My 1988 Ford Ranger, w/4 cyl, 2.3L engine @ 135K miles, engine quit running while traveling about 40mph. When the engine originally stopped running it was hard to tell it stopped. It did not bog down and there was not any rough noise or sound of braking components when it happened. While still moving I attempted to friction start the engine by popiong the clutch and it had no effect. I have recently replaced the spark plugs and the vehicle seemed to be running much smoother. I also replaced the fuel filter in hopes that would fix the problem. The fuel pump seems to be working. The engine is being turned over by the starter, but there does not seem to be any ignition in the engine. I need help in solving what to try next.