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Hello, what year is this expedition? It only does this cruising at a certain highway speed? How many miles are on the truck?
What year is the truck, you have it listed as a 1955
Right, that makes a difference when trying to diagnose the possible cause. Other than the tachometer jumping around does the truck feel like it is driving over a washboard road? Is the transmission fluid level full and condition of the fluid bright red and clean or brown to black and smell burnt?
AN expedition is an suv/truck class. No codes, no odd concerns while driving other than the tach moving, do you feel the engine surge?
Does the concern seem to happen at a shift point? Sorry to ask multiple questions, im looking for something to relate to the tach behavior