If anyone has ANY ideas on what I should try next I`d be greatly appreciative. Once my truck, 2005 F150 5.4 Ltr, 106 000 km, warms up it`ll start running pretty rough when I hit cruising speed, anywhere from 80 to 110 km/h. It`s usually triggered after I do a "highway" merge. I do get a misfire at idle but really noticeable when driving, when rpm`s are low and overdrive is on.

Now in the last couple weeks I`ve replaced all sparkplugs (platinum), replaced the coil for cylinder 3, just had the warranty fuel injector work done on cylinder 3 today and the fuel filter has about 3000 kms since it was last changed as well. I keep getting a PO303 Misfire Message, but I don`t know what else could be causing the misfire, it`s all new parts in that cylinder. I do have an after market air intake and have looked at the MAF as well and it still looks clean, I`m at a loss as to what to try next other than taking the truck to the dealer and getting a blank cheque ready to cover the repair?

 

large bottle of STP fuel injector cleaner in about 3/4 of a tank of gas, take a road trip & floor it a couple times on the interstate & run the tank nearly dry before refuelling.
< $5.00 might do it. Works when my trucks get rough.
Good luck.