Bridge Collapses: Afm To Blame

music · July 3, 2010

I noticed a recent topic derailed (yet again) by the AFM vs non-AFM debate. I don't really 'know' the truth, but I find it interesting. Just to beat the horse, but without totally trashing the last thread where this came up, I figured I'd post my reply in a new thread. I'm going on memory as to how AFM physically works, so I may have something out of whack. I'm sure I'll be corrected if needed...

AFM operates by opening a solenoid and flowing oil through additional passages, diverting pressure from the lifters of four cylinders. The pistons continue to pump, compressing/expanding the air in the chamber, but the valves lock closed and fuel is shut-off to these cylinders. Energizing the solenoids activates AFM, so if they never energize, the system works the same as a non-AFM 5.3 engine (i.e., hydraulic lifters). A custom tune or hand-held can be used to disable AFM, if someone is concerned about that being an issue for them. I recall posts where folks have blamed AFM for their consumption, and then shut it off via a tune, to find that it didn't help. They still blamed AFM, but it doesn't make sense to me. If AFM is disabled, then oil is flowing "normally" to the lifter channels at all times.

I haven't seen any significant comparison of oil usage between the non-AFM and AFM engines. I remember reading very similar threads on the GMT800 trucks regarding oil use years back. It's always been a hit/miss topic... on every make/model in existence. It doesn't seem any more common on the AFM engines to me. TSB's I've seen on this forum and others (for oil consumption) have not been specific to the AFM system, but I understand that GM is working on a TSB (and a part) for folks exhibiting consumption specific to AFM "bypass" oil. Anyone have some actual data or references to support that AFM is the culprit?

music · July 3, 2010

After posting this, I saw the recent AFM debate thread. Been away, and missed it (sorry for the new thread). I've already posted this now, so... anyone have some facts to share on AFM (like real data, statistics, etc... rather than opinion and hearsay)? I realize that some folks probably do have oil consumption due to AFM, but I cannot believe that every (or even most) AFM engines burning oil are doing so due to AFM itself.

kdixer · July 3, 2010

I have never really understood how AFM or so called "overactive" lifters could cause oil consumption. There are 2 main ways for oil to enter the combustion chamber short of being sucked into the intake or catastrophic failure. Those two ways would be via bad valve seals or faulty piston rings. I do not understand why, how active the lifter is, has any affect on oil usage because that is "pre valve seal". Pherhaps overactive lifters cause premature valve seal failure but I have not heard specifics. But an overactive AFM lifter is no more "active" than a non AFM lifter therefore the valve seals should not be vulnerable to failure for this reason. Pherhaps if an exhaust valve AFM lifter is overactive, allowing the piston to expell the air from the cylinder thru the exhaust valve. Then if the deactivated cylinder intake valve did not open (per design) it could create a vacuum and possibly pull oil past the piston rings. But then again I am inclined to believe the piston would suck the intake valve open before it could pull oil past the piston rings. Now my head hurts...LOL.

I would be very interested in hearing other peoples theories on this topic. Exactly how does AFM cause oil consumption?

I personally feel at ease about AFM because my employer has a large fleet of GM AFM equiped vehicles. Some 07 models have experienced oil burn post 100k miles while others have seen over 200k miles with no oil issues. I have heard tale of faulty piston rings to be the blame around 100k miles or so due to improper heat treatment. The rings were supposedly not hardened enough to last. While this rumor makes sense to me it would not explain oil usage in a newer vehicle with much fewer miles.

I would imagine the AFM solenoids have much in common with the various solenoids used in the hydromatic transmissions. And you never really hear people bash them regularly.

nitrohorse · July 3, 2010

I'll post the technical verbiage for the operation of the AFM. It's quite long!

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.

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

Lots of info for sure, but I have had 0 cars/trucks in here with AFM problems, and its been out for 3 yrs now, started on the 05 Envoy V8's. So, as you can see its fairly complex, but again, it has proven thus far to be trouble free. __________________

2008 GMC Sierra XCab Z-71 4x4 Onyx Black

2009 G8 GT Magnetic Gray

GM/ASE Certified Service Consultant

MOE Platinum 04-05-06-07-08

Sir Dan · July 3, 2010

I'll post the technical verbiage for the operation of the AFM. It's quite long!

