An AFM/DFM disabler keeps a GM V8 or V6 running on all cylinders by stopping cylinder deactivation from switching on. Owners use one to avoid the switch to fewer cylinders at light load — the drone, the shuffle in and out of V4 — and because of concerns about the special lifters the system uses. The trade-off is the fuel the system was designed to save.
Below: what AFM, DFM and HEMI MDS are, the ways to turn them off, and the honest pros and cons.
What is AFM?
Active Fuel Management (AFM) is GM’s cylinder deactivation system, used on many V8 and some V6 gas engines in trucks and SUVs. At steady, light load — cruising on the highway, for example — the engine shuts down half its cylinders and runs as a V4 (or V3 on a V6). Press the pedal and it switches back to all cylinders.
It works with special lifters on the deactivated cylinders. Oil pressure, controlled by solenoids in a valve lifter oil manifold, collapses those lifters so their valves stay closed.
What is DFM?
Dynamic Fuel Management (DFM) is the newer version, used on GM’s later 5.3L and 6.2L V8s. Instead of just V8 or V4, it can shut off any number of cylinders in many combinations, changing from moment to moment — so the engine may run on as few as two cylinders under very light load.
AFM vs DFM
| AFM | DFM | |
|---|---|---|
| Modes | All cylinders or half | Many combinations, as few as 2 cylinders |
| Deactivating lifters | On the cylinders that shut off | On all cylinders |
| Found on | Earlier GM V8/V6 trucks and SUVs | Later 5.3L and 6.2L V8s |
What is MDS on a HEMI?
The Multi-Displacement System (MDS) is Chrysler’s version on the 5.7L and 6.4L HEMI V8 in Ram trucks, Dodge and Jeep models. Like AFM, it shuts down four cylinders at light load. Owners turn it off for the same reasons. HEMI tuner →
Why owners disable AFM and DFM
- Feel and sound. The switch between modes can be felt, and some engines drone in V4.
- Lifter concerns. Failed or stuck AFM lifters are a well-known complaint on these engines. Disabler makers claim that keeping the engine on all cylinders reduces lifter wear. There’s no independent long-term data that settles it either way.
- Oil consumption. Some owners link higher oil use to cylinder deactivation; disabling it is a common response.
- Towing and response. Staying on all cylinders means no delay while the engine switches back.
How to disable AFM or DFM
- Plug-in AFM/DFM disabler. A small device in the OBD-II port that keeps the system from activating while it’s plugged in. No wiring, and you unplug it to go back.
- A tune. Many custom ECU tunes can turn cylinder deactivation off in the calibration.
- AFM delete. A hardware conversion: the special lifters are replaced with standard ones, usually with a new camshaft and a block-off for the oil manifold, plus a tune. It removes the system for good — and it’s the most expensive and labor-intensive route.
- Tow/Haul mode. On many GM trucks, Tow/Haul mode keeps the engine on all cylinders more of the time. It’s free, but you have to select it every drive.
Pros and cons of an AFM disabler
| Pros | Cons |
|---|---|
| No switching in and out of fewer cylinders | You give up the fuel the system was designed to save |
| Smoother, more consistent feel | It doesn’t repair lifters that are already damaged |
| Plug-in versions install in seconds and unplug | A plug-in disabler occupies the OBD-II port |
| Cheaper than an AFM delete | It adds no horsepower |
Does an AFM disabler add horsepower?
No. It changes when the engine runs on all cylinders, not how much power it makes. More power comes from a tune. What does a tune do to a car? →
Where Monte fits
Monte is a tuner, not an AFM/DFM disabler: it doesn’t turn off AFM, DFM or MDS. It adds power with a calibration for your engine and year — up to 20% on turbo engines and up to 12% on naturally aspirated ones such as GM V8s and the HEMI. Monte has to be plugged into the OBD-II port directly, not through a splitter, so it can’t share the port with a plug-in disabler. GM V8 tuner → · Truck tuner →
FAQ
What does an AFM disabler do? It stops GM’s Active Fuel Management from shutting off cylinders, so the engine runs on all of them all the time.
What is the difference between AFM and DFM? AFM switches between all cylinders and half. DFM can shut off cylinders in many combinations, down to two, and has deactivating lifters on every cylinder.
Does an AFM disabler prevent lifter failure? Disabler makers claim it reduces lifter wear. There’s no independent long-term data that proves it, and it won’t fix lifters that are already failing.
What is an AFM delete? A hardware conversion that replaces the AFM lifters with standard ones, usually with a new camshaft and an oil manifold block-off, plus a tune.
What is MDS on a HEMI? Chrysler’s cylinder deactivation on the 5.7L and 6.4L HEMI. It shuts down four cylinders at light load, like GM’s AFM.
Can Monte disable AFM or DFM? No. Monte is a tuner that adds power; it doesn’t change cylinder deactivation.