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Discussion Starter · #1 ·
This is my first car with throttle-by-wire, so maybe this is all
normal but it still strikes me as odd.

When coasting from 70mph with the cruise control ON (by holding
the SET - button) the RPM's will slowly track downwards as the car
slows but will suddenly drop from near 2k to 800rpm right at 50mph,
and then jump up again at 40mph. :thinking:

When coasting normally (in drive, cruise control OFF) the engine RPMS
slowly track downwards as the car slows (they do not jump up and
down). All very much as expected.

If the car is put into neutral at 70mph the tach initially drops for a second
but then swings back up to 2k or more and slowly tracks down as the car
slows :eekno:

Its almost like the computer doesn't realize the car is in neutral and is
trying to keep the engine RPMS up with indicated speed. (cruise
control was OFF btw) :screwy: Very odd.
 

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I know that mine compensates for me OVER-speeding down a hill with cruise, so it jumps down a gear for engine braking. Same deal with coasting. I think it does all this to be efficient and save some gas. Why keep the RPM's up when you can coast and be lower RPM. That's what I thought.
 

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Its not unusual to get some air/fuel spikes during coastdown. The PCM will raise rpms to burn off unburned fuel. It monitors the cat converter efficinecy constantly and will make changes to keep CO/NOx down.
 

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Must be an auto tranny thing. I had my cruise on for almost three hours straight with no random changes in the RPM's...
 

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Discussion Starter · #6 ·
[quote author=captainhook link=topic=40109.msg583928#msg583928 date=1140182879]
Must be an auto tranny thing. I had my cruise on for almost three hours straight with no random changes in the RPM's...
[/quote]

Doesn't happen if you just have the cruise on.. you have to take very specific steps to do this (and be looking at the tach).
 

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Remembered this post - the other day i was coasting down (in Drive) from 75 MPH and paid attention to the behavior of the car. As it slowed due to engine braking, occasionally it would stop decelerating and maintain its speed for a second or so, then continue slowing down.

I guess, based on the info on this thread, the engine was buring off accumulated unburned fuel or something. At about 40 MPH it becomes quite noticeable, but only if you pay attention.

Anything, I guess, to make the car run cleaner. It's OK with me!
 
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