DanS

Okay guys I need some serious help. I have a 2005 F150 XLT. I really close do driving this truck off a cliff. I keep getting a misfire in the number 4 cylinder. I have changed the plug 3 times. All were Motorcraft plugs. I have changed the coil twice all Motorcraft. . I know it’s not the coil because I have switched them onto different plugs and no misfire on that plug. I keep getting a shutter around 45-50 mph. When the engine light comes on I go to AutoZone and check it. The programmer keeps saying number 4 cylinder misfire. . Every time I take it to the dealer they say it was a plug there is no way I have received a bad plug 3 times. I seriously really close to mail a letter to ford explaining how incompetent there mechanics are that they can not figure out this freaking problem. What could be causing this to happen? An injector, a computer, or what? Please help! All help is greatly appreciated!

UCF-150

clogged injector causing a lean cylinder burning a plug causing a misfire?

lariatboy

Check your fuel pump sending unit relay - an exposed part under the bed, beside the spare tire, against the frame. Notorious for corroding through and throwing misfire codes due to fluctuating pressure and poor flow. Mine threw a cyl# 8 misfire, my buddy's threw a cyl #4, both exactly the same problem. If it wasn't for a knowledgable mechanic, I would have been chasing plugs and coils for days.

Mod (Ret.)

When the number 4 plug was removed (several times), what was it's condition? Did it appear fouled? You may have a bad injector for that cylinder causing the plug to foul. You may have a burnt or sticking valve, causing the plug to oil foul. It's possible that when the plug is replaced with new, it runs good for awhile, then fouls and produces a code. The cylinder itself could be scored, due to a piece that perhaps broke off when the plug was first changed (?) causing oil to get past the rings and fouling the plug. If the plug is oil fouled, do a compression check on that cylinder. If it's low, remove the plug and squirt some oil into the spark plug hole and re-insert the plug and check the compression again. If it increases, it's the rings and/or cylinder walls. If it does not increase, you have a valve sticking, alowing oil to get to the plug.

Is there any way you can post a pic of the plug itself?

Turkey Hunter

Check the voltage on the # 4 wire going to the coil and compare it to the rest.

cabrego

Any luck solving the problem? I have had a similar problem and I am stumped!@!

gone postal

I'd be checking the injector and the harness going to the COP. It wouldn't be the first time that a wire has broken in the harness going to the COP.