1998 F150 XLT 4.6 missfire

[watermelon-man]

We have replaced sparkplugs, wires, ignition coil pack, and it still missfires. What else could be wrong to cause it to still missfire?We have replaced the EGR valve,MAF sensor now.That didn't fix the missfire either. My wife told me I need to take the radiator cap off and drive a new truck up under it.

[wafrederick]

Could be a fuel injector.When the computer sees a missfire,the computer shuts off that fuel injector.Do a compression check on all cylinders first,it could be a "dead" cylinder too and compressoin test will confirm that.

[clutch492]

Misfire on cylinder #1. LOW COPRESSION.

After going through a nightmare on this 4.6L, I discovered that the intake valve was "cupped" which made the valve stem too long. This would not allow the valve to close completely and only had 50 psi compression. A quick fix is to grind off about .50" of the valve stem. Either that, or remove the head and replace the valve. This valve became deformed due to a misfire that created a lot of heat. An intake valve should not be getting that hot since there is air flowing over it all the time. After the compression was restored, the cylinder is firing normally and the valve should not deform any longer. It should last. The only way to get this cylinder to seal, with air pressure in it, was to remove the rocker arm. After measuring other heads, to see how tall the valve stem was, I determined that there was around 50 to 60 thousands of an inch more valve stem on that intake valve. I ground it down, with magnets and rags surrounding it. Put the rocker arm back on, and had compression. Happy Dance!!!