[T3] Why is it that when I got my 73 SB I never had to turn thekey several times to prime the system .
Jim Adney
jadney at vwtype3.org
Sat Aug 5 09:30:39 PDT 2017
On 4 Aug 2017 at 11:26, William J wrote:
> The so called check valve if it fails one way no fuel output and the other
> way it won't hold pressure in the system after shut down . In the later this
> means to me if it won't hold pressure after shut down this would be every
> time the car is started and shut down one would need to cycle the key even
> if it were a short trip
The purpose of holding the pressure after shutdown is to keep the gas from
boiling in the heat soak after shutdown. It's just like a 15 psi radiator cap on
a watercooled car: The boiling point of any liquid increases at higher
pressure.
> There was some spec on how long the pressure would hold if all if working
> well wasn't there?
Yes, that was twofold: to check for leaks and to maintain the higher boiling
point until the heat soak after shutdown was over.
The following 2 paragraphs are important to understand.
Pressure in the line before starting is not necessary to make starting easy.
What IS necessary is for there to be fuel and no vapor in the lines. If there is
ONLY fuel in the line when you turn the key, the pump will bring the line to
pressure in a small fraction of a second, probably before the starter can get
the engine up to full cranking speed.
If there is vapor or air in the line, it can take a long time to build pressure,
and that may still be vapor pressure, meaning no fuel and a lot of wasted
cranking before the engine has a chance to start.
> what I still can't get past is why I never had this hot fuel soak and vapor
> and never needed to cycle the key before.
I don't know. It could be that your check valve has started to leak, or it could
be that you have other small hose leaks, or it could be that you have an
injector with a tiny leak, or it could simply be a small change in the fuel that
has lowered its boiling point. There's very little elasticity in the fuel ring,
especially once the hoses get old and hard, so it probably only takes about 1
cc of fuel leaking out to drop the pressure to zero.
[Note that later FI systems incorporated an accumulator, which allows much
more leakage before the pressure will drop. I've considered adding one, but
the priming switch is such a cheap/simple solution that I don't think it's worth
it.]
> As it is now the car can sit after hot shut down over night and will start
> in the morning yet I don't drive everyday . I can drive it and shut it down
> and it will fire right up even after a 10 mile trip in hot weather after
> sitting over an hour. I can drive a mile or more and it always starts . I've
> driven it 4 miles 4 starts park in a hot garage and then get in and it fires
> right up.
This seems to contradict your previous statements that you now have to
prime the system with multiple ON/OFF cycles of the key. Now I'm confused.
> I have a momentary contact toggle switch rated at 25 amp figured I'd just
> wire it fused side of a hot all the time fuse tied to the fuel pump hot
> lead where it connects to the pump relay.
The pump normally draws 2-4 A, so that switch will be more than enough. I
like to connect the switch across the relay so that it uses the same fuse as in
normal operation. So you turn the key ON, hold the switch until you stop
hearing bubbles coming up in the tank, and then start the engine.
--
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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