[T3] Why is it that when I got my 73 SB I never hadtoturnthekey severa...

Jim Adney jadney at vwtype3.org
Thu Aug 10 15:17:24 PDT 2017


On 10 Aug 2017 at 13:04, William J wrote:

> Jim. I've got a question . Today I connected my fuel pressure gauge to check 
> the pressure , I use the line that used to go to the cold start valve should 
> still be the same pressure . It read 32 PSI because I was told a while back 
> raising it might help with the ethonal fuel here. I adjusted it back to 28 
> PSI and watched after shutdown it dropped to 22 PSI then after 5 minutes it 
> rose to 23 PSI then got just under 28 PSI then after 10 minutes it was back 
> to 26 . I started it twice with the same result . I've never seen it raise 
> and each time I only ran the engine 5 minutes just long enough to adjust the 
> pressure the engine was not hot yet the fuel line to the gauge was warm. I 
> checked the gauge and it was 22 PSI engine off just keyed on/off to build 
> pressure before I started it . I can't imagine it's the gauge because it 
> read the 22 PSI priming and when adjusting the regulator it rose and fell 
> and held steady.

It's interesting that you see the pressure drop, then rise, then drop again.

I'm only guessing, but I'd say that what's going on is that when you shut off 
the engine, the valve is already open, so the pressure can drop right away to 
the point where the valve can close (~22 psi.)

Once it's closed, with a hot engine, the fuel in the line starts to heat up and 
somewhere it begins to boil. This will raise the pressure, but if the valve is 
the slightest bit sticky it takes a bit more pressure (~28 psi) to open it again, 
but it only opens a tiny bit, letting gas leak back to the tank slowly.

Eventually all the gas that can get hot enough to boil has done so, and all 
the gas that needs to be pushed back to the tank has done so. At that point, 
the pressure falls to the point (~22 psi) where the valve can close completely 
again. This is the point where the engine temp has fallen below the boiling 
point of the gas.

Now the boiling has stopped but you still have hot fuel vapor in the line. As 
the engine cools the vapor also cools, allowing the pressure to fall more. If 
there are ANY leaks in the lines, valves, pressure regulator, or injectors, this 
will cause more rapid falling of the pressure.

Note also that when you attach a gauge to the system, that hose is initially 
filled with air. That air does not go away, because it's higher than the fuel 
ring, so it sits there, acting like a compressed spring on the fuel. This will 
distort the pressure readings you get over time. The steady state pressure 
you read, with the pump and engine running will be accurate, but the air 
bubble will keep the gauge reading from falling as fast as it would without 
the bubble.

You can fix this to some extent by bleeding the hose and the gauge, to 
eliminate as much of the air as possible, but this can be messy, and 
dangerous.

Here are the takeaways: 

The factory pressure for your '73 should be 30 psi; 28 psi was for '68-9. I 
would leave your pressure at 30.

What you're experiencing with your car is all perfectly normal for one of our 
cars. You've said that you tend to overthink things. This is an example of 
that.

Add the primer switch and stop worrying about it.

One more thing: If your pump relief valve is starting to get sticky, it MAY be a 
sign that your check valve is about to fail. It's a common problem at this age. 
If it fails, it will probably not fail while you're driving; it will fail when you've 
stopped and shut down the engine and then it won't want to start again. You 
can take your chances and live with this, or you can bite the bullet and send 
it to me to be rebuilt before it causes you a problem.


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