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Author Topic: Start/Stop & AFM More Detailed Info  (Read 239 times, 7 Replies)
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« on: April 20, 2017, 08:13:22 AM »

Rather than continuing hijacking the New 2018 Traverse thread maybe we can expand the these features here.

With the AFM I know it drops down to 4 cyl when it's not under load and goes back to 6 when needed. But if you have a long stretch of flat road and you're cruising at 65-70 how does it know when to go back to 6 cyl. Someone posted that the newer system is different and better from the older AFM system, how?

The basic concept of Start/Stop is easy to understand, but how long do you have to be stopped before the engine stops. Would stop & go traffic in say Chicago rush hour cause a problem?
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« Reply #1 on: April 20, 2017, 11:07:29 AM »

in the AFM I had on my 2007 Impala.
On long highway trips- it would go into AFM.
If were cruising along at a steady speed---   for a long time-----

the system would come out of AFM- when it met somekind of  parameter.
I dont know if it was time- or temperature.

But it was done to keep the engine at a relatively constant temp-- all over.

thinks of it this way..

with some cylinders turned off---- they would technically be cooler than the others. (Could it lead to warping of the heads?).
So the engine comes out of AFM--- to bring all the parts back up to similar temps.

Read about this way back in 06.
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« Reply #2 on: April 20, 2017, 10:30:33 PM »


The basic concept of Start/Stop is easy to understand, but how long do you have to be stopped before the engine stops. Would stop & go traffic in say Chicago rush hour cause a problem?

My experience with S/S in my Porsche during the very few times I've actually turned it on is it doesn't take very long for it to shut down.  You just need to come to a complete stop with your foot on the brake, and it will shut down pretty quickly.  Releasing the brake, or touching the gas will immediately restart it.  If the traffic is truly stop & go, then I would expect it to activate frequently.  Whether that is considered a problem would be in the eye of the beholder.  The system really shouldn't impact anything in your driving, no matter how many times it is activated.  You certainly don't have to wait for it.  For a normal person that brakes with their right foot, the car will be restarted before your foot reaches the accelerator after releasing the brake.

I don't turn it on, because it is so impractical in a warmer climate.
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« Reply #3 on: April 21, 2017, 06:56:38 AM »

There was a fairly long discussion about this over on the Cadillac SRX forum a couple of months ago.  The SRX follow on, the XT5, has this shut off feature.   I did not get the impression in that discussion that this feature was selectable.  I have plans to rent an XT5 for a few days so should get some direct experience.

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« Reply #4 on: April 21, 2017, 12:01:22 PM »

...
the system would come out of AFM- when it met some kind of  parameter.
I dont know if it was time- or temperature.
...

I believe that parameter was simply a function of engine load. The 8-6-4 was kind of fun. In diagnostic mode (accessed through the climate control), the car could be driven in any configuration desired for any length of time. A Sedan DeVille was really a dog in 4-cylinder mode!!
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« Reply #5 on: April 21, 2017, 12:53:26 PM »

I had driven on long flat highways--- totally flat. No change in speed or anything.
And you could hear the tone of (dual exhaust) and see AFM(on DIC) turn off.....

thats why I believe it was time/temp(engine)
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« Reply #6 on: April 21, 2017, 02:52:28 PM »

Maybe someone with a 17 V6 non Denali trim will post if they notice when it drops to 4 and and how long it stays in 4 cyl mode before it goes back up to 6.

The same with the 17 I4s and tell if they notice anything when the Start/Start comes on and off.
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« Reply #7 on: April 21, 2017, 10:17:07 PM »

   It would certainly be interesting to hear directly from GM on what their acceptable dwell time (engine dead) after lifting right foot off the brake pedal they use for proper evaluation of if the system is operating safely and properly by their "standards"!  Or if one drives as I do in city traffic using the left foot for braking for quicker response reaction time and significantly shorter stops (according to a very old NY Trooper study), does having one's right foot on the gas and "power braking" with the left foot allow you to force the engine to continue to run so there is assurance of no delay in accelerating quickly and hard if one is about to get rear ended by a drunk at high speed? shrug
    I am most certainly not a fan of Stop/Start systems except perhaps on hillbilly mountain golf courses where a gas engine cart is needed.  Given the potential safety issue of delay in restart or no restart at all, I am hopeful Mary Barra, who has proven to be a gifted leader of GM, order a prompt review of the whole Stop/Start concept and at the very least offer a cut off switch to disable the system on those GM vehicles so equipped if they feel it is a liability. Help
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