Hello all. Have a 99 F150 with the 4.2L six. It seems to stumble off idle and when I increase the load as in going up bridges or passing.
I have heard some point to the coilpack. How do I test it?
Fairly new o2 sensors (4) and maf sensor.
Thank you in advance for information. It is my sons truck and we just want it to run correctly.

 

How many miles on the spark plugs and wires, and what brand are they?

The only real way to test a coil pack is replace it. Mine went out at 84k with similar symptoms, not bad enough to trip the CEL.

If you are removing the plug wires for any reason, you should use dielectric grease when you replace them - both ends.

 

The truck has 149K miles. Not sure on the wires, but plugs have 30K. Wires may be 80K old, not sure.
The CEL is on ... again. Even when the MAS was replaced, the light didn't go out. The ABS light is on always too. Can't find anything wrong with the brakes either.

 

If the CEL is on, go get the codes read and post them. Autozone and other parts stores will read them free. We don't need to know what the store says is wrong - we need the actual "P" codes.

What brand are those plugs?

 

I'm guessing you have a vacuum leak somewhere.