[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]

On 3 Aug 2017 at 21:44, William J wrote:

> Why would VW make a system that would force the heated fuel back from
> the lines through the pump leaving you with just vapors. 

The pump's internal valving includes a system that relieves overpressure. It's 
there in case something goes wrong in the lines or the pressure regulator 
that causes the pressure to skyrocket. The part that releases at 18-22 psi 
must have been designed to be above the pressure that Bosch expected to 
see when 1968 gas boiled.

Gas has changed since then.

> Mine never had any sort of covering over the fuel lines.

It did when it was new. That covering got thrown away the first time the 
hoses were replaced, which certainly happened well before you bought the 
car.

> What's the point of the check valve if it does not stop the fuel from
> going back through the pump. 

So that it can relieve overpressure, as I explained above.

> I think the check valve has a small spring under the cover and the
> check valve if I recall is part of an internal passage from the
> discharge port and the return port . Does the valve just regulate how
> much fuel is feed to the engine as in to reduce pressure on the pump
> so it does not work so hard? 

Under that cover there are a spring, a special washer, another spring, a 
piston with a rubber tip, and an O-ring (to seal the cover.) The piston moves 
back to open the outlet once the pump pressure exceeds ~20 psi. It does not 
regulate flow. The piston moves back farther, allowing the gas to return to 
the tank, if the pressure exceeds something like 50 psi, to relieve 
overpressure.   

>  I never has the other issue I've read people talking about on samba when if 
> you drive till hot then park for an hour or so the engine runs a bit rough 
> because the head temp sensor banks heat and needs to cool down .

That's possible, but it's also possible that the roughness is from vapor 
bubbles in the gas that have to be cleared out before the engine can run 
smoothly. We're all guessing here.

> Perhaps it's the unleaded ethanol  diluted fuel we have now yet we've had 
> this garbage well before I had this issue the fuel didn't have quite the 
> amount ethanol in 2006 or 2009 .

Any change that reduced the boiling point would make this problem worse. 
Ethanol is just one of the suspects.

>  Just one more bit . When I got this car in 85 it had a pressure sensor with 
> the proper VW # according to Russ a chart he had , at that time I bought a 
> trashed 72 Cailf auto trans car and stripped it and since it had a new 
> pressure sensor I had one zip tied to the other so I could swap the plug 
> because I could not pass smog and a new part seemed better than old . It was 
> not even close to the same VW # yet both only had the one screw not like the 
> early ones with 2 adjustments and with either the car ran perfect and had 
> the same smog results on our shops dyno. It failed the idle test by a hair. 

Because the D-jet system was used by many auto makers from '68 to '76, 
Bosch made dozens of different pressure sensors for different cars and 
different years,  There were 4 different ones used on Type 3s alone. '68-9 
had a single adjustment, '70-1 had 3 adjustments, and '72-3 (2 different part 
numbers) had a single adjustment.  

Trying pressure sensors of random part numbers is just a shot in the dark: 
You have no idea of what the overall outcome might be, and while the 
outcome at idle might seem good, the results under load could be anything 
from extreme pollution to destruction of the engine, including the remote 
possibility of a great middle outcome.

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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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