Afm Question

508gm1 · March 2, 2011

Hello everyone, relatively new, mostly creeping around for the last few months.

I just purchased a new 2011 sierra 5.3 6 speed and so far I am happy with it except for the standard GM issues - interior rattles. I have been reading alot about the afm and oil consumption issues. I obviously won't know for a while if I will be affected or not. I am writing to ask if anyone knows in detail as to how the afm system turns on and off. I am most interested in what happens to the 4 cylinders when they are de-activated. Do all 4 cylinders deactivate at the exactly the same time, or do the all deactivate individually at a certain cylinder position? If they do activate simultaneously, could that be the source of oil consumption? Let me explain my thoughts.

My understanding is that when the 4 cylinders deactivate, both the intake and exhaust valves close. When the engine is in that state, 2 afm cylinders cancel out the actions of the other 2 afm cylinders. Please correct me if I am wrong.

If all 4 cylinders deactivate at the same time, take worst case scenario. Two cylinders tdc, two cylinders bdc. When the intake and exhaust valves close on the 2 cylinders that are at tdc, and the pistons start on the down stroke, would a fairly substantial vacuum condition not be created? If this is the case, do the piston rings and valves fitted within have tight enough tolerances to withstand a vacuum condition and prent oil from passing by? This would not have an issue on the other 2 cylinders as they would be simply compressing air.

Food for thought, or hogwash? All opinions are appreciated.

Cheers, Brendan

mtiffinal · March 2, 2011

The vacuum thought has been discussed before. There is an in-depth article on here somewhere....try the search function. I don't believe you can have two pistons at TDC.

MS3DALE · March 2, 2011

Hope you like to read.....

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, 7, 6, and 4, 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 cylinder deactivation is commanded, the ECM will determine what cylinder is firing and begin deactivation on the next closest deactivated cylinder in firing order sequence. For example, 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, when all cylinders are active, 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 cylinder deactivation 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 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 cylinder deactivation mode.

During the transition from V8 to V4 mode, the fuel injectors will be turned OFF on the deactivated cylinders. To help prevent spark plug fouling, 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, then return to V8 mode for 1 minute.

Switching between V8 and V4 modes 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.

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 an internal oil 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 a low side driver internal 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 help control 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.

Cylinder deactivation may be inhibited for many reasons including the following:

• Engine coolant temperature out of range for cylinder activation

• Engine vacuum out of range

• Brake booster vacuum out of range

• Transmission gear incorrect or shift in progress

• Accelerator pedal out of range or rate of pedal application to fast

• Engine oil pressure and temperature out of range

• Engine speed of range

• Vehicle speed out of range

• Minimum time in V8 mode not met

• Maximum V4 mode time exceeded

• Decel fuel cutoff active

• Reduced engine power active

• Torque management active

• Catalytic converter over temperature protection active

• Piston protection active, knock detected

• Cylinder deactivation solenoid driver circuit faults

mtiffinal · March 2, 2011

Thanks Rich

508gm1 · March 2, 2011

My eyes are sore now. Thank-you very much for the writeup. I will try and use the search function more often before I post repeat questions.

Cheers, Brendan

cujo8 · March 2, 2011

Man the AFM process seems quite complicated, which makes me wonder just how robust this system will be in the long run. More complicated means more parts and more things that can go wrong. Has there been many problems with the AFM systems, since they were implimented? I just bought a 2011 GMC Sierra that has AFM, so I'm hoping it will work fine for me. My 2006 Sierra did not have AFM and the engine never gave me any problems.

I've read many post on this forum that say the AFM is no good and should be dissabled ASAP. I would like to here what MS3Dale's opinion is on the AFM system, since he seems to know quite a bit about this system. Thanks.

pm26 · March 2, 2011

Switching between V8 and V4 modes is accomplished in less than 250 milliseconds, making the transitions seamless and transparent to the vehicle operator.

In theory.

pm26 · March 2, 2011

Man the AFM process seems quite complicated, which makes me wonder just how robust this system will be in the long run.

About as robust as your stock rear bumper.

cujo8 · March 2, 2011

Man the AFM process seems quite complicated, which makes me wonder just how robust this system will be in the long run.

About as robust as your stock rear bumper.

Now that's not too reassuring.

MS3DALE · March 2, 2011

Man the AFM process seems quite complicated, which makes me wonder just how robust this system will be in the long run. More complicated means more parts and more things that can go wrong. Has there been many problems with the AFM systems, since they were implimented? I just bought a 2011 GMC Sierra that has AFM, so I'm hoping it will work fine for me. My 2006 Sierra did not have AFM and the engine never gave me any problems.
I've read many post on this forum that say the AFM is no good and should be dissabled ASAP. I would like to here what MS3Dale's opinion is on the AFM system, since he seems to know quite a bit about this system. Thanks.

My opinion,and a lot of guys will disagree with me on this.......There is no issue with it,I say that based not only on me not having any issues with my '07 VMAX,But on the fact that we've sold hundreds of cars/trucks with DOD/AFM,and haven't had any issues with any of them.

Now I'm not saying you won't have any issues,I'm just saying that I personally haven't had any issues (keep in mind though that I currently only have 30k on my truck) or any of the vehicles we've sold haven't had any issues with the DOD/AFM system.

Most of the customers I talk to love the system,The only issue around here is all the hills/mountains we have,the system doesn't get to work a whole lot in V4 mode.

