Engine braking/lifting the throttle

Joined
Feb 12, 2004
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1,313
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Western Washington
On a computer controlled fuel injected car, when one lifts one's foot off the throttle and coasts in gear, is any fuel still being injected into the cylinders, or do the injectors shut off? Is spark still supplied to the plugs when no throttle is applied? Would this depend on whether the car uses a cap and rotor or not? I was thinking about this tonight, and realized that I have no idea. Many thanks to anyone who can answer my questions!!
 
Joined
Apr 30, 2003
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Ontario, Canada
It would make sense to shut off the ignition whenever fuel is cut off during coasting, but I'm not sure if any cars do that. Doing that would save energy, and all it would take is a firmware change.
 
Joined
Sep 26, 2002
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Central Arkansastan
Probably not. The energy saved would be negligible, and if the ignition is still firing, It will burn off any blow by or leaks from the injectors therefore keeping the raw stuff from going out the tailpipe. All about emmissions...
 
Joined
Jun 15, 2003
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ME
I've installed aftermarket tachs on FI cars with caps and rotors. They're still getting a tach signal from the coil (-), so the coil is still firing, and there's still spark. Takes less electrical power to bridge the plug gap under no load anyway. If one had a scan tool they could watch injector pulse width. My bet is many cars will drop to zero during the transient when the pedal is being lifted quickly, to blow any fuel out. (Extra idle air bypass is a good bet too, to help lean out.) Perhaps especially stick shift cars which get the motor turned over by the car's inertia will stay at zero FI during a coast-down event.
 
Joined
May 1, 2003
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9,448
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USA
Imagine the engine basicly idleing! It uses this energy to break by makeing the wheels work against this force. This is what slows you down! If it was not putting out power then the car would basicly coast with next to no breaking affect. If you key off the ignition and try to use engine breaking you will find that it is not very effective at all. You also get vey little engine breaking with really agresive high lift long duration cams depending on the amount of over lap!
 
Joined
Feb 9, 2005
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Arizona
On the brand newest of vehicles, this may have changed, but as recently as a few model years ago, the fuel WAS shut off when coasting. However, the engine must be above a certain RPM (~3500 on the cars I've personally driven), and the throttle must have been closed for at least a moment before it'll shut off. The latter is due to an emission spike (NOx, I think) when the throttle plate is shut quickly. To avoid that the throttle is not allowed to completely shut very rapidly and a little fuel is injected for some number of RPMs. So, the answer is YES, the fuel is cut under some circumstances.
 
Joined
Aug 17, 2004
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TX
The fuel injection shuts off when you are coasting, and turns back on at about 1500 RPM. This is for emissions and fuel economy reasons.
 
Joined
Jan 20, 2004
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4,874
Location
MN
The engine still runs, so yes it's getting fuel and spark. However it is getting very little fuel, so it doesn't really burn any gas. -T
 
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