Ford Repair: detonation, mass air sensor, rock tumbler

Question
97 4.6 t-bird 160k+, looks identical in the engine bay to my friends mustangs... The thing detonates completely at random... Some days it'll run like a top, others it sounds like a rock tumbler at half throttle... I run premium gas from dozens of different stations, and have even run fuel treatment just to be sure... The system threw a lean code a while back, so I replaced the used up injectors, and fuel pump with 24#, and a 190lph, fresh plugs, and fuel filter... I'm guessing if I ragged it hard while it was detonating it would throw the lean code again... What is the deal?? Is it new computer time? New mass air sensor? The motor is just about used up and I want to go with a better design then the old 4.6 romeo when it gets changed, but I don't want to put in a PI 4.6 (maybe a 5.4?) and have these same problems... So, Why is it detonating, and what engine choice for the swap would you reccomend for the heavier t-bird that goes 25k miles a year?

Answer
Mark-

  The detonation could be caused by a lean condition or by carbon build up.  I'd check the pressure right after the regulator and make sure it's within spec.  If that checks out you might take it in to see if it needs to be decarbonized.  Do NOT take it to a dealership for this...i can almost gaurantee they'll say it needs it even if it doesn't.  An unmodifed 4.6L motor doesn't really need 24lb injectors.  They'll deliver too much fuel which can also cause detonation (the extra fuel takes up extra space which raises compression and causes excess heat which can cause detonation).  This will also leave excessive carbon build up due to the unburned portion of the fuel.  If it does need to be decarbonized, I'd start running 89 octane (or whatever mid-grade is in your area).  It's better than 87 octane but it'll burn a little cleaner in your motor than premium.  As for which motor to swap, my obvious first choice would be a post-99 4.6L since it'll have a little more power and it'll be a heck of a lot easier to drop in than a 5.4L.  A 5.4L can be done, but I've seen only a handful of 5.4L MN12 (the body style you have) T-Birds so I don't think there's a dedicated swap kit for it out there which means some fabrication will probably be necessary.  If you want to go that route, contact the guys at www.seanhylandmotorsports.com.  They are masters of all things involving Ford's modular (overhead cam) motors and if they don't carry it, they can probably make it for you.  Hope this helps.

Steve