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My brother in law has a 99 f-150 with the 4.2l v6. It had 2 vacuum leaks that I fixed. He had put new plugs and wires in it and it ran like crap. I pulled the plugs and check the gap. They were all at .040-.045 and the gap should be at .052-.056. I gapped all the plugs to .054 and reinstalled them. Reinstalled the plug wires and they felt like they were a little loose on the plugs, they all clicked in place but had some movement. The truck ran and idled great. Just got a call from him and he said that it sounded like it was back firing through the intake. It only does it when you put a load on the engine. Anyone have any ideas or suggestions for me? :dunno: