Chevrolet Repair: Engine, crankshaft position sensor, chevy cavalier

Question
My daughter has a 1999 Chevy Cavalier 4 cylinder. When she turns the car on the engine shakes. She was told that it might be an injector? She put new spark plugs in but that didn't work. Someone told her that the third plug isn't firing. When she gets the car up to speed it seems to stop shaking. Please Help.....

Answer
Hi Tina,
A bad injector is a possibility, but so is a vacuum leak, a sticking EGR valve, a broken valve spring, a bad plug, plug wire, or coil.
If there is no spark to the third plug, check the wire, and the coil pack.
The crankshaft position sensor, or the ECM are also possible, but I hope you don't just start buying and replacing stuff. Swap positions of some things, like the plug wires, maybe coils, etc, and see if the problem moves, or stays with the same cylinder.
Van