Sorry for the stupid question but....?
Thread Starter
On The Tree
Joined: Dec 2011
Posts: 118
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From: Colorado Springs, CO
Sorry for the stupid question but....? 
My roommate and I were talking about the effects of downshifting a vehicle. He has been a mechanic for quite a while, and he said fuel injected vehicles (with standard transmissions) shut off their fuel injectors when they are moving, in gear, and the clutch is not depressed. This feels like it is incorrect. I am only basing this feeling off of the sound of the exhaust during a downshift. It seems like the noise in the exhaust would change a lot more than it does when there is no combustion in the chambers (maybe this is an incorrect assumption?). So can anyone clear this up? I've come up blank on google. Do the injectors turn off completely during a downshift (on a normal - i.e. stock - tune)?
Thanks for the help.
Thread Starter
On The Tree
Joined: Dec 2011
Posts: 118
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From: Colorado Springs, CO
That was exactly my argument. I figured they would still be spraying just a much smaller amount. As for the clutch being depressed during downshift, I was referring to the deceleration after the downshift. Sorry for the confusion.
Does anyone know what goes on with the fuel system during deceleration for sure or is all we have just educated guessing? (Like what I've been doing lol)
Does anyone know what goes on with the fuel system during deceleration for sure or is all we have just educated guessing? (Like what I've been doing lol)
Teching In
Joined: Jul 2015
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I see. You are talking about deceleration fuel cut off, which I believe all obdII cars have. When in gear and engine braking like down a hill for example, modern engines will shut off the injectors.
However not ALL fuel injected engines.
So your roomie is correct other than implying that all fuel injected engines do this.
However not ALL fuel injected engines.
So your roomie is correct other than implying that all fuel injected engines do this.
12 Second Club
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From: From Ohio now in that state up north
Shutting off fule while costing down is a pretty normal practice you can do it in an auto car if the tc is locked or manual if the clutch is engaged. You need to be above a certain speed as well. You are just letting the wheels drive the engine instead of the other way around
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I am going t disagree. If I am going down a long hill with my foot off the gas in gear at whatever speed, if the injectors shut off, I should see no emissions or anything through the O2 sensors because theoretically, the engine is not running since it has no fuel.
This is definitely not the case in my car. This should be easy to detect if you have a wideband and a long hill to test the theory.
This is definitely not the case in my car. This should be easy to detect if you have a wideband and a long hill to test the theory.
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Just to clear things up, DFCO: Deceleration Fuel Cut Off, doesnt literally cut the fuel completely off. It reduces fuel injector pulsewidths by a percentage cell for cell while TPS is at 0%
Basically you can see it working with a wideband. Say you're tuned for 14.7 AFR at part throttle, you let off the gas completely, the car goes from closed loop to open loop, cuts a percentage of fuel (to save fuel), now the AFR is around 15.5:1 to 16.5:1.
If you deactivate DFCO in the tune, when you let off the gas the AFR will stay around 14.7.
Either way, if it cut the fuel completely, the wideband would show something like 35:1 or whatever the max is and the engine would shut off.
Your roomate is not right, but not wrong. But closer to wrong.
Basically you can see it working with a wideband. Say you're tuned for 14.7 AFR at part throttle, you let off the gas completely, the car goes from closed loop to open loop, cuts a percentage of fuel (to save fuel), now the AFR is around 15.5:1 to 16.5:1.
If you deactivate DFCO in the tune, when you let off the gas the AFR will stay around 14.7.
Either way, if it cut the fuel completely, the wideband would show something like 35:1 or whatever the max is and the engine would shut off.
Your roomate is not right, but not wrong. But closer to wrong.