Idle speed vs engine speed

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Dioclese - 08 Dec 2008 13:56 GMT

Little dissatisfied here. Been driving manual tranny cars in the past.
When I let off the accelerator pedal, the engine always immediately dropped
down to the idle speed normally seen at rest. Doesn't matter if the car is
in the driveway, or cruising on the highway and push in the clutch to
observe that idle speed. This asset is effective for slowing the vehicle
down hills and when approaching slower traffic, and slightly longer brake
life.

In this particular vehicle, there is delay from actual engine speed while
traveling on the highway, to dropping to normal engine idle speed. Noticed
this on long downhill grades that the car doesn't immediately deaccelerate
when letting off the accelerator pedal. Rather, it takes about 10 or more
seconds before this occurs. During this 10 second time period, I can push
in the clutch and the engine is running at 1500 rpms or more.

Also, one time I goosed it in first gear to see what she had. The engine
remained at 2500 rpm for a few seconds afterwards while the accelerator
pedal was released and clutch disengaged for observation.

Is this an accelerator cable problem, or, is this normal?
08 Ford Focus coupe, 4 cyl, 5 speed manual.

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Dave

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Chris Whelan - 08 Dec 2008 15:29 GMT

> Little dissatisfied here. Been driving manual tranny cars in the past.
> When I let off the accelerator pedal, the engine always immediately
[quoted text clipped - 18 lines]
> Is this an accelerator cable problem, or, is this normal? 08 Ford Focus
> coupe, 4 cyl, 5 speed manual.

Normal for many modern cars. It's part of the way of dealing with
reduction of emissions.

Chris

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Dioclese - 09 Dec 2008 12:08 GMT

>> Little dissatisfied here. Been driving manual tranny cars in the past.
>> When I let off the accelerator pedal, the engine always immediately
[quoted text clipped - 23 lines]
>
> Chris

Odd, sounds backwards to me. Seems to me that the reduced fuel signal at
normal idle per throttle cable position, and added air would result in a
leaner mixture. Resulting in less emissions. Anyone know the details as to
why they added some modifier to increase time to normal idle speed from
throttle position at normal highway speeds?

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Dave

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Michael Pardee - 09 Dec 2008 15:51 GMT

>>> Little dissatisfied here. Been driving manual tranny cars in the past.
>>> When I let off the accelerator pedal, the engine always immediately
[quoted text clipped - 29 lines]
> to why they added some modifier to increase time to normal idle speed from
> throttle position at normal highway speeds?

That's something I know a little (not much) about. "Lift throttle"
conditions have been a problem for decades; the very low charge makes
ignition problematic and the mixture is (as you say) typically lean but is
not easily controlled. NOx has been a bugaboo in lift throttle, and HC is
ready to pop up when trying to control NOx. In earlier days there was a
separate vacuum line to the distributor to set the timing in lift throttle
conditions, and more recently the designers have made a practice of cutting
all fuel delivery above a certain RPM, after a certain time, under lift
throttle conditions. In some vehicles you can feel it happen. Anyway, in
these days of "by wire" accelerators it is easier to simply let the computer
prevent lift throttle operation when it can... and it can do that when the
clutch is depressed.

Mike

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