I thought it was very interesting to see that Ford was increasing production of the 5.4-liter when they have yet to find out what's wrong with some of them. I've never been as satisfied with a vehicle as I was with my light duty '97 F-250. But when the 14,000 mile mark rolled around I started to hear a very pronounced knock when the engine was started cold.
I received a copy of a fax from Ford that they have received the same complaint across the Triton line of engines. Ford has yet to find out what the problem is after eight months. My dealer mechanic was guessing piston slap. The knock goes away a few seconds after the oil pressure has reached its peak. I, and other mechanics I've talked to, seem to think that it would take a sloppy fitting piston longer than 10 seconds to heat up and expand, pointing possibly to a bad bearing.
You certainly had a technician with a good ear listening to the cold start knock in your 5.4-liter at the dealership. I've spoken to a couple of Ford Techs myself, and received similar responses about this mysterious noise which is currently being worked on by the manufacturer. So I made a few phone calls, and with the help of Ford Public Affairs, I was able to get in touch directly with the engineers in Detroit who are handling the problem. I spoke to the Triton Engine Programs Manager Pete Dowding and Supervisor Bob Moore.
The word is to sit tight because the noise has been diagnosed, and a Technical Service Bulletin will be released shortly. They referred to the condition as a "piston to cylinder bore phenomena". It's actually a piston slap condition emanating from the anti-thrust side of the piston while below TDC (Top Dead Center). Yes, it is an unusual piston knock heard only in Park or Neutral, and then going abruptly silent after a short period of run time. The typical symptoms of excessive piston to cylinder bore clearance would be a knock dissipating slowly as piston temperatures rise while increasing with the application of torque. The service bulletin will be relevant to the complete line of Triton engines from '97 to '99 (4.6L V8, 5.4L V8 & 6.8L V10). Ford is estimating that only one out of every one thousand will be affected.
Unfortunately the repair is going to involve some major engine work comprising the replacement of all eight or ten pistons. The updated pistons have received the appropriate dimension adjustments in order to nullify the knock. The good news is that Ford engineering is confident that the minimal piston slap will not, and has not, inflicted any damage on the cylinder walls.