Date: Mon, 28 Apr 2014 13:52:32 -1000
Reply-To: "SDF ( Scott Daniel Foss )" <scottdaniel@TURBOVANS.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: "SDF ( Scott Daniel Foss )" <scottdaniel@TURBOVANS.COM>
Organization: Cosmic Reminders
Subject: Re: Digifant question...fuel pump effect?
In-Reply-To: <CAHTkEuLf-BC1wez-iyNPZ78XaOFSJWnhU4oO71ZxpQNmUoGRaw@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
ok, I'll bite,
and try to keep it simple.
on the 'hydrualic' part of fuel delivery and injection..
the fuel pump runs constnatly when the engine is running ( they are
suppossed to be wired such that if the engine stalls, like in a crash,
but ign is still on, fuel pump will stop, a safety feature. I keep
meaning to stall my various digifant engines on purpose to see if the
fuel pump stops under those circumstances. )
the pump can produce up to 100 psi if applied to a 'dead end' line ..
fuel pressure is regulated to around 30ish psi .....( see Bentley for
exact #'s ).
the fuel pressure regulator has a vacuum line going to the intake, so
..when throttle is opened more, vaccuum drops a little, and fuel
pressure goes up. It's a standard test to check psi with engine
running, and this vac line disconnected or not. See Bentley.
fuel is returning to the tank constantly ....lots .
It helps keep the fuel cooler in very high temps ( like doesn't get a
chance to heat up while it's in the lines on the engine ) ..
and to just stabilize fuel temp overall. ( super modern cars have
'returnless' fuel systems, btw ) .
so that's the fuel ..
running around and around ...at the right regulated pressure, and being
filtered too.
The ecu doesn't know 'anything' about the fuel btw..
doesn't know fuel temp, or fuel pressure or any of that ...
no can the ecu tell if say, fuel pressure is low . ( more modern
systems ...such as electronic tdi's ...have a fuel temp sensor so the
ecu knows fuel temp too. . ) Dififant is quite crude actually.
then ...
the ecu tells the injectors how long to be open in milliseconds.
more open time in milliseconds, the more fuel is inejcted.
On digifant the 4 incjectors are crudely fired as one 4 - part injector ..
and injection pulses are not even timed to valve or piston position .
...also quite crude.\
On a typical subaru engine ..the injectors are fired sequentially, one
at a time when the should be. The ecu knows cranshaft and camshaft
position ....quite 'smart' that way.
All digifant knows about engine position is that the engine is
turning ( by seeing the ignpilses from the distributor ) and what the
rpm is.
as for throttle position ..digifant is again amazinly crude ....having
only idle and WOT throttle swithces ..
the ecu can't even tell 'how fast' the throttle is opened ( the
increase in fuel pressure due to lower mainfiold pressure making the
fuel pressure regulator lower the fuel pressure helps though ) .
other inputs to help the ecu determin injector open time are coolant
temp, air intake temp ..
and flabber position in the AFM ..an electronic circuit there.... always
a weak area ....wherever something mechanical and something
electric/electronic interface is a weak spot ..switches are, AFM circuit
boards wear etc. The 'next up' and VASTly imporoved air measuring device
is the Air Mass Meter..
which is about 5 times superior to an AFM ..
there are no moving parts at all. Instead the ecu reads the change in
resistance of tiny wires in the Air Mass Meter's air flow..
and ...amazinly ...the ecu can 'see' differeneces in humidity and
barametric pressure ~!
also ...a digifant AFM flapper door is an obvious restriction point in
the intake air stream. ( a square hole to accomodate the flapper thing )
Air Mass Meters are far less restrictive ..are round inside ...just so
much better it's not even funny.
that's most of 'how it works' ...
there's more ...like electronically controlled air idle valve ...
not the subject here of course...
starting in 1990 this is what Subaru engines have that waterboxers don't ...
4 valves per cylinder.
distribuless ignition system
knock sensor ignition
an intelligent throttle switch
air mass meter
sequential fuel injection
'twin spray' fuel injectors ....the stream is divided slightly ...each
half aiming at it's intake valve
Vehicle Speed Input to the ecu
and OBD-II diagnostics ....which is amazingly intelligent ...........for
example...OBD-II can flag 'high current flow' in an injector ..
which is how I identified some stuck injectors on an engine
once.....OBD-I can't do that ...and Digifant doesn't have anything at
all for diagnostic codes.
Welcome to Old-Tech !~
it's a miracle they work as well as they do actually.
The ecu is mostly 'just faking' it compared to modern systems.
I really 'should do something' about a great facility full of vanagons
in southern oregon that I am under-utilizing currently.
On 4/27/2014 4:03 PM, Don Hanson wrote:
> Ok, dumb question here...
>
> My fuel pump has been really noisy for about 1500 miles now and I just
> replaced it with a new one...A quiet one...
>
> Now, with the noisy one, the van ran OK....no glaring issues with it's
> digifant inline four motor....I did 'sense' a slight hesitation as I
> accelerated from coasting...I think....and sometimes as I went from no
> throttle to throttle, like at the bottom of a gully, there was a big
> hesitation, like the motor was OFF for a second or two, then pretty normal
> again...
> I think this slightly "off" behavior has all gone away with the new pump
> installed..I could be imagining it but the motor seems to run better now.
>
> So I was under the impression that the fuel pumps in our digifant VWs
> supply way more than enough fuel to the injectors and some of it is always
> run back into the tank as extra......I assume the noisy old pump was
> supplying enough fuel...I drove back from California to Wa. with a 5000lb
> loaded van up highway 395, through the High Sierras without any
> issues....pump whining away! (spare one under the rear bench seat) But is
> it possible the noisy pump was making the engine run less perfect?
> I should know, but how does the digifant fueling system actually work?
> Could the old noisy pump be supplying inadequate fuel pressure and volume,
> just by a little? It seemed to not matter how much throttle I was
> using...the van ran fine, but just whining and noisy...
>
> Anybody want to give the short version of how this stuff actually works?
> I'm sort of familiar with Motronic, but I never bothered to do anything but
> drive my Digifant inline...I guess I should learn HOW it does it's thing....
>
> Thanks, Don Hanson
>
