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Originally Posted by nglitz
Excellent link, Duke. That adjustment would need to be made after the one I described. That one affects the mechanical operation of the fuel injection that's varying the upper chamber fuel distributor pressure. That adjustment is really only an idle adjustment. It does change the duty cycle, but only at idle. MB calls it the CO at idle adjustment. That's why the steel ball over the screw, to keep you & me out of it.  good idle quality generally means a slightly higher CO at idle. Big deal.
The part that the duty signal goes to is the EHA. The adjustment I described changes the lower chamber pressure in the fuel distributor. This is kind of an "over the whole range" adjustment. It seems to have the most effect on idle & just off idle operation. I'll bet there's an interaction between the two adjustments that the old-timers knew about, but never got written down. Ever get the feeling you're re-inventing the wheel? Or maybe just re-discovering it?
Mike, I just re-checked. It is a 2mm hex drive. It's the only adjustment on the EHA itself. FWIW, it's the second smallest of a Bondhus balldriver set. Smallest in my set is 1.5 mm.
BTW, it worked wonderfully. 90% improvement. Maybe another 45 degree turn this afternoon to make it 100%. |
I think you might have a misconception of how a modern 02 sensor fuel system works. Once the engine and 02 sensor are warmed up sufficiently to allow closed loop operation the system maintains an average stoichiometric A/F ratio except at WOT. A stoichiometric ratio is required to achieve maximum oxidation and reduction efficiency from a three way catalyst. The duty cycle wave form is generated by the ECU as a representation of how much time the system spends slightly lean of stoichiometric, and it should be in the range of 40-60 percent to allow maximum correction authority. If it is near zero or 100 percent the system will not be able to provide sufficient correction to maintain a stoichiometric mixture. When the O2 sensors reads lean the electronics orders the EHA to slightly richen. Then when the O2 sensors reads rich the electronics order sthe EHA to lean the mixture. This feeback control loop operates continuously other than cold start/warmup and WOT at a frequency in the range of 0.5 to 2.0 Hertz.
By using the mixture adjustment screw you are setting the basic mechanical calibration, which determines how the KE system meters fuel before closed loop operation is achieved, and the base setting that is corrected as necessary by the electonics during closed loop operation. Setting it at about 45 percent at idle will be slightly rich, and I have found this aids cold starting in mild weather. The procedure also calls for checking at 2000 RPM no load and both readings should be in the range of 40-60 percent IIRC.
Regardless of where you establish the basic mechanical adjustments, the electronics and 02 sensor will maintain a stoichiometric mixture during closed loop operation unless the mechanical adjustment is biased so far that the electonics runs out of adjustment authority, in which case it will very likely bust an emission test big time.
Duke