[T3] That Fuel pressure regulator

Keith Park topnotch at nycap.rr.com
Tue May 17 17:57:33 PDT 2022

  Well I think Jim finally figured this out!
This is for the Squareback, but it has the 2056 in it now, with stock Djet

Yes, adjustment back where it was when I was done, and what you surmise
makes sense with the much higher pressure on a hot start, as the entire
thing heats up without fuel flowing thru it
Then when the flow starts, it cools and the pressure drops a bit.

This is the same car, but the FI lines have been changed a couple times in
the past 25 years so...  

With the first regulator I don't think I ever messed with it, it happened in
North Dakota and when I got to st Cloud that night, I just changed it out
with the spare.

It think this mystery is finally solved!  Go Jim!  As we never did find a
problem with that first regulator after it was cut open....

Now, the only problem I could think with a hole is that if the diaphragm
ever fails, it would spray gas onto the hot engine, but I don't think ive
ever heard of one of those diaphragms failing.


On 16 May 2022 at 20:17, Keith Park wrote:

> Took a trip to PA last weekend, it's a 1 tank drive and I noticed on 
> the way down the fuel pressure was up at 31-32PSI, not the 29 it was
calibrated to.
> When I did a hot start, it would go up to the mid 30's or higher, then 
> wander down to 32 or so.  This is exactly what the last pressure 
> regulator did on a long trip about 25 years ago, and Jim dissected 
> that one finding no issues.

Not sure which car/engine this is, but 29 is correct for '68-9, but the
later cars tend to run 30-31 psi.

> So I got down there, adjusted the pressure back, which is did, then 
> raised it a bit to get it back to 29 and it took almost the half turn 
> or so It took to get it down, then it was fine, 29, and stayed that way
all the way home.

Are you saying that you think the adjustment ended up pretty much back where
you started?

> Without the gauge, I would never have known except for a little more 
> hunting around with little or no throttle on a coast, it does that 
> when its running rich, and of course. I lost 2MPG on the trip down, 
> which hurt with gas so obnoxiously high.

Since you're the only one with a dash gas pressure gauge, it's possible that
this is something all our cars experience. I have no way of knowing. But if
your tweaking seemed to stop the pressure creep, then it seems like there
must be something odd going on.

> I still wonder what the mechanism is that's causing the issue?  And of 
> course, how long it will be before I have the issue again??

Have each of your experiences been with the same car? If so, could there be
something in the gas lines that's coming loose and blocking the port in the

It's possible that this is some temperature dependence. There are two
chambers in the regulator and gas only flows thru one of them. The outer
chamber, the one where all the adjustment mechanism is located, only has air
in it. The inner one has gas flowing thru it.

I was SURE that the outer chamber had a tiny air vent hole on one side,
pointing down on the Type 3 regulator. If that hole got blocked, allowing
air to pressurize that outer chamber, as the trapped air warmed and
expanded, that would increase the gas pressure by the same increase. I can
even imagine some kind of insect making a home in that hole, blocking it.

But when I went downstairs to check on the size of that hole, there's NO
HOLE on any of the four pressure regulators I have here. That means that the
only thing that keeps the outer chamber pressure at ambient is leakage thru
the adjusting screw threads, and those threads clog up with corrosion over
time. This explains why Keith's regulator "recovered" after adjusting down
and back up.

Now we're getting somewhere.  

If that outer chamber can't come to ambient pressure quickly, let's think
about what could make that pressure deviate from ambient. Temperature would
have an effect. Changing elevation would have an effect. Any fuel leak thru
the diaphram between the chambers, even tiny, would have a large effect, as
the fuel evaporated. Even barometric pressure changes would have an effect,
although those would probably be too slow to notice, unless the thread
blockage was complete.

Okay, I think that outer chamber needs a vent hole, so I tried drilling one,
just to see how hard it would be. Turns out that the steel is soft, so it
was easy to do a 1/16" hole with my drill press with the regulator held in a
This would not be easy if you're trying to do it freehand or in situ.

My future builds will all get this mod. I can do it cheaply for anyone who
wants to send me their regulator. If you've got your engine out or apart,
this is a good time to do this, even if you aren't aware of a problem.

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

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