fuel questions

[quk@sievers.com (Phil Payne)]

In message <199704092147.RAA18701@hammurabi.nh.ultra.net> Huw Powell writes:

> >But it fixed the bulk of the problem.  Now I would like to establish a
> definitive metric for my "old" and "new" valves - so that I can recognise
> the condition again.
> 
> Measure the control pressure being sent back to the fuel distributor, from a
> dead cold start.  It should be about 10 psi and then rise to about 50 psi if
> it's working properly.  You'll need the Bosch tool for this.  handy
> elsewhere in the fuel system, too.

No, no.  This is the whole problem.  On the MB, the "warm-up regulator" is just 
that - a warm-up regulator.  Charles Probst (in his otherwise excellent book) 
calls it a "control pressure regulator" but that is NOT its function on the MB.
The MB has a _separate_ control pressure regulator (that also functions 
as a system pressure regulator) mounted behind the metering head.  It's the 
lack of metrics for _this_ device that bothers me.

> >How can the duty cycle be adjusted?
> 
> Unless your car is really weird (and we know it is!) use a 3mm allen key in
> the little hole between the fuel distributor and the rubber ribbed air
> thingie.  You are adjusting the overall mixture by doing this.  CW = richer,
> CCW = leaner.  I'm not sure if this is how yours will work, since in our US
> cars this interacts with the OXS to maintain appropriate mixture.  The duty
> cycle just represents the computers response to the system, and at idle is
> usually set for about 50% which I presume gives it plenty of room to go
> either way under load conditions.

No OXS.  

> One is actually supposed to set the mixture screw using a CO meter on the
> exhaust.

I do.  I use a Gunson Professional, which works on the thermal conductivity 
principle.

> BTW by changing the screw you are raising and lowering the pivot of the air
> flow plate, which changes the amount of fuel flowing relative to the amount
> of air passing the plate.  Then the duty cycle changes to compensate and
> return the system to the correct mixture.

But this only aligns the curves at idle.  Once the engine leaves idle, the 
enrichment curve is dependent on the shape of the air mass sensor cone, what 
the computer does, and what the control pressure regulator does.  I repeat, on 
the MB, the latter is separate from the warm-up regulator.

--
 Phil Payne
 phil@sievers.com
 Committee Member, UK Audi [ur-]quattro Owners Club