I have the 4.2L v6 in a 97 F150. Last year the motor blew and I replaced it with a 4.2 with a 1 year 36000mile warranty. The motor ran great for about 6 months then when driving I heard a faint knock that over the next few minutes escalated to a loud knock and a puff of smoke. I parked the truck and let it cool down for a little bit. Started it back up and it ran fine. So I drove it home and a short ways before home the faint knock returned, growing to a loud knock BUT only when in gear, in neutral it dissapered, I mostly coasted home with the knock growing louder and tried to coast into my driveway when the truck died. The it wouldnt turn over at all like the compression was to great or something and i parked it and left there for about a week in frustration. Now I start the truck and it runs fine, no knock, no loss of power, untill it warms up then increasing knocking. I spoke with the engine company and thier trying to say its preignition or detonation. Any input would be great, thanks in advance.

 

If you installed it yourself you'll need to get a rep from the engine company to come out and look at it. If a garage installed it, take it back to them. Maybe they'll go to bat for you with the engine company.

 

Rod Bearing