So my friend who has a computer hooked up to his ECU says that he's actually sees less gas being injected while in gear rather in neutral when going down hill.

I was thinking the opposite was true because you're in lower rev range (idling) when in neutral.

If what he is saying is true then how does the car know that its going down the hill and not inject fuel. Does it have a load sensor or some sort? Also if it's not injecting fuel then why do we hear the engine/exhaust sound? Is it just air passing through?

 

that is true.

The doesn't know it's going downhill. What it does know that is your foot is not on the gas pedal, and it doesn't have to burn gas to sustain an idle because the wheels are turning the engine.

The noise you hear is the air being pumped in and out of the engine by the cylinders, and mechanical friction.

 

No joke.. My cousins live and die by the neutral thing... They always think it saves gas and pop it in neutral as much as they can.

One of them even throws it in neutral and shuts of the car so it can roll to the next red light.

 

shutting off the car is stupid, and i doubt your cousin does it. no power brakes, no steering... not smart.

 

This guy has turned into popeye because of it.. Trust me, he did it w/ me in the car and I freaked out.

 

the engine would still be running in Neutral, but at a lower RPM (~750rpm i think?) than when it is in D. The difference is that it is disengaged from the drivetrain. And yes it SHOULD save you gas but the difference would be unnoticable unless you go downhill a lot.

 

^wrong? right? which side are you saying is right?

throw a wideband AFR gauge on your car and see how it reacts to a downhill coast.....or even just coasting.

Stay in gear on a mt = AFR pegs out to max the gauge will read. Means nothing but air is flowing through the exhaust.

Pop it into neutral on a mt = AFR goes to about 14.6 or whatever you idle at. that means gas is running through the motor to sustain operation.

Leave it in gear, the wheels turn the motor for you.

 

Shutting the car off does save gas, but the starter was never meant to be used that much...

False economy.

 

don't you use more fuel to start the car then you do by just idle for a minute? we call the neutral coasting "Mexican Overdrive" lol

 

I think that is a myth.. I'm not sure.

 

pretty sure the ecu injects more on start up..... anyone know for sure????

 

Maybe.. Because when you turn the key onto battery you can hear the ecu injecting fuel ready for the start up.. I'm not sure.

 

start up fuel is a lot more than coasting, i know on my cbr it fills the fuel rail everytime you turn the key on, it's kind of hard to hear on a car though. i do throw it in neutral sometimes but i guess it would be better to just leave it in gear if thats true.

 

Not in a fuel injected car. Def true for older cars though.

 

i see...

thanks for clearing that up

one more thing, how does the ecu know when to transition to inject fuel from when you're in gear to when you go into neutral? Is there a sensor that signals the ecu which gears you're in or is there some sort of load sensor?