GM AFM Explained | How It Works, Why It Fails & How to Delete It
General Motors has a clever system that lets some of its engines “turn into” a smaller engine on the fly. This technology goes by two names: Active Fuel Management (AFM) and Displacement on Demand (DoD). It’s been used on GM’s LS and LT V8 engines as a way to improve fuel efficiency. When the engine’s computer senses the engine isn’t under load, it kills four cylinders to essentially turn the engine into a V4. But there are downsides, especially if your priority is horsepower (and fun).
What is Active Fuel Management? | GM AFM Explained
Active Fuel Management (AFM) is GM’s trademarked term for cylinder deactivation technology that shuts down half of an engine’s cylinders under light load conditions to save fuel. In plain language, when you’re coasting on the highway or cruising with minimal throttle, the engine doesn’t need all its cylinders firing. AFM steps in to temporarily turn a V8 into a V4 (or a V6 into an inline-3) by deactivating cylinders, effectively cutting the engine’s displacement in half. If you need more power (The ECU determines this mainly by throttle pedal position), the system reactivates those cylinders, so you’re back to full power.
The trick to how this is done is in the valve train. AFM-equipped engines use special valve lifters and an oil control system to disable the valves on certain cylinders when AFM engages. Solenoids in the engine valley divert oil pressure to collapse the lifters on pre-determined cylinders. When those lifters collapse, the valves for those cylinders stay closed and the fuel injectors for those cylinders are also shut off. The piston in each deactivated cylinder keeps moving, but because its valves are closed it’s not consuming fuel or producing power.
This technology has been around for a minute. GM first experimented with cylinder deactivation in the early 80s with the Cadillac V8-6-4 engine. Modern AFM debuted in the mid-2000s when GM rolled out what was then called Displacement on Demand on its Gen IV LS engines, and by 2007 many GM trucks and SUVs had this feature. That has continued with the fifth-generation LT engines. And when it works properly, the system really works. EPA tests show a five to seven percent increase in mpg thanks to AFM. And it’s basically the same system across Gen IV (LS) and Gen V (LT) engines.
LS Engine AFM vs DFM | Displacement on Demand Terminology
If you’re confused by the terms “Active Fuel Management” (AFM) and “Displacement on Demand” (DoD), you’re not alone. The truth is, these terms refer to the same technology in GM engines. “Displacement on Demand” was the phrase GM used early on to describe the concept of shutting off cylinders. When the system went into production, GM branded it as “Active Fuel Management.” So, AFM is essentially the marketing name for the DoD system in GM’s vehicles. In practice, whether someone says DoD or AFM, they mean the cylinder deactivation that turns off half the cylinders under light load.
Is Active Fuel Management Good or Bad? | Pros & Cons
Like any technology, Active Fuel Management comes with its benefits and drawbacks. Let’s break down the major pros and cons of AFM in GM’s engines:
Pros of Active Fuel Management
Improved Fuel Economy: The whole point of AFM is to reduce fuel consumption. By shutting down four cylinders when power demand is low, AFM can boost gas mileage by roughly five to seven percent in ideal conditions. It is most effective when you are doing a lot of steady highway cruising.
Seamless Operation: When functioning properly, AFM engages and disengages almost imperceptibly. The switch between V8 mode and V4 mode is designed to be smooth, so most drivers won’t feel the engine shut down cylinders. One could say it’s “free” fuel economy in that sense.
Maintains V8 Power When Needed: When you need it, mash the throttle and the ECM reignites all eight cylinders for maximum power.
Helps Meet Emissions/Regulations: GM has to keep the government happy when it comes to increasingly stricter standards for fuel economy and emissions. This is their way to be able to continue to provide us with big cubic inch, throaty V8s.
Cons of Active Fuel Management
Added Complexity: AFM adds a lot of complexity to the engine. There are special lifters, oil control valves (solenoids), a unique valley cover with circuitry, and programming in the engine control unit to manage it all. More moving parts and systems means more potential points of failure.
Potential Reliability Issues: Unfortunately, AFM’s track record for reliability is mixed. AFM lifters, which are more complex than standard, have a history of failing or sticking, especially as mileage accumulates. When an AFM lifter fails it can lead to serious engine problems. So any money saved with improved fuel economy can be quickly eaten up by the cost of repairing a failed lifter or damaged camshaft.
Oil Consumption: Engines with AFM tend to consume more oil as they age. It’s not uncommon for AFM engines to use a quart of oil between changes once they have high mileage. Excess oil consumption is a nuisance and could contribute to carbon buildup in the engine.
Performance and Sound Trade-offs: When AFM kicks in a V8 essentially runs as a four-cylinder and the exhaust note changes. Then drivers who love the traditional V8 rumble are left with the boring drone of a four cylinder.
