Hi David,

Just piggyback one of the injectors for the pulses as someone pointed out, 
and do some minor processing. That is basically what most OEMs do in their 
fuel computers. As you point out you cannot meter the fuel directly because 
of the return based system, the reasons for a return based system are 
simply because it is the simplest way to regulate pressure, not to cool the 
fuel or release bubbles as it actually has the opposite effect on both 
counts. Fuel recirculation introduces heat, not only from going through the 
pump, but also passing through the warm lines and fuel rail on the 
engine/in the engine bay. If you have major bubbles(large enough to affect 
A/F) you've got other problems, the fuel system should never pickup that 
much air during normal operation, nor should it reach temps where gas 
leaves the solution.  On the Zetecs we can measure the fuel flow directly 
as it is a returnless system with the pump duty cycle controlled by the 
PCM, the problem then becomes accuracy of the fuel flow transducer, which 
isn't easy to find in the low rates of consumption we're talking about 
without spending huge money... so you're better off finding an injector 
based solution anyhow (if someone isn't already doing it) actually, I'd be 
interested in doing it along with you since we need a solution for our trip 
computer option and it would be good to share some of the work and testing.

Jim Akiba

At 01:20 PM 12/1/2005, David Marshall wrote:
>Volks,
>This series of questions involves for me a Diesel TDI engine, but the same
>principals are involved with a fuel injected gasoline engine, so hence the
>cross-posting.
>
>We are going to be starting on a TDI conversion that I want to "kick up a
>notch" as far as instrumentation goes.  The eight function gauge that I have
>installed into our TriStar has the ability to use a few flow meter so that I
>can view the instantaneous amount of fuel being consumed by the engine.  The
>problem is I can't see how to connect the fuel meter so that I can get an
>accurate reading.  The meter is basically a little "box" with a fuel input
>and output on it and it will give an electronic "pulse" per every unit of
>fuel that flows trough it.  The gauge attached to it counts the pulses and
>determines fuel flow from the amount of pulses over time.  Now as we all
>know, modern Diesel and Fuel Injected gasoline engines has an input from the
>fuel tank and any fuel it doesn't use goes back to the tank.  As a result,
>the line from the tank to the engine doesn't show the real fuel consumption
>as there is always fuel going back to the tank.  A the to and from lines are
>never a constant difference ratio, the only thing I can see doing is loop
>the engine's fuel output into the input of the fuel pump instead of back
>into the fuel tank.  The main reason why we dump fuel back into the tank is
>it makes the system self bleeding system by removing air bubbles.  Air
>bubbles are bad for fuel pumps.  The other reason for fuel being returned to
>the tank, especially in a Diesel is it cools the fuel off as the TDI will
>give less power if the fuel is too hot - hence the fuel coolers on the pump
>d�se engines.  So basically I am wondering if there is any such thing as a
>"bubble trap" out there to prevent air bubbles from being trapped in the
>system and if there is a way that I am missing about installing fuel
>metering devices?
>
>For a picture of our prototype engine monitoring gauge see:
>
>http://www.fastforward.ca/gallery/1005instrumentpod.jpg
>
>Cheers!
>
>David Marshall
>
>Fast Forward Automotive Inc.
>4356 Quesnel-Hixon Road
>Quesnel BC Canada V2J 6Z3
>
>Internet: http://www.fastforward.ca
>Email: sales@fastforward.ca
>
>- Engine Conversions and Accessories for classic water-cooled Volkswagens
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