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When you are using the gears to slow the car down, the vacuum in the intake manifold approaches the level of outer space.
There is no air to combine with the fuel to create combustion.
If fuel was being injected it would go out the exhaust as unburned fuel.
You can check this fairly easily if you doubt what I am saying.
Take a phillips head screwdriver. Place the point on the fuel injector and the other end on your ear. You should be able to hear the injector ticking. Use it like a stethoscope.
Rev the engine and the ticking will become more rapid. Then left off the throttle and the ticking should stop for a short time as the engine slows down to idle speed.
If you hear it stop ticking when you let off the throttle you have just heard DFCO working on your car. No ticking=no fuel delivered. The same thing should happen when you are using the engine to slow down the car, as long as your foot is off the gas completely. If that was not the case when you pressed on the gas the unburned fuel in your exhaust would ignite with a big bang.
I can't provide better proof of that.
The brakes work with vacuum assist, the booster holds enough vacuum for a few applications then the assist doesn't work. You still have brakes but the pedal effort is much higher.
Using DFCO downhill means your brakes and steering work normally.
I was almost killed driving in mountains very close to where you live. I was climbing a steep grade and there was an 800 foot drop just past the guardrail on my side of the road. A big rig came around a blind turn (left for me right for him) coming in the opposite direction. Just behind him a Subaru lost it and came across into my lane, missing the guardrail by 18 inches about 200 feet in front of me. If I had been 200 feet further down the road he would have knocked me over the guardrail down the 800 foot drop.
regards
Mech
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