I realize that this is not an issue for my Bullitt, but I figured that somebody here might be able to help. I have a '98 Expedition and I recently put on a 3 chamber flowmaster exhaust and a K&N airfilter. Lately the weather has been colder and it has started to ping horribly. Even if the vehicle is warm, if the air is cold, it pings. One particulary cold morning I noticed that it pinged on almost every acceleration, but later in the day as it got warmer the pinging stopped. That night when I drove it home it was pinging again. What do I do? I took it to a dealer and they ran diagnostics, but to no avail. Any help?