PO 455 on 1998 Explorer

Hi, I have a 1998 sport sohc 4.0 that has 126000 miles on it. The
error code is PO 455. The read out on the reader says " PO 455,
evaporative emission ctrl. sys leak detected. ( gross Leak ). I
replaced the gas cap, dropped the spare tire and the plate that holds
the vapor canister and the vapor valve and inspected by both smell and
sight. I traced the tubing to the engine but was unable due to inspect
the tubing under the battery and AV compressor. Today it warmed up to
75 and I sniffed for gas vapor in the engine compartment and at the
gas cap.

If you have had experience with this system and would be willing to
share with me I would appreciate it. The parts are expensive, so I am
looking for a back yard diagnostic process if possible. I have
searched the internet and have found the ford process of injecting
smoke into the system, and another troubleshooting method of bench
testing the valve and canister but that required a test machine. I am
getting voltage to the valve with the ignition turned on.

Thanks in advance

Everett Cotton