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Old 02-17-2016, 01:31 PM   #59 (permalink)
Calibration Engineer
Join Date: Jan 2010
Location: Canada
Posts: 86

Subie - '00 Subaru Impreza STi JDM
90 day: 22.49 mpg (US)
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Originally Posted by jaysittinback View Post
With the TPS showing "closed" the IAC valve is most responsible for high idling, not increased injector output. It would have to get very lean before the O2 sensor would tell the ECU add fuel. I notice other cars not returning to idle. Did anybody check fuel rate or injection while encountering that condition, because I still wonder what all the ECU does for outputs in that condition..?
The majority of new cars have electronic throttle control (ETC). This means there is no IACV so when the accelerator position sensor (APS) reads full off the throttle controls idle. Some cars control the throttle position based on the bottom 5% or so of APS travel. So if you hit decel fuel cut (DFCO) but are still on the pedal a bit it opens the throttle a bit more and reduces engine braking while using no fuel.

Modern PCMs are not perfect but manufacturers have done a _LOT_ to improve fuel economy in the last 5 years. Many of the ideas I see floating around here are based on incomplete understanding and ultimately result in significantly increased emissions.

As for the quoted post above, fuel follows torque. So if you are at high idle the ECU must request more torque to achieve the higher idle speed. More RPM also means more injections per minute. More fuel used. It will not go significantly lean before the closed loop control (O2 sensor) triggers addition of fuel. This is necessary as a lean mixture is going to put like 5-10x more pollution out the pipe.

The days of sluggish slow responding O2 control are gone in the late 1990's and they would never pass modern emissions standards.


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