Marginal Fuel Savings: While that five percent fuel economy gain shows up in testing, in everyday use the savings might not be nearly as much. If you do a lot of city driving or heavy towing, AFM will rarely engage. Then all you have is an engine with lots of unnecessary complexity that doesn’t help improve fuel mileage.
AFM Problems & Symptoms | How to Know if AFM is Failing
If you have an AFM-equipped LS or LT engine, there are a few common issues and warning signs that the system is about to fail on you. Number one on the list is lifter failure. The most notorious AFM problem is collapsed or stuck lifters in the deactivated cylinders. If an AFM lifter fails to “unlock,” that cylinder’s valves won’t operate at all and you wind up with a dead hole in the engine. Often the first sign is a tapping or ticking noise from the valvetrain, especially at idle or when AFM tries to activate. This noise is the slack in the valvetrain because the lifter isn’t pumping up. It may start as an intermittent tick and progress to a constant tick. A failed lifter will also typically trigger a check engine light due to a cylinder misfire.
Even if you don’t immediately hear a lifter tick, an AFM issue can manifest as a noticeable misfire and loss of power. Again, this will be because a lifter hasn’t reactivated and you have a dead hole, which obviously results in a loss of power and a rough-running engine. If the failure is just starting and not yet permanent, you might notice it during the transition in or out of AFM mode as a shudder or stumble.
We’ve already mentioned it, but an engine burning oil is another symptom associated with a failing AFM system. You might notice you need to top off the oil between changes, or see blue smoke from the exhaust. The culprit can be oil getting thrown into the cylinders during AFM operation.
If an AFM failure isn’t diagnosed early and repaired, it can lead to collateral damage inside the engine. A stuck lifter can grind down the camshaft lobe that drives it. Eventually you may have to replace the camshaft and all lifters, which is a costly repair. If a lifter comes apart (it has an internal spring and locking pins), it can even send shrapnel throughout the engine. Catastrophic failures are less common, but it’s a known worst-case scenario.
Can Active Fuel Management Be Turned Off? | Tuning & AFM Disable Options
Seeing the potential pitfalls, a common question among enthusiasts and owners is, “Can I turn off AFM and just run on all cylinders all the time?” The answer is yes, GM AFM can be disabled, either temporarily or permanently, through a few different methods. GM built the system to default to V8 mode, so preventing it from entering V4 mode is very doable with the right approach.
One of the simplest ways to turn off AFM without modifying any engine parts is to use an AFM disabler device. These are plug-in tuners that connect to the car’s OBD-II diagnostic port and work by tricking the ECU into not activating the AFM mode. It’s a plug-and-play solution so anyone can do it. The nice part is that it’s fully reversible (unplug it, and the car goes back to normal AFM behavior) and it doesn’t change the factory programming on the ECU. So, if you need to bring your vehicle in for warranty work, just unplug the disabler and put it in your pocket. Many people use these devices as a preventive measure, especially if their vehicle is still under warranty or they don’t want to mess with the computer. However, it is important to note that disablers like this should only be used if your engine is in good mechanical shape because they will not correct any existing mechanical problems.
A more permanent solution is to reprogram the engine’s control software to disable GM’s AFM altogether. This is normally done with aftermarket tuning software or handheld tuners. A tuner can go into the ECU settings and switch the AFM feature to “off” so the car will never attempt to enter 4-cylinder mode. Many performance shops can perform this, and devices like HP Tuners or SCT programmers allow knowledgeable owners to do it themselves. This approach is often taken by owners who aren’t worried about the warranty or are already doing other performance tunes. By eliminating the AFM via software, you also ensure the check engine light won’t come on for any AFM-related reasons and the system truly stays inactive. Keep in mind, tuning the ECU will typically be logged and could affect powertrain warranty.
Remember that disabling AFM in software does not remove the hardware. The four special lifters and oil system are still there, just not being commanded to activate. Some folks disable AFM early in the engine’s life as a preventive measure. After all, if the lifters never collapse, they might be less likely to stick or fail. However, if a lifter is already faulty, simply disabling AFM won’t fix it.
How to Delete AFM Guide
Disabling GM’s AFM electronically is one thing, but what if you want to go a step further and remove the AFM system entirely? This is commonly referred to as an “AFM delete.” An AFM delete involves physical modifications to the engine’s internal components to eliminate the AFM hardware. This is a pretty involved process, but it’s done frequently in the performance world. Here’s a step-by-step overview of what an AFM delete entails and its basically the same whether you have an LS- or LT-based engine:
Replace the AFM Lifters: The heart of AFM are the special collapsible lifters on the cylinders that shut down (in a V8, these are usually cylinders 1, 4, 6, and 7). In an AFM delete, these lifters are removed and replaced with standard, non-AFM lifters. The new lifters are just regular hydraulic lifters that cannot collapse. Typically, all 16 lifters are swapped out for consistency and reliability, especially if the engine has many miles. The lifters are accessible only by removing the cylinder heads, so this is an engine teardown job to some extent.
