I have a 2003 F150, I'm having problems with something. It jerks while I'm traveling down the road. Each time I try to speed up, this happens. If I stay at a constance speed, its all good. I change the fuel filter, fuel pump, spark plugs, wires, camshft position sensor and now going to replace purge valve...............HELP!!!
also check for coolant dripping from your heater hose near the firewall (passenger site near center).. a classic _________________ 98 - f150 / 4x4 / 3.55LS
Another possible cause is a flaky throttle position sensor (TPS). Part of its job is to replace the what the old accelerator pump in a carborator did: when you tip-in (press on the accelerator) the TPS sensor detects this and PCM (Powertrain Control Module) will compensate by causing the injectors to put in extra squirts of fuel. If this sensor is stuck or flaky it will cause the engine to "lean out" (fuel/air mixture to be wrong) during tip-ins.
One way to test it is to use a Ford NGS tester and a KOEO (Key-On Engine-On self-test). If you don't have access to an NGS you can hook an ohm meter to the sensor and check for a smooth linear change in resistance as you manually open the throttle-plate. Or you can just replace it (~$35).
