[urq] suddenly lean - the saga continues

[SpotatAshleys]

Yes,yes,I'm tired of this too. Very tired but once again I need help. The "suddenly lean" symptoms have returned. Just to recap they are:

If I accellerate hard in any gear it pulls like a train and the air fuel
meter shows (as expected and desired) a rich mixture. When I lift off ,
change up and re-apply the accellerator the power has gone and the mixture
has gone lean. If I were to keep my foot down this condition would remain.
If I lift off and then reapply the accellerator then the power returns,
the mixture returns to rich and it would  continue to pull until I ran out of
revs. If I keep my foot down instead of changing up the mixture remains
rich so it does not appear to be a case of fuel starvation. (The sediment trap
upstream of the fuel filter is clear as can be anyway). I have removed the
"igloo" and looked at the airflow meter flap and there is no indication
that this sticks in any way - seems to be free to move. The injectors were new
about 2 years ago as were the injector seals. The injectors are held in
place with a steel injector bracket from a non-turbo coupe quattro and can
not move. I have not found any holes or splits in any inlet hoses. The turbo
seems in fine condition, never a hint of smoke and was checked for end play when
the engine was rebuilt about 3 years ago.
It only ever happens when warm but this is probably because I never accellerate
hard until it is properly warm.


I have been looking hard in to possible causes and the finger points at 6 possible components.
WOT switch
Vacuum divert solenoid
Thermo-pneumatic switch
CPR/WUR
One way valve
Metering head

I have checked the WOT switch - it functions as it should, triggering the vacuum divert solenoid at just after 50% throttle.
The solenoid works - I disconnected the pipes and with ignition on, the WOT triggers the solenoid and it diverts to purge the vacuum, causing the CPR to reduce control pressure and richen mixture. The diversion was checked by applying positive pressure and seeing if solenoid allowed the pressure to be diverted (in the right direction). It does.
The CPR/WUR was checked for continuity and to ensure bi-metallic spring heater received power. I am sure this works OK because the mixture leans off as the engine warms - as it should do.
The thermo-pneumatic switch works fine- tested.
The one way valve was removed and tested - but I had already ruled this out because as I understand it,if this failed and allowed movement of air in both directions the mixture would be rich all the time.
Finally I am confident there are no air leaks. (I re-set the basic mixture setting with the breathers disconnected and plugged. Upon reconnection no changes were needed to the basic setting which says to me "no leaks in the breather side of things".
Which leaves the metering head - which is just what I don't want it to be.

So before I set to and remove this and investigate - does anyone have any final thoughts as to a possible cause of these symptoms? Preferably a very simple and cheap solution.....

Sorry for the epic posting - (if having trouble sleeping, read it a second time!!!)
As always, any suggestions gratefully received.
All the best

Keith
'87 WR
'63 Daimler V8