Active Fuel Idle

[imnutz66]

I just laid out some green last week for a 08 Sierra, 5.3 flex-fuel and have been watching the active fuel management on the display jumping back and forth from v8 to v4, wondering why in the world the engine idles on 8 cylinders - makes no sense to me at all. Maybe I am mistaken, but it seems to me that one would be better off running on just 4cyl while setting at a light or at the drive-thru. Maybe there is a way to change the computer??

[NathanL]

I just laid out some green last week for a 08 Sierra, 5.3 flex-fuel and have been watching the active fuel management on the display jumping back and forth from v8 to v4, wondering why in the world the engine idles on 8 cylinders - makes no sense to me at all. Maybe I am mistaken, but it seems to me that one would be better off running on just 4cyl while setting at a light or at the drive-thru. Maybe there is a way to change the computer??

 

 

It only works at rpm's greater than 1,000 rpm.

 

If you had your engine running in V4 mode at a stop light you would hate your truck in short order. TOO ROUGH. Ever have a vehicle that had a miss...not so bad at speed but terrible at idle.

 

At rpm's greater than 1,000 you have the momentum to overcome the roughness of running on 1/2 the cylinders.

 

I guess you could get a tune for it but I'm betting you could probably literally see your truck jump up and down if you walked up to the bumper while it was idling in v4 mode.

[99-2door]

If it was to idle in 4 cyl mode the RPM would have to be higher just for it to stay running. I doubt it has enough HP at 400 rpm in 4 cyl mode to keep the engine and accessories going.

[slimjim2525]

Couldn't they get over the roughness at idle by having different cylinders firing? In v4 mode does the same cylinders notfire, or does it alternate. I gotta tell ya, I had an 04 silverado with the 5.3 and now this 07 with the 5.3 and AFM, and my MPG is exactly the same. What gives?

[iminaquagmire]

As said before, the idle would be rough. The '81 seville had this issue. I believe when it runs in 4 cylinder mode it is cylinders 1,4,6, and 7 that shut down.

[redsled2007]

No, it alternates every 8 secs to different cylinders due to bearing wear.

[iminaquagmire]

Active Fuel Management

All Gen IV Vortec 5.3L V8s feature GM’s industry leading Active Fuel Management technology (AFM). AFM temporarily de-actives four of the 5.3L’s cylinders under light load conditions. It increases fuel economy 7 percent under the federal government’s required testing procedure and potentially more in certain real-world driving conditions. Yet truck owners don’t sacrifice superior V8 power and performance to go farther on a tank of gas.

 

Active Fuel Management stems from a simple premise: most truck owners have more power than they need much of the time. Many choose powerful V8 engines to be prepared for the occasional heavy load, but during routine commuting that powerful engine operates at a fraction of its capability. Volumetric efficiency is impaired, and that means less than optimal fuel mileage. AFM offers a common-sense solution. It saves fuel by using only half of the Vortec 5.3L’s cylinders during some driving conditions, and seamlessly reactivates the other cylinders when a driver demands full power for acceleration or load hauling.

 

Managed by the new E38 engine control module (ECM), AFM automatically shuts down every second cylinder, according to firing order, during light-load operation. In engineering terms, this allows the working cylinders to achieve better thermal, volumetric and mechanical efficiency by reducing heat loss, combustion loss and friction, and lowering cyclical combustion variation from cylinder to cylinder. As a result, AFM delivers better fuel economy and lower operating costs. Perhaps the most sensible thing about AFM is that it harnesses the engine’s existing capabilities, starting with the potential designed into the E38 ECM. The only mechanical components required are special valve lifters for cylinders that are deactivated, and their control system. The incremental cost for the customer is nominal per engine. Active Fuel Management relies on three primary components: De-ac (for deactivation) or collapsible valve lifters, a Lifter Oil Manifold Assembly (LOMA), and the ECM.

 

One of the most sophisticated engine controllers extant, the E38 ECM (below) measures load conditions based on inputs from vehicle sensors and interprets that information to mange more than 100 engine operations, from fuel injection to spark control to electronic throttle control. AFM adds an algorithm to the engine control software to manage cylinder deactivation and reactivation. When loads are light, the E38 automatically closes both intake and exhaust valves for half of the cylinders and cuts fuel delivery to those four. The valves re-open to activate all cylinders when the driver demands brisk acceleration or full torque to move a load. The engine’s electronic throttle control (ETC) is used to balance torque following cylinder deactivation or reactivation. The transition takes less than 20 milliseconds, and can’t be detected by the driver.

 

Valve lifters are operated by the engine’s camshaft, and lift a pushrod that operates the valves in the cylinder head. In the Gen IV Vortec 5.3L, the De-ac lifters are installed in cylinders 1, 4, 6 and 7, while the remaining cylinders use conventional lifters. The hydraulically operated De-ac lifters have a spring-loaded locking pin actuated by oil pressure. For deactivation, hydraulic pressure dislodges the locking pin, collapsing the top portion of the lifter into the bottom and removing contact with the pushrod. The bottom of each De-Ac lifter rides up and down on the cam lobe but the top does not move the push rod. The valves do not operate and combustion in that cylinder stops. During reactivation, the oil pressure is removed, and the lifter locks at full length. The pushrods, and therefore the valves, operate normally.

 

The final AFM component is the LOMA. This cast-aluminum assembly is installed in the Vortec 5.3’s V, or valley, in place of a conventional engine block cover. The LOMA holds four solenoids, control wiring and cast-in oil passages. The solenoids are managed by the ECM, and each one controls oil flow to a De-Ac Lifter, activating and de-activating the valves at one cylinder as required for Active Fuel Management.

 

The Gen IV Vortec 5.3L’s fuel injectors are identical for all cylinders; those feeding the de-activated cylinders are simply shut down electrically by the ECM during de-activation. When the cylinders are deactivated, the engine effectively operates as a V4. AFM operation is load based, as measured by the ECM using dozens of inputs, overlain with the driver’s demand for power as measured by throttle application. AFM’s response time varies with oil temperature, but in all cases is measured in milliseconds. Operation is always transparent to the driver. The engine returns to V8 mode the instant the controller determines that acceleration or load requires additional power.

 

http://media.gm.com/us/powertrain/en/produ...0New/07_LH6.doc

[NathanL]

Bet in a few years it's gone anyway.

[mdub]

No, it alternates every 8 secs to different cylinders due to bearing wear.

Amazing how many times this misinformation has been posted and shot down. Have to wonder where it started, because nobody has ever been able to provide an original source. But why not... I'll ask again, where did the information about this alleged 8-second rotation originate?

[neoh77]

No, it alternates every 8 secs to different cylinders due to bearing wear.

 

I don't believe this is correct...let me quote an article from Super Chevy

 

"Operationally, the LH6 always deactivates the same four cylinders in the firing order (1, 4, 6 and 7). According to Meaghan, lifter design and pushrod length are the same for all eight cylinders, but camshaft lobe profiles are different for the cylinders, which are deactivated. (This seems, in part, to contradict the GM media website, which states, "...in displacement-on-demand equipped engines, half of the cylinders have unique two-piece valve lifters..." -this being an important stipulation for those wanting to swap camshafts.)"

 

And the full article which is a decent read

http://www.superchevy.com/technical/engine...s/0405sc_gmdod/