Install a Non-AFM Camshaft: This may surprise you, but the camshaft in an AFM engine is different from a standard cam. AFM cams have unique lobe profiles for the AFM cylinders that are ground to work specifically with collapsible lifters. They have lower lift and longer duration to accommodate the lifter’s operation. If you leave an AFM cam in place but put normal lifters on it, those cylinders won’t behave correctly and will trigger trouble codes. Of course, this is your perfect opportunity to swap in a performance camshaft.
Remove or Disable the AFM Solenoid System: The valley tray incorporates the Lifter Oil Manifold Assembly (LOMA), which uses four solenoids to direct oil to the AFM lifters when commanded. This connects to four stands, or bosses in the lifter valley that have oil galleries to carry pressurized oil to the AFM lifters. You have two options here. One is to install plugs in the stands cast into the lifter valley and reinstall the valley plate. The second is to use a special valley plate that doesn’t have the solenoids but does have O-rings to seal the connection between the plate and the stands so you won’t leak oil.
Address the Oil System (Optional but Recommended): AFM engines often use a high-volume oil pump and have a special oil pressure relief valve in the oil pan designed to support the AFM system. When AFM is deleted, it’s common to switch to a standard-volume oil pump (since you no longer need the extra oil flow for the lifters and solenoids) and to plug or replace the AFM pressure relief valve.
ECU Tuning: After the mechanical parts are swapped, the final step is to update the engine’s computer tune to disable the AFM programming. With an HP Tuners unit, it’s as simple as clicking a check box to disable the AFM. If you don’t do this, the ECU will think something’s wrong, start throwing fault codes and possibly put the engine in limp mode. And since you’ve probably installed a bigger cam (and made other changes while you’re at it), a complete tune will likely be necessary anyway.
As you can see, an AFM delete is a significant engine modification. It requires pulling off the intake manifold and cylinder heads, and it’s usually done by experienced DIYers or professional shops. Cost-wise, you’re looking at a set of lifters, a cam, new head gaskets/bolts, and other parts, plus quite a bit of labor. Many companies sell AFM Delete Kits that bundle everything you need (engine lifters, trays, valley cover, plugs, a gaskets kit, etc.) for convenience.
Can AFM / DOD Be Deleted Without a Tune
This is a great question that often comes up in discussions about AFM/DoD removal. Let’s clarify the two scenarios:
Scenario 1: If your goal is simply to stop AFM from engaging without opening up the engine, you can do this without a traditional tune by using an AFM disabler device like we described earlier. In this case, you are not deleting the physical AFM components, you’re just preventing them from activating. This approach does not require an ECU reflash, the device itself handles it on the fly. So the AFM hardware stays in place but remains inactive.
Scenario 2: Suppose you went ahead and did the full AFM mechanical delete, but you cannot tune the ECU immediately. Will the engine run? Technically, it can be made to run, but it’s far from ideal. The ECU will still be programmed to use AFM. This usually triggers a diagnostic trouble code (DTC) and could put the car into a limited mode. However, there is a workaround if tuning isn’t immediately available. You can leave the AFM solenoid harness and valve assembly electrically connected (even if you installed a new valley cover, you can tuck the original solenoid pack into the valley or connect it externally) and then use an AFM disabler device to ensure the ECU never actually tries to activate AFM. By doing this, the computer “thinks” everything is still there and the disabler prevents activation. Now you can drive the car without the ECU freaking out. It’s a bit of a hack, but it is a short-term solution until you can get to a tuner.
What Year Did GM Use AFM? Do all LS Engines Have AFM?
General Motors first introduced Active Fuel Management on several engines in 2005, which coincided with the new Gen IV LS engine Beginning in 2005, it showed up on certain models of the Chevrolet Trailblazer, GMC Envoy, Chevy Impala and Monte Carlo and the Pontiac Grand Prix. That list continually grew until by 2010 is was on almost every light truck and SUV and many cars.
When it comes to figuring out what year to avoid AFM, there’s no simple list. Some trims within the same model line included AFM while others didn’t, and during the semiconductor shortage, certain engines that normally used AFM were produced without it. This makes year-by-year identification inconsistent.
The better solution is to check for yourself. The easiest way to tell if an GM engine is equipped with AFM is to check the VIN code and translate the engine code (it changes according to the year). You can also check the valley cover plate on the engine, if you have a V8. You may need to remove the air cleaner box to view it, but if the plate is flat you don’t have AFM. An GM AFM equipped engine will have a valley plate that’s cast aluminum with lots of ridges and oil galleries that are easily visible from the top.
LS Engine Builder Guides
Explore our displacement-based LS engine builder guides to learn more about how to make power in several LS based engine displacements, with or without AFM:
