I've said the same thing about duty cycle and apparently get ignored as I watch people spend lots of money replacing parts in order to get the duty cycle exactly 50% under any and all conditions. This is just not necessary because as long as the duty cycle reading is SWITCHING the system is capable of properly adjusting the air/fuel mixture. Does not matter if it is 30% or 80%. Fact is these cars are no longer brand new and they now have some engine wear and are less than perfect. Duty cycle on these cars is NOT an indication of air/fuel ratio. It is more analogous to what is called "fuel trim" on late model cars. It is an indication of how far in the adjustment range (either rich or lean) the engine computer has to be in order to get the stoichiometric (ideal) air/fuel ratio. Unlike late model cars, these systems have an adjustment for air/fuel ratio, so it is possible to put it within range even on an improperly functioning system. So, yes, it is possible to have the ratio correct at idle, but not at speed or vice-versa. But if the duty cycle reading is CHANGING/SWITCHING within its limits over the engine speed range then the system is able to control air/fuel ratio properly. It does not have to be at exactly 50% (or whatever target value) to be classed as properly operating. IMO some people have the mistaken impression that duty cycle represents air/fuel ratio directly when I have explained above that it does not. Proper duty cycle readings do not, however, rule out problems with the fuel system or base engine, so it is still necessary to be sure that the engine is OK (good compression, valves sealing, ignition system OK) and that the FD and injectors are operating properly before reading too much into the duty cycle adjustment and readings.

As far as your car goes, it is possible to have the adjustment turned too far in the rich direction that it is holding the FD plunger off of the rubber seal when the engine is off. This will cause hard starting when hot. That is easy to check; push down on the sensor plate and there should be a small amount of movement before it contacts the plunger on the FD. Now, if you have the mixture screw adjusted correctly but still do not have the required free play, then it will be necessary to remove the FD and turn the threaded ring around the plunger CW so as to move the ring (and rubber seal) upward. Then reinstall the FD and recheck the free play.

Vacuum leaks are fairly easy to find when monitoring duty cycle (fuel trim) by spraying carb cleaner and watching for changes in the duty cycle reading (that is if the duty cycle reading is switching to begin with).

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Erich Loepke
2010 Ford Focus
Currently Benz-less