Should the AFM kick on sitting at a stop light ?

[Andy King]

I have a 2012' Tahoe 5.3 and I saw when I was sitting at a stop light I will have the V8 still showing on the dash, but I see when diving on the HWY with a light load I get the V4 on the dash like it should.

[juscuzram]

AFM won't kick on at idle. There's a certain set of conditions that have to be met for it to determine V4 or V8, and idling at a stop is not one of them. 

[newdude]

See the info below on the criteria for AFM functionality. 

 

Criteria that put it into V4:

 

Quote

 

To provide maximum fuel economy under light load driving conditions, the engine control module (ECM) will command the cylinder deactivation system ON to deactivate engine cylinders 1, 7, 6, and 4, switching to a V4 mode. The engine will operate on 8 cylinders, or V8 mode, during engine starting, engine idling, and medium to heavy throttle applications.

 

Manifold absolute pressure (MAP) and the conditions listed below determine when cylinder deactivation is enabled.

 

Engine has been running for greater than 30 s

Engine speed is between 700 and 2800rpm

Engine oil pressure is between 187–455 kPa (27–66 psi)

Engine coolant temperature is between 40–129°C (100–264°F)

Engine oil temperature is between 16–128°C (61–263°F)

Throttle angle is 6% or less

Ignition voltage is greater than 11 V

Transmission is not in first, second, or reverse gear

Vehicle speed is greater than 25 kph (15.5 mph)

Brake booster pressure is greater than 42.0 kPa (6 psi)

Vehicle is not in fuel shut of mode

Vehicle is not in heater performance mode

Vehicle is not in tip in bump acceleration mode

Vehicle is not in oil aeration mode

Vehicle is not in low range (if equipped)

 

 

Criteria that disable V4:

 

Quote

 

Cylinder deactivation may be inhibited for many reasons including the following:

 

Engine coolant temperature out of range for cylinder activation

Engine vacuum out of range

Brake booster vacuum out of range

Transmission gear incorrect or shift in progress

Accelerator pedal out of range or rate of pedal application to fast

Engine oil pressure and temperature out of range

Engine speed out of range

Vehicle speed out of range

Minimum time in V8 mode not met

Maximum V4 mode time exceeded

Decel fuel cutoff is active

Reduced engine power is active

Torque management is active

Catalytic converter over temperature protection is active

Piston protection is active, knock detected

Cylinder deactivation solenoid driver circuit faults

 

 

 

[Armo325]

Great info, newdude.  Thanks.  I have wondered about this question before.  I'd be curious to know if there is any fuel savings to be had idling in V4.  I'm guessing the engineers thought of that and determined the answer was "no" but still an interesting question.

[newdude]

1 hour ago, Armo325 said:

Great info, newdude.  Thanks.  I have wondered about this question before.  I'd be curious to know if there is any fuel savings to be had idling in V4.  I'm guessing the engineers thought of that and determined the answer was "no" but still an interesting question.

 

I think there would be lots of customer complaints.  Bad transition on acceleration from sitting still in V4 and then accelerating, rough idle, etc.  I would think that GM tried thinking of that during AFM's original development from the early 2000's and I wouldn't be surprised if they had some mules set up to idle in V4.   

[aseibel]

1 hour ago, Armo325 said:

Great info, newdude.  Thanks.  I have wondered about this question before.  I'd be curious to know if there is any fuel savings to be had idling in V4.  I'm guessing the engineers thought of that and determined the answer was "no" but still an interesting question.

I was just thinking about this earlier in the week when I was sitting in the cold (single digits) watching the engine idle. The idle RPM is only about 500. The earlier criteria said it won't go into V4 at less than 700 RPM, i'm sure GM figured out how slow the engine could spin in each mode while running smoothly.

[Armo325]

27 minutes ago, newdude said:

 

I think there would be lots of customer complaints.  Bad transition on acceleration from sitting still in V4 and then accelerating, rough idle, etc.  I would think that GM tried thinking of that during AFM's original development from the early 2000's and I wouldn't be surprised if they had some mules set up to idle in V4.   

Fair point.  I was thinking about it in a more theoretical sense.  I was mostly curious what the difference in fuel burn would be with four cylinders firing versus eight.  I am sure there are some significant engineering challenges that would come with making it work in practice.

15 minutes ago, aseibel said:

I was just thinking about this earlier in the week when I was sitting in the cold (single digits) watching the engine idle. The idle RPM is only about 500. The earlier criteria said it won't go into V4 at less than 700 RPM, i'm sure GM figured out how slow the engine could spin in each mode while running smoothly.

Interesting observation.  Thanks.