Date: Mon, 5 Mar 2018 21:57:19 -0500
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Fuel pressure too high
In-Reply-To: <CAG12aivOsgYxSPUmCuOWu9E5x0osqwkawgBgggyuXL6=3Zdq=w@mail.gmail.com>
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>
> I hadn't thought of this since I don't really understand the flow of air
> going on. How does the charcoal canister come into play and when? Is
> this really where the tank is getting replacement air as gas is used? What
> is the role of the valve you mentioned, which I take it is that 'thing'
> dangling next to the canister itself with two small tubes connected to the
> engine?
Canister sits there on the tank vent line, with lets call it a hundred
acres of surface area on the activated charcoal. Vapor comes off the fuel
as the tank warms up etc, and sticks to (technically, adsorbs onto) that
enormous surface area instead of passing to the outside.
When you run the engine -- as soon as you move the throttle plate away from
the rest position it uncovers a vacuum port which, working through a tiny
(.006") orifice, starts to pull open that vacuum-operated valve you
mentioned. Once the valve is open, it allows the intake plenum to pull
vapors out of the canister by pulling filtered air over the charcoal bed
and into the intake. On the 1.9l there's a 1.4 mm orifice limiiting how
fast it can pull. On the 2.1l they just use skinny tubing. If that .006
orifice in the valve line is missing, so the valve slams open, I expect it
could cause stumbling off idle.
At the same time enough air passes into the tank to make up for the change
in volume as fuel is used. If it can't, the tank may collapse under the
vacuum, and at some point the pump will start cavitating and making a high
hiss/whine noise as the bubbles collapse; and at some further point if the
tank is strong enough the pump won't be able to get suction and fuel
pressure in the distribution line will fall.
> David, you may be out a nickel -- I can blow air through the return line --
> usually. Strangely, there have been times when I've pulled off the road
> because the engine is stalling, pulled the return line off of the pressure
> regulator and tried to blow through it only to find that very difficult,
> almost impossible. (Which at the time made me think I'd found the
> problem.) But at home, engine cold, I can blow air though no problem and
> hear it bubbling in the tank. And I've traced the line from back to front
> and can see no kinks or other obvious damage. Could there be something
> inside the tank that is creating some sort of intermittent blockage?
There shouldn't ever be a time when you can't blow back through the return
line. The fuel pump circulates a minimum of sixty liters per hour of fuel
through that line, with the engine sipping from the stream as it goes by.
So I think I'll at least claim time-sharing on that nickel.
Normal pressure, controlled by the regulator, is 36 psi (2.5 bar) over
whatever the manifold pressure it. That means that since the injectors
have one end in the fuel line and the other end stuck in the manifold,
there should always be a constant 36 psi pressure difference across the
injector valve, so that the ECU can count on getting x milligrams of fuel
injected for every y milliseconds of injector opening, regardless of engine
load, throttle opening, rpm etc. If the outlet is blocked, line pressure
will rise to something like 100 psi (over ambient air pressure, not
manifold pressure). Further rise will be blocked by a spring-loaded
overpressure valve inside the pump that will recirculate the pump output
internally. That will cause the injectors to deliver roughly three times
as much fuel as the ECU is expecting. If you're in closed-loop mode, the
feedback from the oxygen sensor will cause the ECU to adjust the mixture
leaner over a considerable range, so in closed-loop operation it might not
make a lot of overall difference. I haven't tried it so I don't know for
sure. ECU has considerable range in the richen direction, not sure how
much in the lean direction.
Yrs,
d
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