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HondaDustR HondaDustR is offline
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Join Date: Jan 2007
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Open loop mode is where it reads straight off its fuel delivery maps as a function of rpm, air flow meter position, coolant temp, and air temp. There is no feedback loop as the O2 sensor circuitry trims the injection duty cycle depending on whether it is too rich or too lean. The problem is, it is a rudimentary system tacked on after the fact. the DME was not originally designed for it, and it was added on to allow import to the US. It does not constantly monitor the O2 sensor signal, it only triggers an interrupt when the sensor signals too rich or too lean, meaning the mixture constantly oscillates about ideal. As the sensor response speed slows with age and the range reduces, the mixture can vary more wildly and end up offsetting too rich as the sensor sensitivity declines. Unplugging the sensor puts the DME back on track.

Unplugging the O2 sensor is also a useful troubleshooting trick, as a properly functioning sensor can mask other problems such as dirty injectors and vacuum leaks. If the engine doesn't run well without the sensor plugged in, something is definitely wrong.

Another interesting tidbit...the DME also goes into open loop mode at wide open throttle (assuming your throttle switch is working and adjusted correctly), but WOT open loop mode also disregards the airflow sensor as well. If you have any vacuum leak problems that will cause the AFM to read low, you'll notice the car runs poorly at part throttle, but wakes up and runs fine if the throttle is floored. This reaction will also diagnose a defective O2 sensor. If the car still runs badly at WOT and you've confirmed the WOT switch is properly activating, then the problem can be narrowed down to fuel pressure, crank sensors, fuel injectors, or ignition (or wiring to those systems...), as o2 sensor problems, vacuum leaks, and air metering issues are irrelevant in WOT mode.
1987 silver 924S made it to 225k mi! Sent to the big garage in the sky

Last edited by HondaDustR; 11-12-2012 at 03:16 PM..
Old 11-12-2012, 03:11 PM
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