Cylinder deactivation, like a lot of supposedly new technologies, has been around for more than a century. The first production engine ever to shut down some of its cylinders to save fuel was the 1905 Sturtevant, followed by the 1917 Enger "Twin-Twelve." General Motors revived the idea in 1981, with the Cadillac V-8-6-4, a glorious catastrophe that proved a harbinger of things to come after the advent of sufficiently sophisticted computer controls. Following its rebirth after a century or so of being largely forgotten, cylinder deactivation still had some bugs to work out -- and those bugs have tap-danced giddily in the intake valleys of many a GM truck.
General Motors' cylinder deactivation system, which it calls Active Fuel Management, works very similarly to that used in the old Cadillac V-8-6-4 engine. Under normal conditions, AFM engines allow high-pressure oil to go to the hydraulic lifters on the camshaft. The oil "pumps up" the lifters, allowing them to push up on the pushrods and fully open the valves. This much is standard fare for any typical hydraulic-cam engine. An AFM engine differs in that it has solenoids and valves that bleed off oil pressure from two or four sets of lifters, effectively collapsing them and keeping the intake and exhaust valves on those cylinders shut. The computer immediately shuts off those cylinders' fuel injectors and spark plugs, deactivating the cylinders.
The symptoms for the notorious AFM lifter failure vary from vehicle to vehicle, depending largely on the cause and severity of failure. Commonly, it manifests as a ticking noise on cold startup, when the engine has been sitting for two hours or more. During this time, the oil has cooled down, gotten thick and drained out of the lifters and oil passages. It's not uncommon for an affected vehicle to experience some degree of lifter tap on cold startup, until the oil's heated up, pressurized the lifters and the lifters' hydraulic mechanisms have broken free. It's not a fatal flaw in itself, but it's a sign that bad things may be just around the corner.
AFM lifters tap because they're stuck, and they generally get stuck because they're not working right. Low engine oil levels, dirty, contaminated or old oil, as well as debris and air in the oil can cause this kind of tapping. But more often than not, it's a sign that the lifters are going. Once the lifters begin to fail, you'll see intermittent misfires on the numbers 1, 4, 6 and 7 cylinders, accompanied by a check-engine light and a P0300 diagnostic code. When the lifters go completely, you'll have a permanent misfire on those cylinders, forcing the engine to stay in four-cylinder mode. The computer should recognize the failure and keep fuel and air to those cylinders cut; you'll experience it as a serious lack of power and the maddening ticking of collapsed lifters.
A compression test on the aforementioned cylinders will tell the tale. On a warm engine: If the running compression of the misfiring cylinder stays below 25 psi regardless of whether the AFM solenoid is on or off, then one or both lifters on that cylinder have failed. At first, switching to a lighter-weight, high-detergent synthetic oil and quality high-flow oil filter with anti-drainback valve can help tame the ticking. Some owners have reported success with this approach, perhaps saving the lifters before they failed completely. In most cases, though, this is just buying time before the inevitable lifter failure. Expect a lifter replacement in the near future.
General Motors is aware of the problem, and they've issued a few Technical Service Bulletins to cover it. Note that this isn't the same as a recall. A TSB means you pay for it yourself if the vehicle's out of warranty -- which most with this problem are by now. Look up GM TSB number 10-06-01-007 for further details. Affected vehicles include almost every AFM-equipped GM V-8 vehicle made from 2007 to the 2010 model year. These include all GM trucks and SUVs apart from the V-6 Colorado, the 2007 Monte Carlo, and it's cousins the Pontiac G8 and Saab 9-7X. It also includes L92 6.2-liter engine vehicles built before April 1st, 2006 -- GM built these engines using those defective AFM lifters, even though the AFM itself was disabled.
