My F150 started acting up last fall, it would start very hard in humid weather. Had to keep starter engaged even while firing until it would stay running on its own. Now it simply wont fire I crank it but just wear the battery down. Seemed to have something to do with humidity initially.

 

first off, what engine do you have? are you showing any CELs? from the sounds like it, fuel starvation. if its a V6 its probably the cam position sensor

 

If it's a 4.2 or 4.6, it probably needs a tuneup - plugs, plug wires, and maybe a coil pack.

 

here's what I'm wondering, when was the last tune up done, what was done and what parts were used?

 

I'm new to Fords but I had the same problem with another vehicle. It turned out to be a dirty MAF sensor. Once it was changed out the car started up fine. There is a good how-to article on this forum on just how to do that and doesn't cost anything but a can of cleaner.