If you are planning of buying a vehicle from the General Motors Chevrolet division, such as Silverado, you might be wondering how to tell if a vehicle has Active Fuel Management (AFM) or cylinder deactivation. AFM helps prevent vehicles from using fuel when they aren’t needed.
AFM technology is used in V6 or V8 engines to prevent excessive fuel consumption, hence improving fuel economy. Active Fuel Management System is also called Displacement On Demand as it cuts off the combustion from two or four cylinders when less power is required, thus reducing the net displacement of pistons in the engine.
However, when vehicles get older, people might have observed signs of high wear and tear in cam lobes and oil lifters in the engines with the AFM system. Due to this reason, people want to confirm whether the vehicle has Active Fuel Management System or not.
Also Read: 4.3 Vortec Problems
Table of Contents
What Is Active Fuel Management?
If you have a V8 (8-cylinders engine), half of the cylinders are deactivated i.e. AFM transforms it into V4 under light-load driving conditions. In V6 (6-cylinders) engines, AFM deactivates two or three cylinders and transforms them into V4 or V3 engines respectively.
When AFM is activated, it shuts off the intake and exhaust valves of the cylinders and also prevents the injection of fuel into the respective cylinders. To activate Active Fuel Management, Lifter Oil Manifold Assembly (LOMA) is used. For the L82 engine, OCV (Oil Control Valve) is used to activate AFM.
In V8 engines, AFM deactivates cylinders #1, #7, #4 and #6. In V6, AFM deactivates cylinders 3 and 6. For LTA and LT2 engines, the cylinder deactivation occurs on cylinders 2, 3, 5, and 8.
AFM is also called Displacement On Demand (DOD) or Cylinder Deactivation. In the engine, displacement means the distance a piston moves inside the engine cylinder. Total displacement will be the sum of the displacement of pistons in all cylinders of an engine. When a cylinder is deactivated, the overall displacement of the engine is also reduced. That’s why it is called Displacement On Demand.
Also, keep in mind that Active Fuel Management (AFM) doesn’t kick when your vehicle is idle, just starting, or when driving in medium to high throttle conditions. Moreover, if your vehicle is idling at the stop, AFM will also not deactivate the cylinders in that condition.
What Is Dynamic Fuel Management?
There is a slightly advanced technology called Dynamic Fuel Management (DFM). AFM only deactivates certain cylinders each time when driving in light-load conditions. On the other hand, DFM can deactivate any combination of cylinders to optimize fuel consumption.
Dynamic Fuel Management operates through a sophisticated controller that monitors every movement of the accelerator pedal and runs a complex algorithm that determines how many cylinders are needed to meet the driver’s request for torque. The Dynamic Fuel Management computer can make this calculation 80 times per second. DFM is also called Dynamic Skip Fire.
Unlike AFM, DFM is activated by Oil Control Valve (OCV). You will find DFM on small block engines i.e. L87 (6.2L V8) and L84 (5.3L V8). So, instead of deactivating the same four cylinders, DFM deactivates all eight cylinders of the V8 engine in a rotation.
DFM system is much more complicated compared to the AFM as DFM has 17 firing patterns it can cycle through (16 reduced cylinder patterns plus full V8 mode). The engine control unit (ECU) controls the sequencing of the various cylinder firing orders in the DFM system. The unique firing pattern in DFM means that the engine cylinder fires a fraction of the number of times that it would normally fire for the specific number of engine rotations in V8 mode
Now, in the firing pattern of the DFM system, you will see unusual fractions like 1/3, 2/3, 1/2, 2/5 or 4/9. GM selected these denominators (the digits below the line of fraction i.e. /x) because they all divide evenly into 720 degrees of rotation.
By 720 degrees here, I mean the revolutions of the crankshaft in 4 stroke engine. The complete cycle of a 4-stroke engine is completed when a crankshaft rotates 2 times i.e. 720 degrees. Now, if you divide 720 by any number below the line of fraction in firing patterns of the DFM system, you will get a whole number. For instance, dividing 720 by 9 gives 80. Similarly, dividing 720 by 3 gives 240.
The definition of 2/3 firing fraction means that two out of 3 cylinders are active, leaving one cylinder in the sequence of 3 non-active or deactivated.
Similarly, the 4/9 firing fraction indicates that for every 9 cylinders in the firing pattern, only 4 cylinders will be active or firing. So, for the nine engine cycles (9×2 crankshaft revolutions), each cylinder will fire four times.
In the firing fraction of 1/3, one cylinder will be active out of two cylinders in the firing pattern. So, for 3 engine cycles (3×2 crankshaft revolutions), each cylinder will fire or be active three times.
Here is how the firing pattern grid looks in the DFM system:
In the video below, the person has also explained the firing pattern in the DFM system with great animation:
How to Tell If Your Vehicle Has Active Fuel Management?
If you’re driving Chevy with V6 or V8 engine, you can see Driver Informative Center (DIC) to observe when AFM switches between four-cylinder and eight-cylinder operation. In some vehicles, DIC has buttons next to the instrument panel’s steering wheel.
You can also watch the below video in which you can see whether a vehicle is operating in V8 or V4 mode by AFM. On DIC, you will see a V8 or V4 icon. Switch the DIC to the instant economy (or whatever it’s called) and it will show V8 or V4 above the mpg.
Some vehicles won’t switch back and forth between 8 and 4 cylinders unless they are on a flat road, cruising at a constant speed and under 2000 RPMs. It will maintain full cylinder operation in high-power situations, such as when accelerating from a stop, passing, or merging onto a freeway.
