The LS family of engines produce an amazing amount of horsepower for a given size. And they also generally get good fuel economy if driven conservatively. (Your mileage may vary Mr. Leadfoot). That means they are excellent engine swap candidates.
However, GM has to meet federal standards for their total mix of vehicles so they have developed new ways to squeeze every possible mile out of a gallon of fuel. Shutting off four cylinders of a V-8 is one of those ways. GM first called this system, “displacement on demand” or DOD for short. Now, for reasons unknown, they’ve changed the name to “active fuel management” or AFM. These systems started showing up in Gen IV engines around 2005.
So, what’s not to like? Eight cylinders when you want power, four cylinders for highway cruising sounds like a cool deal, right?
You bet. Until gearheads like us start swapping parts around with unintended consequences.
Let me give you a brief tour of how DOD works:
1. When cruise conditions are right, the engine control module (ECM) does three things to shut off four cylinders–1.) Turns off ignition coils, 2.) Turns off fuel injectors, 3.) Dis-engages valve lifters.
2. Turning off coils and injectors is a simple electronic signal.
3. Turning off lifters is a little more complicated. The DOD lifters have two sections, one of which can slide inside the other. Under normal load, the two sections are held together by small, spring-loaded pins. When it’s time to shut off, extra oil pressure is sent through a special port. The pressure overcomes the springs and the two lifter halves are now separate. The lower half continues to follow the cam lobe while the upper half takes a vacation and just sits around doing nothing.
4. The extra oil flow is regulated by four solenoid valves which are located under the valley cover. And this is where our story takes a nasty turn.
First, let me tell you about my engine. I bought a DOD 5.3 engine out of a 2005 GMC Envoy (same as Chevy Trailblazer) for several reasons: It was low mileage, only 17,000. It was light weight because it is all-aluminum. It was inexpensive because there’s little demand yet. Plus, I thought it would be cool to have the latest high-tech engine in a 20 year old BMW. It would be the perfect engine swap: Older, light weight car with a light weight, high horsepower engine. German handling with American power. Just perfect.
First thing I did to help the 5.3 fit into my BMW was to swap the Envoy SUV oil pan for a Pontiac GTO pan. The GTO pan has a front sump and works like it was made for a BMW.
Here’s the nasty part. What I hadn’t learned yet was that the SUV oil pan has a special oil pressure regulator built into the top of the oil filter ports. I also didn’t understand that the solenoid valves up in the valley cover are not a typical “off-or-on” type valve. They are a “on-a-little bit, then, on-a-lot” type valve.
So use your imagination. If we send unregulated oil pressure to a “always-on-a-little-bit” valve and if enough gets by the valve to overcome the springs on the little pins inside the DOD lifters, and if the lifters disengage without the coils and injectors also disengaging…..well, I can tell you from personal experience, it ain’t pretty.
Here’s scenario for disaster: The intake valve opens normally on one stroke, we take in a fuel/air mixture, both valves close, we compress and fire the mixture, we get a power stroke–OK so far? Now, our exploded fuel/air mixture is expecting to find an open exhaust valve. But let’s just say that this is moment when our exhaust lifter got a little extra oil pressure and decides to disengage and the exhaust valve stays closed. Now we’re re-compressing our exploded mixture and our poor intake valve wants to open. Talk about pressure. It’s enough pressure to collapse and kill the lifter.
I killed one. I replaced it. It ain’t easy. You have to pull the head. It ain’t cheap. The DOD lifter is $70 plus the head bolts are one-time use. I replaced the lifter before understanding why it died. So I killed two more. Then the car sat for two months while I searched for the cause.
Next installment, I’ll tell you how I fixed the problem for good. And I’ll give you my opinions about whether DOD or AFM engines are something to avoid like the plague or something we learn to embrace.
Wayne
