Hi,
I have a problem with a misfire and/or stalling on my 03 F-250 with a 5.4L. It happens only in rainy or foggy weather (humidity related?). I thought I had it figured out after replacing a clogged catalytic converter (which btw blew the manifold gasket and cost me a lot of $$$). It was OK for about 5 or 6 months. Now it's back. But once it warms up it's good. I was wondering if it could be the cat. converter again (???) or if there is something else I'm missing. Thanks for any help.

 

How many miles are on the truck? When the last time the plugs and COP boots were changed? If moisture is getting inside the COP boot you'll more than likely get a misfire. Might be a good idea to replace the O2 sensors also and clean the MAF. The MAF and O2s play god in these engines.

 

The truck is just under 90K. Around a year ago I replaced the fuel filter and all plugs. The dealership replaced two coils (5 and 8) right after that.
I'll have to look at those sensors. Thanks.

 

They usually say replace the O2s every 70k miles but they may not be your problem. More than likely you have bad boots. Many replace the plugs and overlook replacing the boots which are very important. The boots can have a crack in them that you can't see. I'd replace the boots and put a dab of dielectric tune-up grease in each end of the boots. Might as well torque the plugs while your there.

Also I'd check for DTCs also.