Re: Fuel mixture on 5KTQ?

[paul.heneghan@bbc.co.uk (Paul Heneghan)]

From: ampj

>> All your mechanic has done (probably) is compensate for the increased air 
at
>> idle by turning up the fuel.  At high revs, the ratio won't be right.
>> Ratio can be adjusted using long 3mm Allen key at CO adjuster in air flow
>> box.

>    Does a clockwise turn add more fuel like I would presume, or is it
>backwards like most things in my Audi! ;-)  Also, where is that
>confounded 3mm hole in the fuel distributer? That missing hole location
>will bug me forever even if I don't buy a long 3mm Allen. And although I
>don't own a 3mm yet, every time I'm in the engine bay I still like to
>'check' it, or more appropiately look for it, to make sure it's still
>there even though I have NO IDEA where it is! :-) So please help my poor
>lost soul! :-)  Thank you! Allan

Find your air flap/fuel distributor.  It appears to be in two halves.

The air flap half (usually at the back) is covered by a black rubber hose 
type thing about 6" in diameter with waffle type patterns on it - this 
routes the air (after the flow has been measured) to the inlet manifold.

The fuel dist half (usually at the front) is a black (usually rusty on older 
cars) metal contraption with lots of fuel lines bolted to it.

Between the two halves is a small vertical tube with (hopefully) a cap 
(yellow or blue or in some cases a metal plug) in it.  This cap has not been 
present on any of the four cars with Bosch FI that I have bought - this 
allows unmeasured air to get into the system - not a good idea - obviously 
many mechanics who work on these cars haven't a clue about Bosch FI.  If you 
remove the cap, and insert a long 3mm Allen key, you can adjust the position 
of the fuel control arm relative to the air flow measuring arm.  This 
adjusts the mixture.  DON'T REV THE ENGINE when the Allen driver is inserted 
 - I think it can put strain on the bearings (I've done it once or twice by 
mistake, and haven't noticed any ill effects, but all the manuals carry this 
warning).

The mixture is normally set to give you something like <1% CO content at 
idle (this figure varies between different models and different countries). 
 If the mixture is too rich, then you lose economy and power.  If it is too 
lean, then you might get overheating, lack of power and shorter life of 
spakplugs.

If there is an air leak (split hoses, hard injector o-rings etc.), then 
extra unmeasured air gets sucked into the system and makes the mixture too 
lean.  Most mechanics don't bother to find out why the mixture is now wrong, 
they just enrich the mixture by raising the fuel control arm relative to the 
air flow measuring arm (from memory, I think the adjusting 3mm Allen screw 
is turned clockwise to do this).  This means the mixture is correct at idle, 
but is now wrong at other speeds/engine loads.

Don't try to adjust the mixture unless you have an exhaust gas analyser, or 
at the very least, a colourtune sparkplug kit.

Paul
paul.heneghan@bbc.co.uk
1984 Audi 80 quattro
1989 Audi 100 Avant