I have a 2002 Ford Ranger Xlt v6 4.0l which suddenly lost power to the point I couldn't go up a steep incline. The engine will start up with no problem. I have changed the plugs, wires, coil pack, and one upstream O2 sensor on the driver's side. The engine seems to idle ok when you're in the park position. If you try to throttle up quickly, the engine will bog down. I pulled the EGR valve off and used a vacuum pump to see if the valve would move and it did. Does anyone have suggestions on what else I should try?

 

How many miles on the odometer, are there any codes that you know of? You might wanna try putting in the other O2 sensor.

 

"... which suddenly lost power to the point I couldn't go up a steep incline." ...any circumstances which could shed light?

Vacuum leakage?
Fuel filter (lame suggestion, I know, but you never know).
Stretched accelerator cable?
Clogged cats?