My truck has a 351W with a ZF S5-42 4x4 trans. I have replaced the timing chain, clutch, starter, and the alternator Has been replaced twice. I am wanting to know what I can do to max out the general performance of my truck. I have been having a few problems with the engine at idle speed. When it starts cold, it revs up to the 1000-1250 mark then once warm knocks down to about 750-850 where it should be, but then after driving it for a while, when I pull up to a stop sign the idle will stick at about 1500, but then if I tap the gas fast it will rev up and knock back down to normal idle around 750. Could this be a problem internally with the throttle body's butterfly valves? I have been told that I should swap the Speed Density sensor for a Mass Air Flow sensor. Could this fix the problem with the idle?