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96 F150 with a 4.6? Does it feel like you are drving over rumble strips? Is your truck 2 orXXXXX Has the check engine light come on? Other the over drive problem how does the trans shift?
The trans in the 1997 F150 4.6 has an issue with torque converter chatter. What we do is either flush the transmission and install the Mercon V fluid and a shudder additive. Instead of flushing you can drop the pan and drain the converter to get all the fluid out. Replace with the Mercon V and install a shudder additive. This may have to be done twice in order to totally eliminate the problem.