[T3] Why is it that when I got my 73 SB I never had to turnthekey several times to prime the system .

William J catnine09 at dslextreme.com
Sat Aug 5 12:37:06 PDT 2017


----- Original Message ----- 
From: "Jim Adney" <jadney at vwtype3.org>
To: <type3 at vwtype3.org>
Sent: Saturday, August 05, 2017 9:30 AM
Subject: Re: [T3] Why is it that when I got my 73 SB I never had to 
turnthekey several times to prime the system .


> 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.
>
   Jim I have a pertronix this is why I don't like to leave the key on . It 
may not hurt it few the time it takes to prime the system yet i rather not 
risk it.
> -- 
> *******************************
> Jim Adney, jadney at vwtype3.org
> Madison, Wisconsin, USA
> *******************************
>
> _______________________________________________
> VWType3.Org mailing list - type3 at vwtype3.org
> To unsubscribe or change subscription options, visit:
> http://lists.vwtype3.org/listinfo.cgi/type3-vwtype3.org
> If you need more help, contact: gregm at vwtype3.org
> 


---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus




More information about the type3-vwtype3.org mailing list