old relay though :-(.

> I know that normally there is an air flow switch
> that keeps the fuel pump running continuously when
> the car is running (sucking air).  Do you think
> the "switch" in the Audi is the FICA (fuel injection
> control unit)?

It's not an "air flow switch".  On older Audis (primarily those that
have separate ECUs for fuel and ignition) the ignition signal
is fed directly from the coil to the FP relay; on cars with an integrated
fuel and ignition ECU, the ECU controls all aspects of FP relay operation.

> My next step is to see if I have an intermittant
> fuel pump connection, or a bad Audi control brain.

If you have a bad fuel pump connection your car would stall too.
So that's not likely.  ECU failure is even more unlikely.
If this bothers you try replacing the FP relay with a good one.

-Ti
-- 
    ///  Ti Kan                Vorsprung durch Technik
   ///   AMB Research Laboratories, Sunnyvale, CA. USA
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 //////  http://metalab.unc.edu/tkan/
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