Quote:
Originally Posted by DonR
According to my Scanguage II, my 3.0l Mitsubishi uses about .45 gph idling in drive once warm. In Neutral it uses 0.35 gph.
My initial thought on the subject is that by slowing down the engine it would use less fuel. Which is probably applicable to carburators, not sure.
New cars with Idle Air Control valves try to keep a constant idle rpm. So when you load the engine down in gear, it opens a small secondary throttle & gives it more go juice.
This is only my thoughts, if they are wrong, someone please correct me.
Don
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Carb will still use more fuel because the engine is partially loaded, so it needs more power at either the same speed or even a lower speed to maintain it's condition (running). More power out = more fuel in.
IACV's are the only idle source on FI cars, the throttle plate is fully closed when you're not using the pedal, so the IACV just opens further to compensate for load.
Regarding OP's question - It does use less fuel, but not as much less as just shutting your car off. If you can time the light and shut the engine off and coast up to the light from 500 feet away (provided you're going to sit at it anyway), you'll save several seconds worth of fuel until the opposing light turns yellow, then you restart and you're ready to go by the time your light is green again.
Either method is relatively little gain, but they both work to some extent. I almost always coast to lights. Hell, I turn the engine off when I'm coasting to bleed off speed.