I rolled under my truck tonight to check the air in the spare. I thought I knew most everything about my truck. Above the spare there appears to be some type of plastic box maybe 5" high about 18" long and 7''wide. Has (1) 2 wire connection, and several small hoses and 1 large soft hose coming out of it. They go to the frame rail and I can't follow them very well cause the fuel tank is there. Will be interesting to know what this does. Thank you... Spotty...
98 F-150 4.6 4-wheel 8' bed extended cab automatic.

 

Its the EVAP purge canister. Its a fuel vapor filter. If you ever have check engine light codes like P0447 or P0448 this could be the problem.

 

It is part of the evaporative recovery system for the emission system.

 

Thanks LXman... Is it tied into the fuel tank or do those lines run up to the engine bay?

Thanks... Spotty..

 

It should be connected to the fuel tank and filler neck as well as running up into the engine compartment and connecting to the rest of the emissions control system. If Im not mistaken that is your charcoal canister...it basically allows for the movement of vapors from so that they arent released. Kinda like the expansion tank in your cooling system.
-Chris

 

Top view of what you're seeing from below.

 

Thanks everyone, nice pic NcRanchero, that's it exactly! Man, I can't believe how rusty my truck seems underneath, that's Minnesota winters I guess, still no rust on the outside though.

Spotty..