[urq] suddenly lean - the saga continues

[Jim Haseltine]

Keith,

I had something similar with my MB a year or so ago - didn't matter how long 
I spent getting the mixture and idle speed correct, once I revved the engine 
the mixture would be all over the place when it settled back down to tick 
over. With the help of a very friendly tester I managed to get it through 
the emmissons part of the MoT (set it, set it, don't touch it, test it 
quickly) but still couldn't get it to idle correctly. Gave in eventually and 
exchanged the metering head for a rebuilt one - it's perfect now.
Regards,

Jim Haseltine

----- Original Message ----- 
From: "SpotatAshleys" <spotatashleys at hotmail.com>
To: <urq at audifans.com>
Sent: Wednesday, February 14, 2007 7:57 PM
Subject: [urq] suddenly lean - the saga continues


> 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
>
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