[T3] Diagnosis help for a badly running 1971 VW 411

Keith Park topnotch at nycap.rr.com
Tue Aug 13 14:06:12 PDT 2019

I did, all T4's with D jet have the diaphragm in the pressure sensor.


Topnotch Restorations
topnotch at nycap.rr.com
71 Squareback  “Hothe”
65 Notchback  “El Baja Rojo”
93 RX7  “Redstur”
95 Chrysler Cirrus Lxi "Cirfogsalot"
"hanging out at the tail end of the bell
curve, and loving every minute of it!" 
-----Original Message-----
From: type3-vwtype3.org [mailto:type3-vwtype3.org-bounces at lists.vwtype3.org]
On Behalf Of Jim Adney
Sent: Tuesday, August 13, 2019 1:11 PM
To: type3 at vwtype3.org
Subject: Re: [T3] Diagnosis help for a badly running 1971 VW 411

On 12 Aug 2019 at 17:28, Jens Vagelpohl wrote:

> Maybe I am misunderstanding what youTMre saying, but how could a system
> controlled by manifold pressure go without that pressure sensor? How could
> you oeskip that?
> Needless to say, that engine has a manifold pressure sensor like all fuel
> injected Type 3s.

Yes, all D-jet engines have, and need, a pressure sensor. And all pressure 
sensors have a sort of bellows, more accurately called an aneroid, but of
6 FI years on Type 3s only the '70-1 pressure systems have that extra 
diaphram. Those pressure sensors are easily identified by the slots (or 
rarely, round holes) in the cover, that allow atmospheric air access to one 
side of the two-year-only diaphram.  

Sometimes that diaphram cracks and leaks air into the inner space where 
the aneroid lives, disabling the pressure sensor. When this happens, the 
pressure sensor aneroid is surrounded by (nearly) atmospheric air and 
thinks the throttle is floored. Thus it tells the brain to deliver max fuel
to the 
engine, which makes the mixture extremely rich, unless the throttle actually

IS floored. Usually, this causes poor running and fouls the plugs in just a

Since this issue concerned a '71 Type 4, it seemed possible that it might 
also have that style of pressure sensor, with the slots or holes in the
but I thought I'd heard someone mention on the samba that Type 4s never 
got this version of the D-jet system.

That's why I asked about the diaphram. I was hoping someone like Keith 
would know and respond.

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

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