Subject: URQ freq valve trigger

[E. Roy Wendell IV]

On Oct 3, 2005, at 2:46 PM, quattro-request at audifans.com wrote:
> My new to me black #472 URQ 10V K26 is puzzling me a bit. The Frequency
> valve will not work consistently. I just spent the last week removing 
> and
> replacing the down pipe & 02 sensor but no change. The valve itself is 
> not  the
> issue. If I plug other known good units into the wiring harness they 
> don't  buzz
> either.
> Other than the 02 sensor and the ECU (BTW I'm using the one from my 
> old car
> that worked fine) what other items/sensors could be the culprits?
> If I unplug the 02, the valve buzzes and the car runs  crappy. 
> Plugging the
> 02 back in it shuts down again. The 02 I'm using  was the one from my 
> old car
> that ran great.
> TIA
>
> Dennis
> Denver

This one's easy. Your base fuel mixture as determined by the mechanical 
parts of the CIS system (pump, regulator, control pressure valve, fuel 
distributor, airflow plate) is either too rich or too lean to the point 
that the "frequency valve" can no longer compensate for it. The 
"frequency valve" is a variable duty cycle solenoid valve similar to a 
standard EFI injector that varies the pressure in the upper chamber of 
the fuel distributor in order to maintain the mixture near 
stoichiometric as determined by the O2 sensor. If the base mixture is 
too rich or too lean the valve goes to 0% duty cycle (closed all the 
time) or 100% duty cycle (open all the time). In either case it stops 
buzzing because it is no longer transitioning from closed to open which 
is what creates the vibration. You either have a fuel system problem, 
vacuum/air leak, or the CO adjustment on the airflow plate arm is off. 
This is also why the engine doesn't run very well with the O2 sensor 
disconnected. When the computer detects an open circuit O2 sensor it 
defaults to 50% duty cycle rather than getting feedback from the sensor 
and trying to compensate for the incorrect mixture. You have to look at 
the basic mechanical fuel injection system as the coarse adjustment and 
the O2 feedback system as the fine adjustment. If the coarse adjustment 
is too far off then the fine adjustment doesn't have the authority to 
overcome it and achieve the correct mixture. Since your engine starts 
well (which I assume it does as you didn't state otherwise) then it's 
probably running rich. If you don't have any other problems then you 
may simply have to tweak the mixture adjustment (CO) screw on the 
airflow plate arm. To do it properly you need a meter that reads duty 
cycle and aim for 50% with the engine warm and running. I've had very 
good success just turning the screw one way until the valve stops 
cycling, turn it the opposite way until it stops cycling again, and the 
set it midway between those two points. If you are on the lean side it 
won't want to start or will lose power under high load. It is also 
normal operation for the valve to stop buzzing during deceleration 
because the fuel injection system shuts off due to the deceleration 
valve, the mixture goes very lean, and the valve goes full open (or 
full closed, I can't remember which) in an attempt to compensate. When 
the rpm drops to just above idle, fuel injection resumes and the valve 
goes happily back to buzzing away as it controls the mixture.

Roy Wendell
Morgantown WV, USA
turbo quattro type 44 times 3