Question: I have a 2005 Ford F-150 5.4-liter Lariat with 150,000 miles on it. For the last two months, I've noticed a bump or small bang like the ignition was turned off for a split second. This happens only between 40 and 50 mph, and only when I accelerate. If I do a slow acceleration, it doesn't bump or bang. This noise also happens with cruise control on when I come to a hill. If it downshifts on the hill, the bump/bang stops. Again, this happens only between 40 and 50 mph. The transmission has been serviced twice since I bought the truck in 2005. I had it serviced two weeks ago and the bump/bang stopped for about a week, then came back. The transmission works fine above 50 mph or below 40 mph.
Answer: It's really tough to evaluate the condition going by your description. "Ignition was turned off for a split second" sounds like an engine shutdown or a misfire. But you're talking as if it's a transmission problem. And the lock-up torque converter does engage at 40-50 mph. I strongly suggest paying the $100 diagnostic fee at a Ford dealership. If the condition can be duplicated regularly (not random, but once a month), a technician may be able to diagnose the problem right off the bat on a test drive. If not, they can at least narrow it down to the transmission or engine, and take it from there.
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