[alfa] re no more than 5000 rpm

[Joe Elliott <jee@xxxxxxxxxxxxxx>]

The fuel pressure goes up not because of the increase in RPM but 
because of the increase in throttle opening.  There is a vacuum line 
that runs to the fuel pressure regulator, and as the vacuum 
decreases, it increases the fuel pressure.  This is done for two 
reasons.  For the fuel to leave the injectors at a consistent rate, 
the pressure DIFFERENCE between the fuel rail and the plenum has to 
remain constant.  However, the air pressure in the plenum is not 
constant, because with the throttle closed, it's much lower than 
atmospheric.  That's the main reason that fuel pressure goes up with 
throttle opening--to keep the pressure drop across the injectors 
constant.  The other reason is the same concept as the "accleration 
pump" in a carburetor--it can be beneficial to enrich the mixture 
slightly upon rapid throttle opening, but the L-Jetronic's "software" 
doesn't take this into account.  The fuel pressure regulator, 
however, does, as you've observed.

As for isolating a potential electrical problem, I guess you could 
try measuring the voltage either at the battery or at the terminal 
block on the left front fender, but the voltage could be just fine, 
and a dirty connection there could still be your problem.  I suppose 
the real test would be to put an ammeter between the alternator and 
that terminal block, but I'm not entirely sure what to look for. 
Maybe if the current never increases above a certain value, you know 
you have a problem somewhere that could prevent the ignition from 
working above a certain engine speed?  My approach would probably be 
to check every connection and make sure it's not dirty, and bypass 
the ignition switch to eliminate that variable.

Joe Elliott
'82 GTV-6


At 2:18 AM +0000 9/6/04, alfa-digest wrote:

> Date: Sun, 5 Sep 2004 19:17:00 -0700 (PDT)
> From: Alan Lambert <lechefgerard@domain.elided>
> Subject: [alfa] re re re no more than 5000 rpm
>
> Hello everyone--
>
> First, a non Alfa question. If you look at the address portion of the
> header, I have sent this to two different Alfa Digest addresses. Are
> they both good? Or is there a preferred one?
>
> I have now checked fuel pressure up to and including the "cut-off"
> point of 5000 rpm. It's fine, nothing changes as the engine begins to
> sputter. I am curious, however as to why the pressure goes up from some
> 30 psi to about 45 psi as I rev the engine up. Is there some signal to
> the pressure regulator or pump that tells it to increase pressure as
> rpm's go up?
>
> It has been suggested that the problem may be electrical in
> nature--that somehow the drain on the electrical system goes up as the
> rpm's do and therefore we lose spark. How would I check that?
>
> Any other suggestions????????
>
> Thanks, all
>
> =====
> Best Regards,
>
> Alan Lambert
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