You can use the Diagnostic Link Connector (DLC) to see the firing patterns of the DFM system while driving. You can watch the below video for such an example:
Whichever Chevy vehicle model you have, if you have its owner’s manual, you can check if it has Active Fuel Management or not. Also, when you step on the gas pedal, you can hear something happening through the exhaust. It’s like a drone sound until the RPMs come up. In that case, the engine is switching from V4 to V8.
Components To Identify Active Fuel Management
You can also identify if your vehicle has active fuel management by looking at the following components.
Engine with Active Fuel Management is installed with a Lifter Oil Manifold Assembly (LOMA). The LOMA consists of 4 electronically operated solenoids and is bolted to the top of the engine block beneath the intake manifold assembly. It also has an electrical connector on the back in a dark color as shown in the image below:
The purpose of bridges on the valley cover is to direct the flow of pressurized engine oil to the active fuel management intake and exhaust valve lifters. You can check this link to see the other side of the valley cover. Engine without AFM has a plain cover. It has no bridges.
AFM Towers On Engine Block
The engine block with AFM has an oil gallery that routes the high oil pressure to the valve lifter oil manifold assembly. Also, on cylinders 1,4,6 and 7, there are 2 vertical, cast-in block oil passages called AFM towers. They permit oil flow from the manifold assembly to the valve lifter bores.
Active Fuel Management can only be available in engines with automatic transmission. GM has been using controlled slippage in the torque converter in automatic transmission to absorb some of the vibrations resulting from running an engine on a reduced number of cylinders. The torque converter mechanical clutch is modulated continuously on and off during V4 mode to absorb the additional vibration when V8 is operating as a V4.
Special Oil Lifters
In AFM engines, there are special tall oil lifters in the cylinder block that receives the oil through the AFM towers. You can find these special oil lifters on the four cylinders, namely 1, 4, 6, and 7. AFM lifters have spring mounted on the top.
Valley Cover Gasket
If you remove the valley cover of the AFM engine, you will see an orange-colored gasket with more curves and notches. On the other hand, engines without AFM have a straight gasket without any curves and corners.
Engines with AFM have lifter guides with one notch that indexes on a tab in the block to ensure that it’s installed in the right location. Lifter guides in engines without AFM have a single notch since all lifters are the same. These lifter guides keep the lifters from rotating, thus keeping the roller of the lifter going correct direction on the cam lobe.
High Volume Oil Pump
GM introduced a new oil pump for engines with AFM. The new oil pump has 33% more capacity which pumps 1.26 cubic inches per revolution.
If the standard oil pump is used in engines with Active Fuel Management, it may lead to engine noise and no AFM operation at cruising speeds. Moreover, low oil pressure to the Oil Lifter Manifold can also damage the AFM lifters.
Active fuel management should allow cylinder activation and deactivation are both supposed to occur on the base circle of the cam lobe, making the transition from four to eight-cylinder mode unnoticeable to the driver.
The failure of camshafts and lifters has become more common with the AFM system. The symptoms usually start with chirping and squeaking noises and then progress to misfire conditions.
Due to this reason, the lobes of AFM cams are slightly taller to turn on/off the cam lifters. The cam lobes of AFM engines are ground differently for a smooth operation of AFM lifters.
In AFM engines, camshaft lobe lift is slightly greater to compensate for the AFM lifter spring compression.
Oil Pressure Relief Valve
Engines with AFM have an oil pressure relief valve on the oil pan rail that prevents excessive oil pressure from negatively impacting the AFM components, such as lifters. The oil pressure relief valve bypasses extra oil pumped by high-volume oil pump when AFM is not active.
Which Engines Have Active Fuel Management?
In 2007, General Motors introduced Cylinder Deactivation System i.e. Active Fuel Management. Presently, GM manufactures the following four different engines with Active Fuel Management technology:
|GM Engines Family||GM Engines Models With AFM|
|GM Small Block Gen IV engine family||
|GM Gen V small block engine family||
|First-generation GM EcoTec3 engine family||
|Second-generation GM High-Feature V6 DOHC engine family||
|First-generation High-Feature V8 DOHC engine family||4.2L V-8 LTA Twin-Turbo|
- Vortec 5.3-liter V-8
- Vortec MAX 6.0-liter V-8
- 3.9-liter V-6
- 5.3-liter small-block V-8
Which Vehicles Have Active Fuel Management?
GM includes AFM in the following vehicles:
- Chevrolet Avalanche
- Chevrolet Impala
- Chevrolet Silverado
- Chevrolet Suburban
- Chevrolet Tahoe
- Chevrolet TrailBlazer
Common Problems With Active Fuel Management
Lifter noise and excessive oil consumption are the most common problems with Active Fuel Management.
The oil spray that is discharged from the pressure relief valve in the crankcase may result in carbon deposits in the piston ring grooves that stick to the rings and cause excessive oil consumption. When the oil hits the crankshaft, it is thrown up on the cylinder wall. Due to this, excessive oil is used and can cause carbon deposits.
The AFM lifters collapse or become stuck if the oil is not regularly changed or suitable motor oil is not used for the engine. When motor oil is not changed regularly (normally it should be changed after 5000 miles), the lifters used in the DOD/AFM cylinders gum up and get stuck.
Deactivating AFM On GM/Chevy Engines
You can either use Range tech 003 disabler to deactivate AFM or install AFM delete kit. You will have to replace lifters, camshaft, lifter guides, gasket and valley cover to delete the AFM kit. Also, you need to switch to a standard oil pump.
AFM disabler is plugged into the OBD2 port of the Chevy engine and prevents the ECM from switching over to four-cylinder mode by turning off the ACM setting. However, deactivating AFM using a disabler only works if the lifers and cams are fine. If you feel that the lifters and cams are going to fail, you should use the AFM delete kit to deactivate the AFM.