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Just when I was about to give up and go buy a toyota I came across an article about open loop and closed loop modes.
This is what I found:
Closed loop mode means the engine control module (ECM) measures the A/F (air/fuel) ratio and uses this information to maintain the A/F ratio at a certain constant value. This mode uses feedback from an (oxygen) O2 sensor to close the loop. The A/F that the system tries to maintain is 14.7 to 1. Also, it just so happens that the standard O2 sensor is most accurate at 14.7 A/F ratio, which gives a good feedback signal.
The bottom line is that the ECM control logic tries to maintain a 14.7 A/F ratio during normal conditions. Normal conditions are a fully warmed engine and other than full throttle.
Open loop mode differs from closed loop in that the O2 sensor is ignored and the engine can be managed to run at A/F ratios other than 14.7, usually richer or lower than 14.7. The ECM controls the fuel injectors without getting any feedback that the calculated fuel delivery rate actually matches what the engine received. Without feedback, the loop is open, hence the term "open loop". A good example of open loop is when the engine is first started on a cold day.
My engine runs rough when first started until the oil temp reaches about 80 degrees, open loop mode. After that the engine runs ok except for the hesitation in third gear, closed loop mode
If an injector is clogged it might not be able to supply enough fuel causing a lean condition in open loop mode. In closed loop mode that same injector might still have trouble supplying enough fuel but the fuel feedback control system will compensate once in closed mode making the symptom less noticeable.
Tomorrow I'm pulling injectors.