Cylinder deactivation is not exactly a new engine technology, and while it was never widely adopted, GM first used it on some Cadillac models more than 20 years ago. If truth were told though, the operation of the first iterations of cylinder deactivation systems was not only harsh, but these systems were also extremely prone to failures and a result, most customers disliked and distrusted cylinder deactivation systems. However, several car manufacturers have gone to great lengths to iron out the bugs, with the result that modern cylinder deactivation systems are now fairly reliable. Thus, in this article, we will take a closer look at some modern cylinder deactivation system in terms of both how they work, and what can go wrong with them, starting with this question-
For the purposes of this article, cylinder deactivation refers to various strategies to deactivate some cylinders on V-type engines during light engine loads in attempts to meet ever-more stringent emissions regulations in some markets or jurisdictions, and most notably, in the state of California. As a practical matter, the development of the first cylinder deactivation systems by GM predates the trend to downsize engines, and then to boost the power output of downsized engines with forced induction and other engine and fuel management strategies.
Nonetheless, the idea of deactivating some cylinders on V8 and V6 engines went a long way towards meeting the emissions regulations that were in effect when GM released what it called "Displacement on Demand" systems in 2005. This system was first implemented on the 2005 Chevrolet Trailblazer and GMC Envoy models, whose 5.3L Generation IV engines could deactivate cylinders 1 & 7 on the left bank and cylinders 4 & 6 on the right bank, leaving four cylinders to produce power under certain operating conditions.
When this system worked, it worked well; in fact, since Trailblazers, Envoys, and similar SUV’s only required about 25HP to maintain cruising speeds above 110 km/hour on level roads, these engines provided excellent fuel economy and greatly reduced emissions when 50% of their cylinders were deactivated. We need not delve into the complexities of the enabling conditions that restored full engine operation here, except to say that the same cylinders were always deactivated and that both deactivation and reactivation of cylinders occurred automatically when all the respective enabling conditions were met.
Soon after 2005, GM renamed their Displacement on Demand system to “Active Fuel Management”, mainly as a result of a), Jeep/Chrysler/Dodge introducing an almost identical cylinder deactivation system they called “Multiple Displacement System” on Hemi engines, and b), Honda/Acura introducing their own cylinder deactivation system, which they called “Variable Cylinder Management”.
You may ask why any of the above is important. You may well ask because while the above systems represent ancient technology in terms of automotive engineering, these systems form the basis of new cylinder deactivation systems that no longer always disable the same cylinders. One example of such a system is known as "Dynamic Fuel Management", which debuted early in 2019 on GM’s 5.3L and 6.2L engines, otherwise known as the L84 and L87 engines, respectively, which begs the question of-
Before we get to the specifics of this system, it is worth mentioning that high oil consumption was not only a common feature of all cylinder deactivation systems- it was also a condition that resisted all attempts at rectification on all applications.
Briefly, the problem involved the fact that piston rings need both compression and combustion pressure to work since the spring tension of rings is not enough to push them against the cylinder walls with sufficient force to establish and maintain a positive seal between their sealing surfaces and the cylinder walls. Therefore, pistons are designed so that both compression and combustion pressures are able to exert a significant force from behind rings to push the rings against the cylinder walls, which is the principal mechanism that enables piston rings to a), contain combustion pressure, and b), to largely prevent oil on cylinder walls from seeping past the rings into combustion chambers.
Given the above it is hardly surprising that engines fitted with cylinder deactivation systems should suffer from high oil consumption, simply because the mechanism that ensures the effective operation of piston rings is removed when cylinders are deactivated. Put in another way, this means that when cylinders are deactivated, oil control is dependent on the spring tension of the rings alone, which, as it turned out, was a less than successful strategy.
However, GM in combination with Delphi Technologies seems to have found at least a partial solution to the problem of high oil consumption. The Dynamic Fuel Management system, which Delphi Technologies refers to as “Dynamic Skip Fire”, has the ability not only to trap a low-pressure combustion charge in deactivated cylinders but also to deactivate different cylinders in both fixed and almost countless combinations or patterns.
In practice, this system uses eight oil control solenoids, each of which controls both valves on each cylinder. So when any given cylinder is deactivated, the control system disables the fuel supply to that cylinder, but not before combustion has taken place in that cylinder. The result of this is that since the system deactivates both valves, much of the combustion pressure is retained within the deactivated cylinder, where the pressure assists the rings with effective oil control. When a deactivated cylinder is reactivated, the control system opens the exhaust valve first to evacuate the trapped exhaust gas, before reactivating the intake valve to allow reactivated cylinders to revert to the normal operation of the compression/combustion exhaust/intake cycle.
Nonetheless, the greatest advantage this system has over previous cylinder deactivation systems is that by being able to deactivate different cylinder pairs in subsequent engine cycles, the system can reduce, if not eliminate NVH issues that occur at different engine speeds and loads when fixed cylinder pairs are deactivated. Although most V8 engines are inherently well-balanced, the amplitudes of torsional vibrations that occur within a V8 engine are for the most part determined by both engine speed and load, so when the same cylinders are always deactivated regardless of the engine speed and load, the loss of power contributions by deactivated cylinders can compound some vibrational frequencies.
The trapped exhaust gas in deactivated cylinders acts as a kind of “spring” that goes a long way towards balancing the upward inertial movement of pistons in deactivated cylinders against the normal compression pressure of functioning cylinders, which reduces some torsional vibrations. While this does not address vibrations resulting from the loss of power contributions by deactivated cylinders, the control system can “tune out” most engine vibrations by deactivating different cylinder pairs in each engine cycle, with the current engine speed and load determining which cylinders are deactivated in any given engine cycle.
