I own a 1993 F-150 w/the 4.9-6. It has the distributor located on the lower left (Drivers side) of the engine.
Recently I noticed that when it rained, the engine would miss badly on start up. But would smooth out after engine got warm. And would run OK on dry days.
Figured I had a crack in the Dist cap. So I bought a new cap & rotor. Upon changing them, I found that the old rotor was in pretty bad melted shape. Evidently a lot of electrical activity going on inside that cap. Found no cracks in the old cap. But did notice that both the new and the old cap have some kind of vent on them. A black cap fitted loosely on the top on the Dist. cap. Which I'm thinking that due to the low location of the distributor, is allowing water to get inside.
So far no trouble since replace cap & rotor. In fact it has been a marked improvement. But what's the deal w/that black vent thing?

 

It is simply that a VENT.. it allows the condensation that the heating and cooling cycle creates to escape..



I should not admit water unless you are driving around in creeks...

 

White XLT said it perfectly. Even the cap on my 351w has the vent. Its just there to vent the inside.

 

I wonder if the low location of the distributor on my engine might expose the cap vent to more outside moisture than it was designed for. There is no doubt that water was getting inside.

 

The isnt going to be mnuch of a moisture difference in a foot unless you are driving through creeks as White XLT said. Just going throu a puddle and splashing a bit isn't going to kill it. You may check in the inline six forums for more engine specific help, but I wouldn't get catch up on the vent issue.

Ford Inline Six, 200, 250, 4.9L / 300 - Ford Truck Enthusiasts Forums