[Vwdiesel] Why gasss cars mpg sucks

Roger Brown r.c.brown at ieee.org
Sun May 15 11:24:39 EDT 2005

Val Christian wrote:
> When you top a hill, and start down, coasting, the injection level 
> commanded by the ECU can drop to zero.  It's a function of power commanded.
> Taking your foot off the pedal, it will drop to zero.  Depending upon
> the hill, not moving your foot may cause some fuel to be commanded
> to be injected.  But simply stated, yes, fuel can (and often does)
> drop to zero.  It's not freewheeling, per say, because the entire 
> engine (altenator, compressor, power steering, pistons and valvetrain)
> keep turning.  So when the available power exceeds those, you get a 
> free ride.

My fuel injected '85 Toyota does this as well and even when you just take your foot off the gas coast up to a stop sign in gear 
or when you hit the brakes above 1200 RPM.  You can watch the O2 meter drop to full lean when you do so and the smell of the 
exhaust changes for sure.  It is done with a combination of sensors that monitor RPM and throttle position.  Every once in a 
while you can get it to do this out of gear and it sounds almost like the engine died, but then once RPM drop below about 1200, 
the fuel is kicked back on and the engine pops back to life.  And if you mistakingly have the idle speed set too high you can 
get into a strange up-n-down idle cycle if sitting with the brake on:


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