Tale of two AFM systems

[4WD]

2017 Tahoe 4WD 3.42 gears GVW 7300
2018 Z71 LT 4WD 3.08 gears GVW 6800
Both with 5.3L and 6 speed auto
Did not buy either expecting great MPG …

Obvious pattern difference in the AFM
Tahoe will drop to V4 always at 60 mph, often at 65 mph, sometimes at 70 mph
Z71 will use V4 at 55 mph sometimes and never above …
Z71 engine speeds are lower of the two … only 1450 rpm at 60 mph and 1750 rpm at 70 rpm.

Is there anything to be concerned with … or just assume staying V8 almost always is good for engine life ?

 

[The_Nuke]

I don't believe the AFM tech and how much it is activated while engine is operating has any bearing on the engine's total life expectancy. The engine should last as long as it is going to last regardless of how much the AFM was utilized.

As far as what causes the two engines to have different profiles for the AFM activation, I don't know. What are the requirements for the AFM system to kick in?

Identify those, and I bet you can walk them backward far enough to figure out wha'ts causing the different behavior between engines.

 

[Snagglefoot]

I'd say the 2017 is better matched with the higher 3.42 gears. I believe the ECM is getting a vacuum signal and RPM reading. Could be the vacuum of the lower reving Z71 is too low to kick in the system. The 2017 is acting like a higher revving four banger which is what it probably should act like. Maybe low gears AND AFM, is just too much.

 

[4WD]

Yeah, I've never had less gear in any truck … without a 6 speed auto and 2 speed transfer case I would of looked for a lower gear … but it actually handles deep sand very well …

 

[double vanos]

Agreed: it's the gearing. Interesting that the Z71 came with the 3.08 and the plain Tahoe came with the 3.42s. You'd think it should be the other way around. Maybe the ‘18 was supposed to get better mileage ( corporate edict).

 

[4WD]

Could be because the Tahoe has the better trailer package …
There was an extremely limited inventory when shopping the pickup due to body style

 

[Railrust]

My 2018 Silverado is almost always in V4 mode on the highway unless I'm going uphill...doesn't matter if it's 68 or 78...that thing loves dropping into V4. I'm actually getting better fuel economy in this truck than my wife's Honda CRV. It's ridiculous.

Trying to figure out why your trucks are engaging into V4 at different times - aside from the gearing - I'd wonder about rim/tire size. I'm assuming you're using the same octane fuel in both and same oil type in both too.

I can tell you right now that the V6 version of this platform does NOT go into V4 mode nearly as much as the V8 does, and it shifts much differently because of it. Much smoother. I hear that's why a lot of people do the AFM delete...it makes the shifts much smoother...and much less hunting and holds onto gears longer. I haven't thought about doing that yet because I'm loving the gas mileage I'm getting...I can drive over 550 miles a week on one tank of gas. Unreal for a truck.

 

[4WD]

Interesting … What is your gear ratio ?

 

[K20FA5]

I'm surprised your Z71 came with 3.08? I though it was part of the package that they came with 3.42?

Mine has the 3.42 and it behaves more like you say your Tahoe does. It's so smooth going between V8 and V4, I don't even notice it much anymore. I just leave it on the digital MPH screen and drive it. I do get better MPG than my wife's 2WD 4Runner though.

 

[4WD]

It's 3.08 … but also lighter than most being regular cab + short box

 

[clinebarger]

Per the general DOD Enable conditions in GM service manual......2017 L83 Truck & 2017 LT1 Camaro have the same description

*Engine has been running for greater than 30â€s
*Engine speed is between 700 and 2800â€rpm
*Engine oil pressure is between 187-455â€kPa (27-66â€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 11â€V
*Transmission is not in first, second, or reverse gear
*Vehicle speed is greater than 25â€kph (15.5â€mph)
*Brake booster pressure is greater than 42.0â€kPa (6â€psi) (No vacuum pump on Camaro, This value is zeroed out)
*Vehicle is not in fuel shut off 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)

When compared to the actual calibration of a 2017 LT1 Camaro SS.....The above IS just generalizations!!!

The most interesting being the Vacuum Delta DOD cutoff threshold, Which from the best I can discern means.....
If the manifold Vacuum varies +/- 1.476 inches within 12.5ms....DOD/V4 mode will be disabled.
The high TPS DOD cutoff value is 40%......Lighter vehicle, Bigger engine, & Less aerodynamic drag......I assume a truck/SUV is set way lower!
RPM disable.....3,200 RPM.

 

[CKN]

In my 2018 Silverado (5.3) I can get a solid 25 mpg at 70 mph on the flat Interstate. My computer records my "best" mpg (over 50 miles-that's what I have it set at) and that was actually 29.6 mpg. That 29.6 was at around 60 mph and I had a tailwind.

 

[clinebarger]

I do have a 2009 Tahoe calibration file(Gen IV 5.3L/6L80E/3.08:1)......Probably not the exact same as yours, But closer than a LT1 Camaro.

The Max TPS cutoff is 6%.
The Vacuum Delta is even tighter at +/- 0.185 inches of mercury.

 

[4WD]

You'd think the Z71 would be in V4 between 60-70 mph … flat road and no wind …

 

[spasm3]

I wonder if there is more drag on the pickup with air coming off the roof and down onto the bed. The tahoe may be better aerodynamically. That might explain why the truck does not go in v-4 at higher speeds, more drag.

 

[WillB]

I plan on unloading my Ram when powertrain warranty expires - whether it GM AFM or Dodge MDS - the lifters are the failure point.

Not worth the hassle when the lifters take a dump.

 

[4WD]

Could have some effects … it's the Chevy version of this

 

[clinebarger]

Originally Posted by 4WD
You'd think the Z71 would be in V4 between 60-70 mph … flat road and no wind …


Not necessarily, The faster you go...The more aerodynamic load is placed on your truck.

Checking the actual Calibration & Data Logging your truck would tell us for sure.

 

[clinebarger]

Originally Posted by WillB
I plan on unloading my Ram when powertrain warranty expires - whether it GM AFM or Dodge MDS - the lifters are the failure point.

Not worth the hassle when the lifters take a dump.



I've seen more NON MDS/AFM lifters take a dump than the actual deactivation lifters on BOTH brands!

 

[WillB]

Its so bad in the Hemi world that the aftermarket is no longer producing MDS lifters. I have heard the same about the Hellcat non MDS lifters.
Originally Posted by clinebarger
Originally Posted by WillB
I plan on unloading my Ram when powertrain warranty expires - whether it GM AFM or Dodge MDS - the lifters are the failure point.

Not worth the hassle when the lifters take a dump.



I've seen more NON MDS/AFM lifters take a dump than the actual deactivation lifters on BOTH brands!