562cobra

WTF??? Once the engine cranks, the fuel injectors stay feeding the combustion chambers with fuel.

The engine is what keeps the wheels turning. Unless its on a downhill, gravity will turn them.

yakirg

dude, as i said, i am talking about what happens when you use the motor to brake... in this case, the motor does't turn the wheels, it's trying to make them slow down, and the wheels (or ectually the car's inertia) keep the engine Rev up...

it doesn't look like anyone here knows if the function exist in the mustang EFI computer...

2006_BLACK_GT

There is no cylinder deactivation (or total engine deactivation) on any mustangs (or Fords I think) if that is what you are getting at?

dennis_112

Dont mean to add fuel to the fire here, but when you slowing down the motor is still getting fuel, otherwise you wouldnt here the car running when your slowing down. The car doesnt shut down when you hit the brake or press the clutch down therefore the car is still using fuel. Its simple. Back to what he is ACTUALLY asking here. The mileage problem, whats your rpm when cruising at 60? Are you under 2k still? Your motor is probably still stock and just running rich, which would cause your mileage and would add to the popping, must of which is just because you are holding all that rotating mass on your clutch. Which will shorten the life of the clutch and flywheel, which in turn slows your car by engine braking or clutch braking which ever you prefer. Just press the clutch in and use your brakes or you will be on here asking why your clutch is slipping soon. Also get your car's air/fuel ratio check on a dyno. If your curious if your running rich. And while there see how much hp your making. That should answer all your questions.

mustangman281

Wow does anybody in this thread know anything about engines or ford engine managment. I don't think so.

Yes the injectors do get cut during "overrun". You would see that if you ever ride in a car that has a wideband O2 gauge and it goes max lean when you lift off the gas above 3000 rpms. Overrun is when you lift off the throttle above 3000rpms which means the motors rpm is turning faster than the fuel provided can support, so the motor begins to spool down. Its also called engine braking.

The popping noise is caused by fuel burning in the exhaust. The throttle blade snapping shut reduces the air and fuel flowing into the motor, and increases the vacuum (lowers the pressure). This results in less air and fuel in the cylinder during the power stroke, which in turn results in a lower pressure in the combustion chamber. The A/F mix burns faster in proportion to the pressure applied. Well, when we reduce pressure this way, the mix burns slower. The lower burning fuel generates less heat, and the cooling effect of the non-burning fuel tends to “quench” the flame front, or slow it down even further. Because the mix is burning much slower, the exhaust valve can open before all the fuel is consumed, and the unburnt fuel is ejected into the exhaust. Timing is also retarded to smooth idle. If you want to lessen the popping noise you can add more fuel during deceleration in the tune. This has the effect of raising the chamber pressure slightly, which burns a little more before the exhaust valve opens.

Bottom line is the popping noise is completely normal.

yakirg

Hmm ... I am new to this forum, but I had expacted some admin to close this question ...
So what we have so far ...
2 people say that the fuel is shut off during engine brake ... both sound knowledgeable.
4 people say no, but most don't understand how this could happen anyway ...

I know for a fact that other cars (I think most EFI cars ever) shut off all fuel to the engine during decelleration.

mustang281, you said it shuts off only above 3000 RPM, I tested this morning, and it still pops when I am above 4000 RPM ...

dennis, do you stop in 5th gear?

10x, all!

2006_BLACK_GT

yakirg - Regardless of the off-topic fuel discussions, I think most people with aftermarket exhaust will tell you the popping is normal especially at higher rpm.

If you do have higher gears, this would contribute to below average mileage, but 12 on the highway is pretty bad. There are a few good gear guesstimators on the net if you want an idea of whats back there.

_Slowbra_

the fuel injectors should cutoff fuel when the throttle is closed (your foot is off the accelerator...aka deceleration) when the engine revs are above idle...so from my understanding it will shut fuel off whenever deceleration is occurring before you fall to idle RPMs.

and to yakirg: i have straight pipes on my car, the popping and rasp is normal, the mpg is not...check your A/F