There is not an abundance of technical information available to the independent repair trade in terms of maintenance, servicing, repair, and programming issues on GM’s Dynamic Fuel Management system but since this system is still very new, it is unlikely that many independent workshops will encounter these systems anytime soon. Nonetheless, the same cannot be said for older cylinder deactivation systems, so let us look at some-
In case you need a reminder:
From an engineering perspective, the AFM system used by GM and the MDS system used by Jeep/Chrysler/Dodge is for all intents and purposes indistinguishable from each other. Both systems use oil control solenoids to move spring-loaded pins in valve lifters to effectively shorten the valve lifters. In practice, the valve lifters that control cylinders that can be deactivated consist of inner and outer parts that can be either coupled or decoupled from each other by moving the locking pin with pressurised engine oil.
When the two parts of the valve lifters are locked together, the lifters’ effective length is reduced, meaning that the lifters cannot open the valves. However, it should be noted that when cylinder deactivation is initiated, the oil control solenoid’s operation is timed to allow the deactivation of the exhaust valves first, thus allowing a deactivated cylinder to trap the exhaust gas from the last combustion event in the cylinder. Note also that while this feature is also present on GM’s new DMF (Dynamic Fuel Management) system, the older AFM and MDS systems are not able to disable different cylinder pairs at different times.
As mentioned elsewhere, trapping exhaust gas within a disabled cylinder reduces both oil consumption and engine vibrations, but a feature of these systems is that cylinders are deactivated only for ten minutes at a time, before full V8 operation is restored for one minute, with the transition from one mode of operation to the other taking less than 250 milliseconds. In theory, the 10-minute/1-minute cycle can be continued indefinitely- provided of course that all enabling conditions are met, but note that spark is maintained during cylinder deactivation to prevent spark plug fouling.
Similarly, the VCM system used by Honda/Acura also uses moveable locking pins that are managed with oil control solenoids. However, the major difference is that instead of causing valve lifters to collapse, the VCM system uses the locking pins to decouple the rocker arm assemblies from the camshaft followers, in much the same way that locking pins in rocker arms are used to activate/deactivate Honda’s VTEC system.
Note though that limited space precludes even a cursory discussion of the enabling criteria/conditions for any cylinder deactivation system, except to say that these conditions are lengthy, complex, and too numerous to list them all here, which brings us to-
Since all cylinder deactivation systems use pressurised engine oil and oil control solenoids, all such systems require a plentiful supply of clean, uncontaminated oil to work as designed. In fact, most issues with these systems stem directly from oil supply and/or oil pressure control issues, which means that the symptoms and problems you are most likely to see could include one or more of the following-
Excessive oil consumption and/or fouled spark plugs
Although the trapping of exhaust gas in deactivated cylinders has gone some way towards reducing oil consumption, many drivers of GM and Jeep/Chrysler/Dodge vehicles that feature cylinder deactivation still consider the oil consumption of their vehicles to be excessive. In fact, excessive oil consumption usually manifests from around 50 000 or so km, and to address this issue, GM has released TSB 10-06-01-008F, which states that in cases where oil consumption reaches about 750ml per 3 200 to 4 800 km of normal driving, you should check the following-
The remedies for these conditions include replacing tappet (valve) covers with redesigned units, and/or to fit an oil deflector in the crankcase near the pressure relief valve to prevent oil mist from passing into the valve. Other remedies include stripping down the engine to clean out piston grooves, or to replace pistons when the rings have seized in their grooves. Note that replacing rings and/or pistons generally does not resolve the issue over the medium to long term, since the cylinder bores/walls are no longer round or parallel.
Misfires and/or mechanic noises
These conditions are almost invariably caused by dirty, degraded, or contaminated engine oil that prevents the free movement of locking pins in rocker arms and valve lifters-regardless of the application or OEM manufacturer involved. The problem is that even though one or more cylinders may fail to deactivate fully as the result of sticking or binding mechanical parts, the ECU will still execute various torque-control strategies to prevent the driver from detecting the transition into reduced power mode.
In practice, the ECU will continue to execute all required torque-control strategies for all cylinders that have not deactivated, which could result in the vehicle surging or stumbling. Other likely symptoms could include severe misfires and/or valve lifter-type noises and other mechanical sounds, and particularly on Honda applications such as Odyssey models. It should be noted that there are no quick and easy fixes for these issues: effective repairs usually require disassembly of affected parts to clean out oil sludge and other deposits.
Note though that Honda/Acura addresses "normal" cylinder deactivation noises through the use of highly advanced engine mountings, and noise suppression via the factory fitted audio system.
Feeling the transition
Some drivers complain that they can feel the transition between normal and reduced-power mode, and in most cases, there is nothing you can do to resolve this. However, manufacturers have taken note of this complaint and as a result, have developed software updates that incorporate several strategies to smooth out the transition.
In some cases, these include momentary changes to ignition timing and fuel delivery strategies, as well as adapting enabling conditions to reduce the number of times the cylinder deactivation system can or will operate. Whenever you run into this kind of complaint, the first step would ideally be to check for TSB’s that describe these and similar complaints, but having said that, obtaining the required software patches and updates from OEM manufacturers could be challenging, to say the least.
Nonetheless, although cylinder deactivation systems have improved greatly over the past decade or so, many drivers are willing to put up with reduced throttle responses, or even delayed throttle responses and other persistent problems only for only so long, and in these cases, deleting the cylinder deactivation system from the ECU might be a viable option.
Excessive oil consumption on engines with cylinder deactivation systems is the most common issue customers complain about. Therefore, it is important to understand that a), oil consumption is a function of these systems’ operation, and b), that educating your customers on how these systems work can go long way towards reducing, if not eliminating some of the most common issues your customers are likely to experience with the cylinder deactivation systems on their vehicles.