Frequency valve life

[quk@isham-research.demon.co.uk (Phil Payne)]

In message <9709021944.AA07909@aztek-eng.com> DeWitt Harrison writes:

> Phil, I fear this will be a "coal to Newcastle" exercise, but ...
> 
> I believe the CO frequency valve - as used in the US on the late '80s
> MC engines to control closed loop fuel mixture in lieu of the  KE-III
> style differential pressure regulator built into the fuel distributor - to
> be a long lived part based on the infrequency with which my mechanic
> has had to replace them. I think these parts to be sturdy and simple
> much like the ISV.

OK.  We're partly there.  The differential pressure regulator is indeed 
missing from the metering head, and is essentially replaced by the frequency 
valve and its friend the external pressure regulator.  However, the MB engine 
has no emission control provisions and no OXS sensor.  The frequency valve 
appears to be the only way in which the ECU can influence the fuel injection 
system - in essence, it's the only actuator with which the ECU can respond to 
input from the WOT switch.

> In a different set of the Bentleys (for the 85-88 5000s),
> I once saw a way to make the valve buzz by initiating
> a diagnostic output sequence from the ECU. I will try
> to find this again if you wish.  For what is was worth,

Burnt into the brain.  Stick a fuse in the top of the fuel pump relay and 
switch on the ignition.  Pull the fuse for four seconds, and reinsert for four 
seconds.  Repeat.  It cycles the fuel pump, frequency valve, cold start valve 
and wastegate frequency valve.

It buzzes just fine.  I'd like to verify:
 
a) That the duty cycle changes under full load.
 
b) That this has some effect on fuel delivery.

-- 
 Phil Payne
 Committee Member, UK Audi [ur-]quattro Owners Club