Cylinder Deactivation (Active Fuel Management) System Description

Lots of info for sure, but I have had 0 cars/trucks in here with AFM problems, and its been out for 3 yrs now, started on the 05 Envoy V8's. So, as you can see its fairly complex, but again, it has proven thus far to be trouble free. __________________

2008 GMC Sierra XCab Z-71 4x4 Onyx Black

2009 G8 GT Magnetic Gray

GM/ASE Certified Service Consultant

MOE Platinum 04-05-06-07-08

I for one wouldn't say it's trouble free... what with all the jerking in and out of v4 mode and strange vibrations. The oil consumption issue is just another nail in the AFM coffin for me.

Stex · July 3, 2010

I'll post the technical verbiage for the operation of the AFM. It's quite long!

Cylinder Deactivation (Active Fuel Management) System Description

Lots of info for sure, but I have had 0 cars/trucks in here with AFM problems, and its been out for 3 yrs now, started on the 05 Envoy V8's. So, as you can see its fairly complex, but again, it has proven thus far to be trouble free. __________________

2008 GMC Sierra XCab Z-71 4x4 Onyx Black

2009 G8 GT Magnetic Gray

GM/ASE Certified Service Consultant

MOE Platinum 04-05-06-07-08

I for one wouldn't say it's trouble free... what with all the jerking in and out of v4 mode and strange vibrations. The oil consumption issue is just another nail in the AFM coffin for me.

I have no problems with AFM and cannot tell when it goes from 8-4-8 except by the dic...maybe you have other issues...my friends, driving 5 other such trucks say the same...no issue, seamless...

BigBlueLB756 · July 3, 2010

Un-technically.....if oil is being consumed or otherwise diminished, wouldn't it be burned and show up as smoke or deposits on the tailpipe, or be found captured in the catch can from by-passed vapors (you do use a catch can, don't you)?

Does the de-activation of these cylinders cause a negative chamber pressure that draws oil vapor up from the pan? (No compression and ignition to provide positive cylinder pressure).

music · July 3, 2010

Does the de-activation of these cylinders cause a negative chamber pressure that draws oil vapor up from the pan? (No compression and ignition to provide positive cylinder pressure).

It shouldn't. The exhaust valve closes to capture the last combustion event, pressurizing the cylinder. At peak "up" stroke, the internal pressure should equal the original pressure generated from the fuel/air combustion, less any losses through the rings. The combustion charge acts as an "air spring" inside the cylinder. If the rings aren't sealing tight, then this could allow pressure to drop over time. Maybe engines with poor ring clearances are more prone to leak down, but if it passes exhaust gas easily then poor compression would more of an issue than oil consumption.

nitrohorse · July 3, 2010

I'll post the technical verbiage for the operation of the AFM. It's quite long!

Cylinder Deactivation (Active Fuel Management) System Description

Lots of info for sure, but I have had 0 cars/trucks in here with AFM problems, and its been out for 3 yrs now, started on the 05 Envoy V8's. So, as you can see its fairly complex, but again, it has proven thus far to be trouble free. __________________

2008 GMC Sierra XCab Z-71 4x4 Onyx Black

2009 G8 GT Magnetic Gray

GM/ASE Certified Service Consultant

MOE Platinum 04-05-06-07-08

I for one wouldn't say it's trouble free... what with all the jerking in and out of v4 mode and strange vibrations. The oil consumption issue is just another nail in the AFM coffin for me.

I have an AFM 5.3 as does my neighbor and my boss. None of us have felt the jerking and strange vibrations that you mention.

Also, none of us have any oil consumption issues.

nathan60 · July 6, 2010

sir dan,mine too jerks and vibrates.no oil usage 5.3l

nitrohorse · July 6, 2010

sir dan,mine too jerks and vibrates.no oil usage 5.3l

Have you taken your truck to the dealership?

music · July 6, 2010

Have you taken your truck to the dealership?

+1. Not normal. I can barely detect it, and only then because I've trained myself by watching the DIC. If I'm not paying very close attention, it's completely transparent to me.