My 90 Laredo, 4.0 auto, is sparking from the 2nd from the rear spark plug to the valve cover right beneath the heater control valve. The heater valve control looks like it may have a small leak in the line/fittings, I cant see liquid but i can smell it and see some residue. Could the vapor be hitting the spark plug wire connector boot and cause a short? (new heater control valve $32)

 

If any part of the plug wire is arcing to anything else under the hood means that the plug wire needs to be replaced. It's broken down enough to not insulate properly.

Replace that wire.

 

+1 it's time for a new set of wires. You suspect a leak from the HCV but can't see anything... What do you think happens when you drive in the rain?

(On the HCV note, if it's leaking just eliminate it. You can go as ghetto as some barbed fittings with hose clamps or purchase heater hoses from a '97+ XJ. Chrysler stopped shipping them as they are unnecessary and unreliable.)

 

Makes sense, thanks. These wires aren't but about 2 yrs old. Get what I pay for I guess.