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

[Jim Adney]

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.

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