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Discussion Starter #1
I was wondering if this was true or not? I would think I just misunderstood what was posted on another board. I would think the way afm works is the lifters pump up just as much with afm off. It is just that the cylinders do not fire. You are carrying around dead weight since the pistons and valves are still opening. That is way afm is not a true 4 cylinder type mechanism and it is just worth a mile or so per gallon more when activated. I do notice a slight vibration when it kicks in since I changed to a custome x pipe.
 

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Stock I felt it vibrate IMO you tend to notice it more when you have any exhaust mods. As soon as I put my Borla on I noticed it a lot more. Since I went with my Cortex tuner I turned it off and that annoying DOD/AFM vibration disappeared.
 

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had the same problem. went in with my cortex and deactivated the AFM. i have magnaflow catback exhaust on the car. i heard it more when i had the cruise control on going down the highway.
 

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The valves don't open when AFM is activated. There is a pin in the lifters that is controlled hydraulically and it disengages the lifter from the pushrod, so the valves are stuck closed. The ECM disengages spark firing in the AFM cylinders while AFM is activated as well. The only thing happening in the AFM cylinders while it's on is the piston slides up and down.
 

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Discussion Starter #5
thanks for the explanation.

questions - when the lifter which sits on top of the cam is engaged for afm how does the pin work? I am trying to visualize this and I can not. does the push rod just sink into the lifter when the pin is disengaged?

also whether you have afm on or off I assume you are getting the same lift and duration with the valves. Correct?
 

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The lift and duration should be the same since these cars don't have VVT or Variable Valve Timing, like some of the high end 6 cyl cars. My Tacoma had it, but honestly I could never really tell when the vavles timing was any different from normal.

Being that we don't have VVT, there is just the single lobe for the lifter to ride on and therefore the lift and duration cannot change.
 

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Brewtus
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when the lifter which sits on top of the cam is engaged for afm how does the pin work? I am trying to visualize this and I can not. does the push rod just sink into the lifter when the pin is disengaged?
Yes, I believe the rod sinks into the lifter. That's how I'm understanding and visualizing it at least. I think that's why the lifters are considered "weaker" on the AFM cylinders: because they're hollow.

Edit: By hollow I mean they are allowed to collapse downward on the inside so that the pushrod can sink into the lifter.

also whether you have afm on or off I assume you are getting the same lift and duration with the valves. Correct?
If you're talking about the non-AFM valves/cylinders, yes, I believe lift and duration doesn't change... if for the simple fact that it's the same camshaft and there's no variation of of VVT on the L76.
 

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thanks for the explanation.

questions - when the lifter which sits on top of the cam is engaged for afm how does the pin work? I am trying to visualize this and I can not. does the push rod just sink into the lifter when the pin is disengaged?

also whether you have afm on or off I assume you are getting the same lift and duration with the valves. Correct?
Think telescope.
I hope that other info helps.



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If you want visualizations, go here: http://www.g8board.com/forums/showthread.php?t=19612

Great write-up by bobthemonkee that includes removal of the lifters. I asked a question similar to yours desertg8:

Me said:
Slightly off-topic but...

Can someone tell me what makes a DoD cam, a DoD cam? And what else occurs besides shutting off fuel injectors when AFM is activated?
I learned a LOT in that thread. :thumbsup:
 

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had the same problem. went in with my cortex and deactivated the AFM. i have magnaflow catback exhaust on the car. i heard it more when i had the cruise control on going down the highway.
I have magnaflow axle-backs and just put a MF X pipe on, I already was annoyed w/ the sound/vibration now it just flat out sux. Did you notice much change in MPG w/ the AFM off? I could probably tune a couple more HP out of the x pipe anyhow so I was thinking of bringing it in for that + tune out the AFM.

Do you gain any additional HP w/ AFM off? If no gain did you notice any change in response?
 

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The valves don't open when AFM is activated. There is a pin in the lifters that is controlled hydraulically and it disengages the lifter from the pushrod, so the valves are stuck closed. The ECM disengages spark firing in the AFM cylinders while AFM is activated as well. The only thing happening in the AFM cylinders while it's on is the piston slides up and down.
Spark plugs are always firing, whether or not AFM is engaged or not. I remember the ignition hardware engineers complaining that it put undue stress on their parts.
 

