Update: I drove the rear wheels up on ramps, set the jackstand just in case, and examined the vapor (charcoal) canisters today. On the '02 4-doors these are laid on top of the fuel tank and separated by at least a foot and 90-degrees of rotation. My goal was to solve my fuel fill problem without dropping the tank, if possible. (NB: I do NOT fault Ford for squeezing these things on top of the tank. I suspect--a guess--that the engineers did this as part of the gen 3 re-design to lower the center of gravity and therefore possibility of rollover. I actually applaud anything to do that!)
First off, it seems nigh on impossible to get the small rusted nuts (9mm?) off to remove these canisters. You can get a closed end ratcheting wrench in there, but the nuts are just too crudded up to move. Its possible that with a long-nosed vice-grips you could accomplish something, but the one I have is from Harbor Freight (pittsburgh brand, a deceptive name for Chinese crap) and worthless.
I decided to remove all of the hose connections from the canisters. My theory is that they did not both fail simultaneously. If I could figure out which one was clogged, I could cut my time/expense of replacement in half.
While I mentioned above that I thought I could get the rear one out w/o tank removal, but not the front one, I now think the front one might be easier to remove (after using a decent long-nose vice-grips or breaking the tab). At a minimum, for the rear one, you'd have to remove the drive shaft (which is likely a lot easier than removing the tank).
I have to give Ford engineers/suppliers credit for the connectors that snap into the canisters. I was able to get all four out by pulling/wiggling (used a small prybar to get an angle on more remote of the rear ones) without breaking any of them. The o-rings looked good and they snapped back in w/o incident. Hooray!
After removing the hoses, I came up with a way to "blow test" the canisters. I took a 12-inch or so length of 7/16 i.d. gas line, the outside diameter of which is just a bit smaller than the inside diameter of the canister ports, and built up the end of the hose on one side with electrical tape. I started further back from the end than the width of the tape, so that toward the end of the buildup I could edge toward the end of the hose and create a taper. My goal was to have an end that would fit securely in the canister ports for blowing (w/o losing air from that port). Here are pictures:
More pix below.
With this modified hose, I was able to lay on my back and blow into one port of each canister. For the front canister, there was very little resistance. I even wet a finger of my free hand and could feel the air blowing full force out of the other port. NB: this front canister attaches to the top of the fuel tank (allowing gasoline vapors to enter), and the other port connects to a three-way connector that connects to both the purge valve in the engine compartment and the rear canister.
The same exercise on the rear canister revealed that blowing into it created noticeably more resistance. I repeated the comparison several times (didn't even have to move) to verify. This told me that if I was going to replace either canister, it should be the rear one. Note that this rear canister connects to the front canister (and purge valve) through one port, and to the rear "fuel vapor separator"/vent hose through the other.
When I originally did this test on the rear canister, I still had the hose to the separator/vent hose connected. I therefore wondered whether the extra resistance was from the canister itself or that length of hose (which is impossible to remove with the tank in). This is what emboldened me to remove the outside connector on the rear canister attached to that hose--even though it was very hard to reach and I was afraid I would break or be unable to reconnect it. After removing it, I repeated my "blow test" and felt the same resistance. Ergo, the hose was not blocked. While it was disconnected, however, I blew some compressed air through it from the other end (where it connects to the vent solenoid) for good measure.
Several times during this exercise, I crawled out and went to the engine compartment, where I had removed the lower, larger-diameter hose from the vent purge solenoid (on the wheel well below the fuse box). I blew compressed air backward through that hose with the canisters disconnected to be sure I got any crap out of the hoses.
Another advantage to constructing/using the hose shown above is that I was able to go back and blow compressed air through each port of each canister. I think I felt some debris come out of the rear one when I blew into the outside port (forcing air back through and out the port that goes to the front of the truck).
After all of this, I put everything back together. I noted one of the vent lines for the rear diff was split, so I cut out the bad part and spliced with a straight barbed connector. I zip-tied the vent hoses back up alongside the fuel fill hose/neck. My plan was to drive it for fifteen miles, try to fill the tank, and if there were no improvement, make arrangements with a buddy to use a hoist, remove the driveshaft, and try to get that rear canister out and replace it.
Well, I drove about 25 miles, went to Costco, and . . . filled up 6.6 gallons full blast without a single hiccup. I'm still not declaring victory, however. I'll wait a few more tank fill-ups before I do that. In fact, my guess is that I temporarily ameliorated the problem with that rear canister, and that it will return. But at least I can order a $60 canister from Rockauto, wait a few days for it to come, and go to the garage on a weekend with the part and a plan for a permanent fix. I sure hope I don't have to drop that tank, though.
