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Edward, ASE Certified Technician
Category: Ford
Satisfied Customers: 2753
Experience:  ASE certified. Auto Transmission rebuilder for Ford Motor Co. from 91 to 2000
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Ford F-150 XL: Hello, I have a 2002 F-150 V6, 4.2L. I had a

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Hello, I have a 2002 F-150 V6, 4.2L. I had a misfire on cylinder 5. I changed the plugs and wires. It drove fine for 30 miles before it started running rough and the CEL began to flash. Now I have a misfire in cylinder 3 (p0303). What should I try next? Thanks.
Hello and thanks for your question. My name is XXXXX XXXXX I will do all I can to help. Just so I'm clear on this, was the original code (the P0305) all that was stored originally? then you replaced the plugs and wires, and got a P0303 after that? and that P0303 wasn't set with the P0305 initally right?
Customer: replied 3 years ago.

Correct, the original code was P0305. After driving it for 30 miles and then getting the flashing CEL, all I have now is the P0303 code.

OK, is it a dead miss? misfiring all the time and running poorly? or runs good most of the time? Do you feel it misfire at all? under load maybe?
Customer: replied 3 years ago.

I did feel it misfire under load. What is a dead miss? It runs poorly as I accelerate.

OK, a dead miss is a constant misfire, you feel it all the time. There is a few things that can cause this Jeff, I will list them, but it will take some fault tracing steps to narrow it down.

I'm going to first suggestion, removing plug number 3, take a good look at it, make sure you didn't crack it puttin it in, (sometimes that happens) If it's not cracked, Get a spark tester or old plug, and pull the wire off cyl. 3.
Make sure number 3 is firing (ground the plug or spark tester to engine metal) We want to make sure the coil pack is proving spark to cyl. 3. at all times the engine is running,

If we do not have spark to cylinder 3, see if the coil pack is arcing or "leaking" spark out the side of it anywhere, and you may spray it down with some water to help see if it's shorted out anywhere.

If we do have spark to cylinder 3, then you'll want to check compression on that cylinder just to be sure compression is alright, If compression is alright, check for injector pulse. with a noid light. If you are getting injector pulse, then swap injector 3 for number 2, see if your misfire moves from P)303 to P0302. If it does, then you have a bad fuel injector.

Last but not least, if you don't have any spark to the plug, The PCM grounds the coil pack to provide spark for the spark plugs, It isn't uncommon for the coil drivers in the PCM to fail. I know you have power to the coil pack if it's running, so I would suspect a failed PCM if you do not have spark to number 3 plug and the coil pack isn't failed.
Edward and 4 other Ford Specialists are ready to help you
Customer: replied 3 years ago.

Thanks for this information. How do I check the compression?

You will need a compression gauge Jeff. A compression gauge screws into the spark plug hole and you crank the engine over and then read the gauge. you want at least 140 psi on your v6.
Customer: replied 3 years ago.

OK. Thanks again for all of your help, Edward.

absolutely, Thank you. let me know if you have anymore questions.