I have a 2000 5.4 Expy and have been troubleshooting a P303 cylinder 3 misfire so far to no avail. I have swapped COPs with a couple of other cylinders to make sure that it was not a coil and found that the truck runs good until it gets up to normal temps and then it will hesitate when pressing the accelerator indicating a miss. I just pulled the plug to confirm it looks fine (3000 Miles on the plugs) and did find a small amount of what looked like coolant on the COP boot. To be sure, I stuffed a rag in the hole to make sure that there was no moisture before reinserting the plug and a differenct COP. There is no CEL, but I get a P303 on a code reader. I just pulled the fuel injectors and checked resistance before swapping the #3 out with the #1 just to confirm it was not an injector. At a glance, there is no broken wiring, however it looks like I will need to look at the COP connector and the injector connector wiring next. Any suggestions on what to do next would be appreciated.

 

I've got to ask only because this throws so many people off, you are doing this to the passenger side, correct?

 

Alloro, Yes, passengers side. Thanks. I know there are instances where folks are working on the wrong cylinder. The third cylinder is the third one in from the front of the engine on the passengers or right side.

 

Correct, you're all set on that point I guess.

 

Another clarification to try and help. You did clear the P0303 code AFTER you changed COPs, but before re-starting, correct.

I have seen many boots crack, ground out the spark, and give you this code, even after flipping COPs.
Also, once you move them with removal, you really need to inspect closely. Also, plenty of dielectric grease.

Not uncommon to have a slight coolant leak and give you fits like what you describe.

Here's one of mine.

 

You could also get a set of noid lights to ensure the fuel injector is getting a pulse under both cold and hot conditions. I think the noid lights might work to check the power going to the coil as well but don't quote me on that one.