Does EEC-V PCM shut off injectors when COASTING?

[fixizin]

Gads, I hope so, since my old carbureted pre-computer Mazda did so with some simple vacuum mechanism.

There's some acronym for this mode of operation, but I fer-got it... "free ride down the mountain" = FRDM?... nope, that's not it.

Of course as with most fuel-saving ideas, this is only possible with a MANUAL tranny.

 

[AllanD]

Since Ford Introduced that Feature with the early EFI escorts and the 1983-1/2 thunderbird turbocoupe I doubt they have even considered making an EFI engine without it.

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[fixizin]

Oh... you'd be surprised what gets left out of the firmware when the memory wall is hit... ;')

Trying to think what OBD-II param to check while coasting... hmmm...

 

[shadetree]

Fuel is shut off during deceleration, not coasting with the accelerator pedal up, else the engine would die. When decelerating, the fuel will be shut off until around 1200 rpm, then fuel will return for the idle circuit. shady

 

[fixizin]

So if you're coasting down a hill, in gear, @ 3,000 RPM, engine braking as it were, it's still got a finite injector pulse width?... must be pretty dang short.

By "die", do you mean heads would cool off too much? Otherwise, if the vehicle is driving the engine, instead of the other way around, why does it need fuel?

 

[Earl43P]

Here are my ScanguageII observations from a highway trip to SW PA (mountains):

At normal downhill coasting, foot off the pedal, the injectors are still delivering gas, PERIOD. You can gain speed going down a hill, foot off the gas pedal.

With Cruise set, once you coast downhill ~4-6 mph more than the set speed, the PCM cuts the injectors OFF and you can feel the dramatic difference in engine braking. It'll actually slow the truck closer to the set speed, but depending on the hill, it may not have enough "authority".

I base this observation solely on looking at the MPG readout. It'll show ~ 66 mpg without the Cruise being on, rolling downhill. Do the same downhill run with cruise set and it'll register significantly higher MPG (though I don't recall the exact reading). As you approach the set speed, you'll feel it "come back" and the MPG readout drops simultaneously.

Pretty sure the ScanguageII will let you program an additional parameter (like injector pulsewidth?) but I haven't dug into that booklet enough to actually program mine (I did HP, but that is hokey). I guess it COULD be that the PCM is cutting off SPARK, rather than injector pulse, but my intuition feels like it is fuel. You can also hear a change in the exhaust note.

I can't do any definitve testing because it is FLAT here, no hills!

 

[Bob Ayers]

then fuel will return for the idle circuit. shady

EFI doesn't have an "idle circuit" like a carb does, only the IAC valve!!

 

[shadetree]

So if you're coasting down a hill, in gear, @ 3,000 RPM, engine braking as it were, it's still got a finite injector pulse width?... must be pretty dang short.

By "die", do you mean heads would cool off too much? Otherwise, if the vehicle is driving the engine, instead of the other way around, why does it need fuel?

Think about it. Why would you want to continue with fuel with the throttle plates closed?

When an engine said to "die" that means the engine is no longer running under it's own power. It stops turning. It has nothing to do with cylinder heads. shady

 

[Bob Ayers]

I guess it COULD be that the PCM is cutting off SPARK, rather than injector pulse, but my intuition feels like it is fuel. You can also hear a change in the exhaust note.

Your intuition is correct, if the spark was cut off, and the injectors were still pulsed, there would be raw gas into the CATs, a definite NO-NO.

 

[Earl43P]

And a nice "explosion" when the spark came back, DOH!

Like I used to do with my 64 Fairlane, 260 V8, 2 speed automatic, cherry bomb muffler. Put it in low to decelerate (OK, really to make that exhaust rumble loudly) turn the key off for a while then back ON....BANG.

 

[Wicked_Sludge]

EEC-IV turns the fuel injectors OFF any time the following conditions are all met:

ECT shows temp. over 140 degrees.
TPS shows closed throttle.
VSS shows vehicle is moving.
RPM is over 1500.

in addition to shutting the injectors off, the computer also advances the timing in an attempt to slow the engine even more (using trace amounts of air/fuel mixture to push against the rising piston).

i dont know about you guys, but i can easily tell when my PCM goes into and out of the deceleration strategy...the exhaust note is entirely different, and i can feel the deceleration effects.