I have disabled a couple of AFM units years back, there is a kit available to mechanically disable them out of a place in Texas, works fine. I found Suburbans could still give 18 mpg or more on flat ground with AFM disabled. Once the laws about smog checks tightened up I have gone to a factory stock repair, installed a new AFM manifold with new lifters and oil pressure sender on each rig I have fixed. When working properly the AFM does offer better mpg. I know, you have to be willing to pull the heads to do this work. I also pull the oil pan and replace the oil pressure relief valve that is near the oil filter and the Oring on the oil pick up tube. It is a lot of work but once done your engine is good for many more reliable miles and will pass smog in all 50 states and eliminates pesky oil pan leaks. The states are cracking down on modifications.