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Old 04-12-2001, 08:26 AM   #23
edekker
Scooby Newbie
 
Member#: 143
Join Date: Aug 1999
Location: Kanata, ON, Canada
Vehicle:
1999 Impreza 2.5RS
Supercharged - and STOLEN

Post

NebScoob,

An interesting observation can be made if you have a fast-acting, LED-ladder A/F meter (non-numeric, non-LCD display) connected to the front O2 sensor (the reading of rear O2 sensor is too sluggish). You will be able to identify the feedback mode the ECU runs in, by observing the behavior of the A/F indicator. A closed-loop ECU mode is confirmed whenever the A/F meter puts on show of that playful dance around the stoich – indicative of a closed-loop system behavioral response. The ECU is likely running open-loop whenever the A/F indicator remains fixed at a certain position along its scale (or may disappear altogether).

At around 4000+RPM and higher, the A/F indicator switches, from its playful dance around the stoich, to a staid position near the stoich (or slightly higher) and remains there as long as the revs are kept that high. It will do this even on the lightest of throttle positions, in any gear. When the throttle is made heavy enough (~50% or greater), the indicator jumps rich (ECU recognizes a WOT or near-WOT and adds fuel).

One of the advantages (and there are several) of having a knock sensor is that it allows the ECU to take advantage of aggressive timing to make more power than it could have otherwise. But it can only do this for the range of RPMs through which the knock sensor can be depended upon. As the RPM goes higher, the will be a point where engine noise become too substantial. The noise floor of the knock sensor will have risen too high, making it difficult to discern actual knock signal from the noise (among where the knock signal is buried).

If that’s true, the ECU must enter a different mode of operation, most likely one that utilizes conservative power-robbing timing (to avert detonation, etc.), which doesn’t depend on the knock sensor for ignition advance. And, from what I had observed, chances are the ECU runs open-loop as well (not necessarily meaning it ignores the MAS or MAF however).

Ed.
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