My cylinder 3 is hungry for spark plugs.

[99RangerOffRoad]

My cylinder #3 has gone through 2 plugs working on a third in less than 20,000 miles.

With a fresh plug it runs fine, but after awhile it starts to run rough. It progressively gets worse until it throws a p303 code. Pop in a new spark plug it runs fine. The plugs aren't cracked and they aren't black. I will get some pictures later. I have a feeling it is the coil, but I don't want to throw money at it, I want to fix the problem right the first time.

So does anyone have any suggestions on how to test the coil or what else could cause this problem?

Thanks!

 

[RonD]

Coil is easy to test with an OHM meter.
Google: How to Check an Ignition Coil Pack the easy way
Nice video

While you have the OHM meter out test OHMs on spark plugs wires #1, #2 and #3, see if they are all the same.

Running lean could melt it, but that would show and you would hear pinging noise.

Must be motorcraft plugs in a 4.0l, it will eats other brands.

 

[99RangerOffRoad]

Yeah, I only use motorcraft plugs in it. I will check the ohms when I get some time... Damn summer classes are rough.

So, I just checked the ohms they seem to check out. The secondary resistance is 14 and the manual says between 13 and 15. The primary resistances are consistent across the board.

Here are the spark plug pics that I just took.

 

[RonD]

They look a little brown, but it's a stretch.

Have you tested a bad spark plug with an ohm meter and then a new plug?
Can you clean the plug and have it work again?

You could swap #2 and #3 injector and see if the problem moves over to #2.
Don't forget to release pressure, if you do.

 

[adsm08]

Ohm testing will not reveal a weak coil, or one that only misses under certain loads.

I would just replace the coil pack, especially if it is original.

 

[99RangerOffRoad]

Yeah... I guess I will get a coil pack. They aren't terribly expensive and I did break off the retention tab on mine when I pulled the harness off to test the resistances anyways. Old plastic was just too brittle.

Although I did notice that I hard time completing the circuit with the multimeter so I sprayed some electronics cleaner on the contacts. I don't know if that makes a difference or not on the spark plug.