... Does a 0% duty cycle indicate any potential issues other than running very rich? ...
If you checked the Lambda control adjustment (at X11 across pins 2 & 3) with a voltmeter and read out “a constant roughly 14.1 to 14.2 volts“
(instead of measuring a duty cycle of 0% with a duty cycle meter), then:
- either the engine is running very rich
- or there‘s a problem with the ECU
- or there‘s a short circuit on the wire to pin 3 of X11
So should I adjust my Lambda control adjustment? I've previously been instructed by a mechanic to never adjust it as he has set it where it needs to be ...
Very good question … much better than rashly changing the Lambda control adjustment when the meter shows a too low/high duty cycle. … And I hope that the mechanic made sure that everything else was in order before he set it.
I suggest to check a few things that might cause the 14 volts (0% duty cycle) reading … starting with measuring voltage, with “igniton on“ (engine not running), between X11 pin 3 & ground … and between X11 pin 6 & ground. The quotient of the pin 3 voltage and the pin 6 voltage should be about 0.3. … If that's the case, the third of the above-mentioned possible causes can be excluded (and the second one is less probable).
The next thing I‘d suggest, is to measure the o2 sensor voltage at the (connected) o2 sensor connector under the passenger side‘s floor panel (green cable). ... 0.8 volts or higher indicate “rich mixture“.
If that‘s the case, I suggest to check the CSV (cold start valve) for tightness.
If the CSV is tight, I suggest to check the fuel pressure.
With your engine running at idle, SP (system pressure) should be about 5.3 - 5.5 bar (77 - 80 psi) and LCP (lower chamber pressure) should be about 0.4 bar (5.8 psi) below SP ! … SP higher than 5.5 bar (80 psi), or LCP more than 0.4 bar (5.8 psi) below SP are possible causes for too rich air/fuel mixture.
Let‘s see the results of these test prior to further suggestions.