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Brewtus
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Spark plugs are always firing, whether or not AFM is engaged or not. I remember the ignition hardware engineers complaining that it put undue stress on their parts.

My bad. You are correct sir. I must have added spark plugs to the list of things AFM turns off in my mind. I found this for those interested:

Cylinder Deactivation (Active Fuel Management) System Description

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 and 7 on the left bank, and cylinders 4 and 6 on the right bank, 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.

When commanded ON, the ECM will determine what cylinder is firing, and begin deactivation on the next closest deactivated cylinder in firing order sequence. The Gen IV engine has a firing order of 1-8-7-2-6-5-4-3. If cylinder number 1 is on its combustion event when cylinder deactivation is commanded ON, the next cylinder in the firing order sequence that can be deactivated is cylinder number 7. If cylinder number 5 is on its combustion event when cylinder deactivation is commanded ON, then the next cylinder in the firing order sequence that can be deactivated is cylinder number 4.

Cylinder deactivation is accomplished by not allowing the intake and exhaust valves to open on the selected cylinders by using special valve lifters. The deactivation lifters contain spring loaded locking pins that connect the internal pin housing of the lifter to the outer housing. The pin housing contains the lifter plunger and pushrod seat which interfaces with the pushrod. The outer housing contacts the camshaft lobe through a roller. During V8 mode, the locking pins are pushed outward by spring force, locking the pin housing and outer housing together causing the lifter to function as a normal lifter. When V4 mode is commanded ON, the locking pins are pushed inward with engine oil pressure directed from the valve lifter oil manifold (VLOM) assembly solenoids. When the lifter pin housing is unlocked from the outer housing, the internal pin housing will remain stationary, while the outer housing will move with the profile of the camshaft lobe, which results in the valve remaining closed. One VLOM solenoid controls both the intake and exhaust valves for each deactivating cylinder. There are 2 distinct oil passages going to each cylinder deactivation lifter bore, one for the hydraulic lash-adjusting feature of the lifter, and one for controlling the locking pins used for cylinder deactivation.

Although both intake and exhaust valve lifters are controlled by the same solenoid in the VLOM, the intake and exhaust valves do not become deactivated at the same time. Cylinder deactivation is timed so that the cylinder is on an intake event. During an intake event, the intake cam lobe is pushing the valve lifter upwards to open the intake valve against the force of the valve spring. The force exerted by the valve spring is acting on the side of the lifter locking pins, preventing them from moving until the intake valve has closed. When the intake valve lifter reaches the base circle of the camshaft lobe, the valve spring force is reduced, allowing the locking pins to move, deactivating the intake valve. However, when cylinder deactivation is commanded ON, the exhaust valve for the deactivated cylinder is in the closed position, allowing the locking pins on the valve lifter to move immediately, and deactivate the exhaust valve.

By deactivating the exhaust valve first, this allows the capture of a burnt air/fuel charge or exhaust gas charge in the combustion chamber. The capture of exhaust gases in the combustion chamber will contribute to a reduction in oil consumption, noise and vibration levels, and exhaust emissions when operating in V4 mode. During the transition from V8 to V4 mode, the fuel injectors will be turned OFF on the deactivated cylinders. The ignition system secondary voltage or spark is still present across the spark plug electrodes on the deactivated cylinders. If all enabling conditions are met and maintained for cylinder deactivation operation, the ECM calibrations will limit cylinder deactivation to a cycle time of 10 minutes in V4 mode, and then return to V8 mode for 1 minute.

Switching between V8 and V4 mode is accomplished in less than 250 milliseconds, making the transitions seamless and transparent to the vehicle operator. The 250 milliseconds includes the time for the ECM to sequence the transitions, the response time for the VLOM solenoids to energize, and the time for the valve lifters to deactivate, all within 2 revolutions of the engine crankshaft.

