Re: [eng] mixture adjustment & stumbling

[Brett Van Sprewenburg <brett@netacc.net>]

Foxx has an 8v, and does not have a differential pressure regulator
like the 16v's.  My best understanding is that an exhaust gas analyzer
or a way to measure the dwell or duty cycle of the lambda sensor at
the test harness location is the only sure way to set the mixture.
See the Bentley, or the Bosch fuel injection book for more particulars.

On the specific mention of the what the current readings mean below, it
is incorrect.  When a meter is hooked up inline to the DPR, it is measuring
the amount of current being fed to the DPR by the lambda (o2) computer
as a correction to get the engine to run as close as possible to
stoichiometric (14.7:1) at idle.  When you twist the CO adjuster clockwise,
which richens the mixture, you'll see the mA readings go _down_ on
the meter.  This is because with more mechanical enrichment 'dialed-in'
to the system, the O2 / computer sniffing the exhaust must make a smaller
correction to get the car to run at stoichiometric.  Think of it as if
you're not directly affecting the mixture because the computer is always
fixing it for you back to as close to stoichiometric as it can get,
all you're measuring is the amount of correction the computer is
applying at idle.  It took me a while to understand this concept, and
how to reconcile it with the fact that when the computer wants to make
the car run richer, the mA jumps to 21mA+, but think about it a bit and
it'll probably come to you. :)  When that happens, the computer is
essentially ignoring O2 input, and enriching the mixture a fixed amount
(21mA or so) on top of what ever mechanical fuel 'curve' has been
dialed in with the CO adjustment at the fuel distributor.  Imagine that
the DPR can only do one thing, enrichment by
adding more fuel and you'll start to get it.  Yes, the _engine_ can
still run lean under certain circumstances, but the only thing
the CIS-E system can do about it is either add more fuel, or add less
fuel on top of what the mechanical fuel distributor system is already
putting in.  This is _why_ adjusting that CO screw by such a tiny
amount can affect the engine throughout it's operating range, as it's
the base fuel setting or curve upon which the computer & DPR tweeks the
mixture based using internal RPM tables and O2 sensor input.  Anyway,
the intricacies of how it really works is pretty cool. :)

==Brett


>To Adjust the Co mixture I used a multimeter and hook it up in series with
>your differential presure regulator and the, then start the car.  Leave the
>O2 sensor hooked up. The meter should  be oscillating between 4 to 6 milli
>amps, averaging 5 milli amps, after a minute or two.  This is the stock
>setting.  If it is more current then it is too rich and if it is less
>current then it is lean.
>
>More info can be found at   http://www.iders.com/alexweb/vwtuning1.html
>
>Robert
>----- Original Message -----
>
>  i adjusted my mixture the other night as per a vw mechanic:
>disconnected the oxs, adjust to rich side until engine started dying,
>adjusted to lean side until engine started dying, set mixture screw in
>the middle of those two setting and hooked back up the oxs. now i seem
>have a stumbling feeling about 2k-3k, when i shift from 3rd to 2nd going
>around a corner. i cannot say this for sure, though, as i only drove it
>once. also, i have some rough idling when i first start the car up. i
>think my idle might be set too low. yes? the only other thing i did was
>replace the timing belt and pulley when i replaced the motor mount. we
>lined up the marks on the crank with the other pulley and the camshaft
>in-line with the valve cover, as per bentley. ideas? thanx.
>
>--
> /\_/\   Foxx (in a box)

 \/  '84 Scirocco (ITB racer 2B) | "Hot VW's, take two home. They're small"
\/\/ '88 Scirocco 16v (Show), '92 Passat 16v (Winter+) | - brett@netacc.net

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