First time here,
Bought a used 2003 F150, 4.6 with 87,000 miles on it, ran great until 92,000 miles. Service light came on, took it my local Auto Zone to read codes, it showed P0303 code, number 3 cyl. misfire. I moved the coil to the number one cyl. to check and see if coil ok, reset light. 130 miles later it came on again, still showing number 3 code. I replaced the plugs all eight, with motocraft Plat's., checked the gap all set at .54 also cleaning all connectors and coils, checking for corosion. After completing replacement of plugs and starting the truck the light did not come on, so great problem solved. The truck still seemed to have lag when I stepped on the gas but other wise seemed ok. 106 miles later the light came on again, re-checked the code's and sure enough the number 3 cyl. code showed up. MPG is at 8.6 but running smooth, every once in a while still has bump when ideling. I doubled check to see if I connected hose and wire's up, all ok. No smoke, or fumes seem to be happeneing.

Thanks for any help,
Bigbluebox3

 

I think you hold the forum record of duplicating posts. Relax, someone will come along and help out. I don't have the 4.6 engine to know the quirks about it. With relocating the coil and misfire stayed in original cylinder, one would thing the coil is eliminated. Have you done a compression check, would be nice to know the condition of the cylinder in relation to the rest. Is this a COP engine or does it have conventional plug wires? As I said I don't have this engine for any experience. Be patient and help will be along.

 

Good Old Bill,
I have read a ton of posts on already on this, it always seem to be bad sparks or assoiated with wires. My 4.6 in not conventional wires, has coil atop each plug.
Bigbluebox3

 

Good Old Bill,
I will locate compression testor to find.

 

Good old bill,
update drove the truck short while ago and the light was on, stopped the truck, re-started and the light was out. Could it be bad wire???Would rather pay the money to fix it the first time instead of all the mystery.

 

The actual wire that connects to the COP, is it clean and tight. Don't just check the terminal but check the wire where it goes into the terminal, the crimp, is it good.

 

Is the engine really misfiring?? I'm reading some of the posts dealing with misfire codes, such as this one, but very few mention that the engine is actually misfiring, just that the CEL comes on and a code is generated.
I've seen some reports that the misfire detection system in these vehicles is not the most reliable performer and might decide to tell some lies.....curious.

Mileage at 8.6? Is that a misprint? If not, then you must be running uphill in 2nd all the time. That ride should be giving close to 20mpg.

 

Fuel mileage numbers have to be wrong. Another area to look at on your engine is the PCV hose may have a small hole in it and these are known to cause misfire codes and intermittent misfire codes. Might want to check it out.