The cylinder deactivation system consists of the following components:

• The VLOM assembly

• Eight special valve lifters, 2 per deactivating cylinder

• The engine oil pressure regulator valve for cylinder deactivation operation

• Gen IV cylinder deactivation engine block

• The ECM

Valve Lifter Oil Manifold (VLOM) Assembly
The cylinder deactivation system uses an electro-hydraulic actuator device called the valve lifter oil manifold (VLOM) assembly. The VLOM is bolted to the top of the engine valley, below the intake manifold assembly. The VLOM consists of 4 electrically operated Normally Closed Solenoids. Each solenoid controls the application of engine oil pressure to the intake and exhaust valve lifters on the cylinders selected to deactivate. Engine oil pressure is routed to the VLOM assembly from a passage on the rear of the cylinder block.

All 4 VLOM solenoids are connected in parallel to a fused ignition 1 voltage circuit, supplied by the powertrain relay. The ground or control circuit for each solenoid is connected to the engine control module (ECM).

When all enabling conditions are met for cylinder deactivation, the ECM will ground each solenoid control circuit in firing order sequence, allowing current to flow through the solenoid windings. With the coil windings energized, the solenoid valve opens, redirecting engine oil pressure through the VLOM into 8 separate vertical passages in the engine lifter valley. The 8 vertical passages, 2 per cylinder, are connected to the valve lifter bores of the cylinders to be deactivated. When vehicle-operating conditions require a return to V8 mode, the ECM will turn OFF the control circuit for the solenoids, allowing the solenoid valves to close. With the solenoid valves closed, engine oil pressure in the control ports is exhausted through the body of the solenoids into the engine block lifter valley. The housing of the VLOM incorporates several bleeds in the oil passages to purge any air trapped in the VLOM or engine block.

To control any contamination to the hydraulic circuits, a small replaceable oil screen is located in the VLOM oil inlet passage, below the oil pressure sensor. The oil pressure sensor is a 3-wire sensor which provides oil pressure information to the ECM.

During service, use extreme care in keeping the VLOM assembly free of any contamination or foreign material.

Engine Control Module (ECM)
The engine control module (ECM) is responsible for the management and control of all engine functions. Each ECM comes equipped with a specific set of software/calibrations designed for that engine and vehicle application. The ECM will determine engine operating parameters, based upon information from a network of switches, sensors, modules and communication with other controllers located throughout vehicle. Internal to the ECM is an integrated circuit device called a low-side driver. The low-side driver is designed to operate internally, like an electronic switch. An individual low-side driver controls each valve lifter oil manifold (VLOM) solenoid. When enabling conditions for V4 mode are met, the ECM will command the low-side driver to ground each VLOM solenoid control circuit, in firing order sequence. Internal to the low-side driver is a fault detection circuit, which monitors the solenoid control circuit for an incorrect voltage level. If an incorrect voltage level, such as an open, high resistance, or short to ground, is detected, the low-side driver, along with the fault detection circuit, will communicate the condition to the central processor in the ECM. The ECM will then command a return to V8 mode, set a corresponding DTC, and illuminate the malfunction indicator lamp (MIL) on the instrument panel.
 

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Brewtus
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and...
Cylinder Deactivation Inhibit Reasons
Listed below are the powertrain conditions that will inhibit V4 mode, while operating under light load driving conditions:

• Engine manifold vacuum low

• Brake booster vacuum pressure low

• Accelerator pedal position rate of increase too high, electronic throttle control

• Accelerator pedal position too high, electronic throttle control

• Ignition voltage out of range

• Engine oil pressure out of range

• Engine oil temperature out of range

• Engine RPM out of range

• Transmission gear incorrect

• Transmission range incorrect

• Transmission gear shift in progress

• All cylinders activated via scan tool output control

• Minimum time in V8 mode not met

• Maximum V4 mode time exceeded

• Engine oil aeration present

• Decel fuel cutoff active

• Fuel shut-off timer active

• Minimum heater temp low, HVAC system

• Reduced engine power active, electronic throttle control

• Brake torque management active

• Axle torque limiting active

• Engine metal over temperature protection active

• Catalytic converter over temperature protection active

• Piston protection active, knock detected

• Hot coolant mode active

• Engine over speed protection active

• Fault Active or Fault Pending--cylinder deactivation is disabled for the following faults:

