I have a 2001 Ford Expedition with approximately 150K miles on it. I am the second owner. The vehicle is in good condition and was free of any mechanical defects until a few months ago. My problem...Check engine light came on and I was told it was for a EGR valve. I had it fixed. A few days later the light is still on and now the vehicle has what I initially thought was a skip. I have had 2 tune ups. The first tune up was successful for about a month then the skipping started back....it was a busted boot. Ok, so I got that fixed. The fuel pump and fuel filter were also replaced. During this entire ordeal, I've noticed that the vehicle drives smooth but continues to have a skip at around 50-55mph. I usually drive faster than this and I only notice it around this speed. I had another mechanic check it out and was informed that after all of the repairs that he now believes that my overdrive is not working. His theory, when the vehicle is not in O/D the there is not any "skipping". I have not noticed any other issues with the transmission. My overdrive light is working and is not blinking. Could this be a sensor? What is the cheapest yet most effective way of trouble shooting this issue before I end up with an unnecessary costly repair?

 

I suspect you have another boot that is allowing spark to leak through the side of it.