sigmanf150

Guys I've got a significant miss (stumble) at idle after I go through a really large puddle or even an automated car wash. Once I let the truck sit for a few hours and dry, the miss goes away. It seems like moisture is causing the problem. I'm thinking about letting it dry out and taking a spray bottle and run the truck and start spraying till it stumbles.

It has to be a coil or plug or electrical in nature I think. A vacuum leak would not be intermittent like this I would think.

Thoughts?

Summers22

iTrader: (2)

Well COP's would do that, but not to the extent of a puddle. Most likely your FPDM on your rear axle. They get corroded and short out.

sigmanf150

Hmm. Hadn't thought about the FPDM. Truck does have 293k. Is there a way to test it?

Summers22

iTrader: (2)

Yeah, drop your spare and pull it. It has a aluminum housing and when resting against the axle that is steel, it corrodes. It will look real nasty or okay. A visual inspection after removing it will tell all.

sigmanf150

Well, I changed the FPDM with a new Doorman unit with the stand offs, but I still had the problem of rough idle and misses after a car wash, so I used a spray bottle and sprayed the first COP on the passenger side and sure enough, it started rumbling and missing. So I blew out the cavity with compressed air, removed the coil pack and the spark plug, changed it with a new Motorcraft SP-515 plug, new boot and spring, greased it up, tested the COP for primary and secondary resistance, and put everything back together. Fired right up and no more missing!