- Brake Booster Vacuum Sensor

- Manifold Absolute Pressure Sensor

- Engine Oil Pressure Sensor

- Engine Coolant Temperature Sensor

- Vehicle Speed Sensor

- Crankshaft Position Sensor

- Engine Misfire Detected

- Cylinder Deactivation Solenoid Driver Circuit

The scan tool output control is used to deactivate half of the engine cylinders, V4 mode, by commanding all of the solenoids ON, or deactivate one cylinder switching to a V7 mode, by commanding ON one solenoid. Listed below are the powertrain conditions that will inhibit V4 mode, or V7 mode, with the engine running, while using the scan tool output control function:

• Engine speed out of range

• Manifold absolute pressure (MAP) sensor fault

• Accelerator pedal position too high, electronic throttle control

• Piston protection active, knock detected

• Engine oil temperature out of range

• Engine oil pressure out of range

• Engine oil aeration present

• Engine metal over temperature protection active

• Accelerator pedal position rate of increase too high, electronic throttle control

• Cylinder deactivation solenoid driver circuit fault

• Engine coolant temperature sensor fault

• Catalytic converter over temperature protection active

• Brake booster vacuum pressure low

• Brake Booster Vacuum Pressure Sensor Fault

• Axle torque limiting active

• Brake torque management active

• Vehicle speed sensor fault

• Engine coolant temperature too high

• Engine not running

• Vehicle speed not zero

• Engine coolant temperature low

• Reduced Engine Power Active, electronic throttle control

• Transmission gear incorrect

• Transmission range incorrect

• Ignition voltage out of range

• Maximum V4 mode time exceeded

Listed below are the powertrain conditions that will inhibit a cylinder deactivation solenoid from being energized, with the ignition ON and the engine OFF, while using the scan tool output control function:

• Engine speed not zero

• Vehicle speed not zero

• Transmission not in park or neutral

• Ignition voltage out of range
 

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No problem. I agree that it is logical to turn off the spark plugs under AFM, but the engineer who wanted them on got listened to.
Out of uncontrollable, morbid curiosity, why exactly did he want them to stay on??
 

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Out of uncontrollable, morbid curiosity, why exactly did he want them to stay on??
Obviously I have no credibility in saying this (I can't find a damn article on their reasoning for it), but maybe the lack of sparking combined with repetitive compression of the exhaust gas in the cylinder would cause a carbon buildup on the "contacts" of the spark gap?
 

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The one thing that I never understood, being a mechanical engineer and all, is why they do not rotate the firing cylinders. Basically, when AFM kicks in, the same 4 cylinders remain powering the vehicle. To me, it would make sense to rotate which 4 where powering the vehicle, this way the system would wear evenly, but hey, I don’t work for GM, and bumper to bumper for a 100K...Oh well

Sorry for jacking
 

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Out of uncontrollable, morbid curiosity, why exactly did he want them to stay on??
The engineer I did talk to was the hardware guy who was against them always on. It could have been there was a good reason for it, but he wouldn't admit. Or it could have just been politics/bureaucracy getting in the way. I don't know for sure.

Obviously I have no credibility in saying this (I can't find a damn article on their reasoning for it), but maybe the lack of sparking combined with repetitive compression of the exhaust gas in the cylinder would cause a carbon buildup on the "contacts" of the spark gap?
That could be the reason, except for the exhaust gas. The valves are deactivated during the cylinders compression stroke, when it's fresh air in there.
 

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The one thing that I never understood, being a mechanical engineer and all, is why they do not rotate the firing cylinders. Basically, when AFM kicks in, the same 4 cylinders remain powering the vehicle. To me, it would make sense to rotate which 4 where powering the vehicle, this way the system would wear evenly, but hey, I don’t work for GM, and bumper to bumper for a 100K...Oh well

Sorry for jacking
Gen IV Firing Order:18726543

Due to the firing order, the cylinder positions, the crank, the harmonics, etc., running on cylinders 2, 3, 5, & 8 is the least unbalanced way to run a V8 on four cylinders.

I don't recommend this, but if you want to see what a random firing is like, drain all the coolant out of your engine, and go for a drive on a hot day. There is (maybe only was) a limp mode where it cuts fuel to half the cylinders, and your engine is cooled by the air coming in. The engine shakes like hell, makes hell of a lot of noise, but it's better than getting stranded in the middle of the desert from overheating.

Oh, and looking at engines that simulated 100k miles, there's little if any wear difference between AFM and not.
 
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