[T3] That Fuel pressure regulator

Jim Adney jadney at vwtype3.org
Tue May 17 07:34:43 PDT 2022

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 vice. 
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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