[urq] suddenly lean - the saga continues

Louis-Alain Richard larichard at plguide.com
Wed Feb 14 15:43:32 EST 2007

Hi Keith,

Two or three things I can think of:

- something moves and let air in (oil fill cap, oil dipstick, rear main 
crank seal, ) and then is replaced by vacuum

- have you tried to push on the air meter flap, to see if it sticks 
down there as when you close the throttle rapidly ?

- check the air inlet of the turbo, the little 90 degree rubber elbow. 
Mine was cracked and it was difficult to see because it is so deep in 

- have you removed the air filter, to see if there's no paper sheet 
that gets suck and blocks the air filter and then drop back ?

- is your throttle cable and throttle body OK ?

- finally, it could be ECU related, because the ECU controls the 
ignition, and if the ECU is having strange inputs (engine revs, 
throttle position, vacuum/boost signal), it can retard ignition quite a 

And don't be so sorry, at least you're providing us some food for 
thoughts in this real quiet period !


Le 07-02-14, à 14:57, SpotatAshleys a écrit :

> 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
> 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
> _______________________________________________
> Audifans urq mailing list
> Send posts to: mailto:urq at audifans.com
> Manage your list connection: 
> http://www.audifans.com/mailman/listinfo/urq
> Have an urq question?  Check the Audifans Knowledgebase!
> http://www.audifans.com/twiki/bin/view/Audi/UrQuattro
> Have an urq answer? ... Please help others by adding to the KB ... all 
> contributions welcome!

More information about the urq mailing list