[urq] suddenly lean - the saga continues

[Buchholz, Steven]

... one more thing comes to mind ... check the intake plumbing around the airflow sensor plate to see if there is a path that bypasses the airflow sensor plate.  I know on the MC there is some sort of decel valve which bypasses the airflow sensor plate to cause it to drop to zero when decelerating to save fuel and improve emissions ... if this turned on at the wrong time it would cause air to bypass the sensor plate and screw up the mixture.  

I don't recall reading this as being done yet ... One thing you should be able to easily do is to monitor the control pressure when this happens ... the mixture is directly controlled by the control pressure, and the control pressure does get input from the manifold ... 

Steve B
San José, CA (USA)
> 
> ... ISTR mentioning this before, but since I'm not sure I'll pose it again.
> 
> The stock WX/WR has no means to measure airflow sensor position, and that
> isn't something that's going to be obvious from monitoring the system and
> control fuel pressures.
> 
> The thing I've been wanting to investigate is whether or not the fuel
> distributor/airflow sensor mounting plate on the WX/WR is similar enough to
> some other CIS Audi application which has the potentiometer on the pivot for
> the airflow sensor (which is used for the fuel economy estimation on the trip
> computer) to support a swap.  I've always figured the MC engine might be a
> good candidate ... some day I'll sit the plate from each application side by
> side to see how easily this would be to do.  With this you could know whether
> the sensor plate position was the difference between the two situations.
> 
> The other data that would be interesting would be to know the ignition advance
> angle in both conditions ... but, here again, I'm not sure how you can get
> that info easily from the WX/WR ...
> 
> When you shift do you know whether or not you're getting a big pressure pulse
> from closing throttle for the shift?  Some sort of impulse back into the
> intake might force the airflow sensor plate down and not allow it to recover.
> You could test that theory somewhat by leaving the clutch engaged but release
> and reapply the throttle as if you had shifted ...
> 
> Steve B
> San José, CA (USA)
> >
> > Hi Louis-Alain
> > Thanks for the speedy reply.
> > I feel fairly confident there are no leaks on the engine block/breather side
> > of things.
> > Re: the meter flap - this did not seem to stick at any point when I took the
> > igloo off recently to inspect - this was my first thought when this all
> > began and I have checked it several times since  - to be sure...
> > No problems in the air filter department - have checked that.
> > Throttle body and cable are fine.
> > I don't think it's ecu, I feel sure it is purely a mixture thing rather than
> > timing. The air fuel meter shows it to be a sudden leaning of mixture.
> > Now the inlet to the turbo is a good idea as is the turbo to intercooler and
> > I/C to throttle body hose. I am about to replace this latter hose and will
> > check all the others again - i.e. take them off and look not just a visual
> > with a mirror and torch.
> > Thanks for the suggestions.
> > All the best
> >
> > Keith
> > ----- Original Message -----
> > From: "Louis-Alain Richard" <larichard at plguide.com>
> >
> > 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
> > there.
> >
> > - 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
> > bit.
> >
> > And don't be so sorry, at least you're providing us some food for
> > thoughts in this real quiet period !
> >
> > Louis-Alain
> >
> >
> > 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
> > > 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