441G · March 2, 2011

Man the AFM process seems quite complicated, which makes me wonder just how robust this system will be in the long run. More complicated means more parts and more things that can go wrong. Has there been many problems with the AFM systems, since they were implimented? I just bought a 2011 GMC Sierra that has AFM, so I'm hoping it will work fine for me. My 2006 Sierra did not have AFM and the engine never gave me any problems.
I've read many post on this forum that say the AFM is no good and should be dissabled ASAP. I would like to here what MS3Dale's opinion is on the AFM system, since he seems to know quite a bit about this system. Thanks.

My opinion,and a lot of guys will disagree with me on this.......There is no issue with it,I say that based not only on me not having any issues with my '07 VMAX,But on the fact that we've sold hundreds of cars/trucks with DOD/AFM,and haven't had any issues with any of them.

Now I'm not saying you won't have any issues,I'm just saying that I personally haven't had any issues (keep in mind though that I currently only have 30k on my truck) or any of the vehicles we've sold haven't had any issues with the DOD/AFM system.

Most of the customers I talk to love the system,The only issue around here is all the hills/mountains we have,the system doesn't get to work a whole lot in V4 mode.

rich - do you stock part #12571595?

MS3DALE · March 2, 2011

Man the AFM process seems quite complicated, which makes me wonder just how robust this system will be in the long run. More complicated means more parts and more things that can go wrong. Has there been many problems with the AFM systems, since they were implimented? I just bought a 2011 GMC Sierra that has AFM, so I'm hoping it will work fine for me. My 2006 Sierra did not have AFM and the engine never gave me any problems.
I've read many post on this forum that say the AFM is no good and should be dissabled ASAP. I would like to here what MS3Dale's opinion is on the AFM system, since he seems to know quite a bit about this system. Thanks.

My opinion,and a lot of guys will disagree with me on this.......There is no issue with it,I say that based not only on me not having any issues with my '07 VMAX,But on the fact that we've sold hundreds of cars/trucks with DOD/AFM,and haven't had any issues with any of them.

Now I'm not saying you won't have any issues,I'm just saying that I personally haven't had any issues (keep in mind though that I currently only have 30k on my truck) or any of the vehicles we've sold haven't had any issues with the DOD/AFM system.

Most of the customers I talk to love the system,The only issue around here is all the hills/mountains we have,the system doesn't get to work a whole lot in V4 mode.

rich - do you stock part #12571595?

Yes,That is the AFM lifter (12576400 is the non-AFM lifter) I stock 2 of them,GM recommends I do so under the RIM parts ordering system,I have some "lost sales" of that number,which is why GM recommends I stock them.

441G · March 2, 2011

Man the AFM process seems quite complicated, which makes me wonder just how robust this system will be in the long run. More complicated means more parts and more things that can go wrong. Has there been many problems with the AFM systems, since they were implimented? I just bought a 2011 GMC Sierra that has AFM, so I'm hoping it will work fine for me. My 2006 Sierra did not have AFM and the engine never gave me any problems.
I've read many post on this forum that say the AFM is no good and should be dissabled ASAP. I would like to here what MS3Dale's opinion is on the AFM system, since he seems to know quite a bit about this system. Thanks.

My opinion,and a lot of guys will disagree with me on this.......There is no issue with it,I say that based not only on me not having any issues with my '07 VMAX,But on the fact that we've sold hundreds of cars/trucks with DOD/AFM,and haven't had any issues with any of them.

Now I'm not saying you won't have any issues,I'm just saying that I personally haven't had any issues (keep in mind though that I currently only have 30k on my truck) or any of the vehicles we've sold haven't had any issues with the DOD/AFM system.

Most of the customers I talk to love the system,The only issue around here is all the hills/mountains we have,the system doesn't get to work a whole lot in V4 mode.

rich - do you stock part #12571595?

Yes,That is the AFM lifter (12576400 is the non-AFM lifter) I stock 2 of them,GM recommends I do so under the RIM parts ordering system,I have some "lost sales" of that number,which is why GM recommends I stock them.

rich - do you remember why you had lost sales on that part number?

MS3DALE · March 2, 2011

Man the AFM process seems quite complicated, which makes me wonder just how robust this system will be in the long run. More complicated means more parts and more things that can go wrong. Has there been many problems with the AFM systems, since they were implimented? I just bought a 2011 GMC Sierra that has AFM, so I'm hoping it will work fine for me. My 2006 Sierra did not have AFM and the engine never gave me any problems.
I've read many post on this forum that say the AFM is no good and should be dissabled ASAP. I would like to here what MS3Dale's opinion is on the AFM system, since he seems to know quite a bit about this system. Thanks.

My opinion,and a lot of guys will disagree with me on this.......There is no issue with it,I say that based not only on me not having any issues with my '07 VMAX,But on the fact that we've sold hundreds of cars/trucks with DOD/AFM,and haven't had any issues with any of them.

Now I'm not saying you won't have any issues,I'm just saying that I personally haven't had any issues (keep in mind though that I currently only have 30k on my truck) or any of the vehicles we've sold haven't had any issues with the DOD/AFM system.

Most of the customers I talk to love the system,The only issue around here is all the hills/mountains we have,the system doesn't get to work a whole lot in V4 mode.

rich - do you stock part #12571595?

Yes,That is the AFM lifter (12576400 is the non-AFM lifter) I stock 2 of them,GM recommends I do so under the RIM parts ordering system,I have some "lost sales" of that number,which is why GM recommends I stock them.

rich - do you remember why you had lost sales on that part number?

We had an independent shop asking for 2 of them,I will actually be sending them back to GM after next month (April) if we don't sell them.