[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.
Keith
Topnotch Restorations
topnotch at nycap.rr.com
http://www.topnotchresto.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
the
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
few
hours.
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
cover,
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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