It's so hard to find good info on this topic, so I wanted to share how I was able to replace the fuel filler hose on my 1991 Isuzu pickup (and for only about 30 dollars). The original hose developed a leak in the lower middle to the point that I could not fill up without quite a bit spilling, and attempts to patch it up did not help much.
I bought a 1 and a half foot section of 2 inch diameter fuel-rated hose from a local NAPA store, about 26 dollars with tax. Although it is not particularly flexible, neither is it totally rigid, so it can do a slight curve as needed. I ended up using about 14 inches of it. The diameter need was determined by borrowing calipers from the NAPA attendant and measuring the fuel intake pipe that the top of the fuel filler hose connects onto (I will refer to this as the fuel intake pipe/plate); the digital calipers showed about 1.96 inches. I also used a couple of stainless steel hose clamps (the kind you tighten with a flathead screwdriver) that I already had on-hand, in place of the original wire clamps that were difficult to open/loosen (especially the one nearest the gas tank; that one took me nearly half an hour of trying to reach and squeeze together the two ends with my slipjoint pliers).
I tested loosening the 3 screws that you find just inside the fuel intake door on the side of the truck; about halfway thru loosening they were hard to turn, so I sprayed some WD-40 on them the day before, and I also found a more substantial phillips screwdriver for taking them completely out. Note on that the nuts that they screw into are fixed to the plate of the fuel intake, so you only need a screwdriver; you don't have to hold the nuts in place while turning.
To allow room to work, I took off the rear passenger tire, after blocking the front passenger and rear driver wheels for safety. I also found having a good flashlight was helpful. Even though working outside, since there would be a few gas vapors, I set up a fan to provide a strong current of fresh air toward me at all times. I also had goggles to protect my eyes just in case.
After removing the 3 screws holding the fuel intake plate/pipe to the side of the truck, I pulled off the small vent hose connecting to it. After moving the small clamp down a ways on this small hose, I carefully used pliers to twist it a few times to loosen it enough to be able to pull it free. After this, I closed up the end of it temporarily with some electrical tape, so that gas vapors from the fuel tank would not come out of it while it was detatched. After this, I was able to move the top of the fuel filler hose downward (after pushing the part you see from the outside inward past the hole it comes thru). Of course I had to first take off the filler cap to do this. The next thing to do was to pull the top of the hose off of the fuel intake plate/pipe. It was on tight, so I took a utility knife and cut a slit into the hose for the length of the pipe underneath, about 3 inches. After pulling the fuel intake pipe/plate free, I temporarily taped the fuel intake cap to the hose top, again so that gas vapors would not come out of it during the rest of the procedure.
The next thing was to free the lower end of the hose, where it connects to the gas tank. The hardest part, as mentioned earlier, was getting hold of the two ends of the hose clamp (with the pliers) so that I could pull the clamp further up the hose and thus off of the pipe section (about 2 inches) that portrudes from the gas tank. Took me quite a few tries but thankfully succeeded. I was careful to watch out for other small hoses and such in that area, so as not to pull, nick, etc, anything else under there. Incidentally, I had only about 1/8 of a tank of fuel, but I was still careful not to do anything to create a spark, since I was still working in the vicinity of at least trace amounts of gas vapor.
Once the old hose was pulled free, I put the lower end of the new hose onto the pipe coming out of the gas tank, with the screwdriver-tightened clamp already loosely in place. I clamped it just below the end of this pipe, which again is only about 2 inches in length. Tightened the clamp as much as possible. Then I determined how much, if any, I needed to shorten the top of the hose to be long enough but not too long for connecting to the fuel intake plate/pipe. In my case, I ended up using about 14 inches of the hose, although I could have gone as much as 15 inches. I wanted at least a couple inches of upper hose on the intake pipe. I then positioned the intake pipe/plate so that I could reattach it to the intake door with the screws. I found that I was able to firmly reattach it with the two screws on the right, without the left screw; after trying to position it flush to handle all 3 screws, it seemed that it was "close enough" with just those two, and likely required less strain on the hose overall. All completed, the hose was nearly straight from lower to upper, with just the slightest of bending along the way. Not bad for replacing the original hose that was obviously custom formed with a couple of bends. After completing the main hose attachment at the lower part (gas tank) and the upper part (fuel intake pipe/plate), I also of course reattached the small vent hose that connects to the fuel intake pipe/plate. Both hose clamps tightened as much as possible. On the upper part, the hose clamp was placed about a half inch below the end of the hose, but plenty enough above where the pipe extended into it from the fuel intake.
Hope this helps someone, even without pictures to go with it. It took me about 2 and a half hours, but I'm sure it could be done in less time. Having all tools handy will save time; for example, I had to hunt around for a stubby flat-head screwdriver for tightening the lower clamp in a very small working space. Time also includes taking off and putting back on the rear tire.
Finding a good hose replacement was the main challenge up front. No salvage yard in the area had one, and neither did the closest auto parts store (Advance). The closest Isuzu dealership was far off. Googling around, I noted that NAPA might be a solution, and sure enough, that is where I got the "generic" fuel hose. I knew not to try to use anything else (radiator hose, etc) as it had to be rated for handling fuel. It was sold by the foot, and so I estimated that I needed just short of a foot and a half. 26 bucks, not bad. I thought about having a car repair shop do the actual work, but then decided that, given sufficient planning and time, I could do the job myself. Really, the hardest part of it was working in a relatively cramped space. Otherwise, it was technically not a whole lot different than changing, say, a radiator hose. I've heard of some fuel filler hoses being some sort of "hose within a hose" but at least in this case, it was a simple 2-inch hose.
Well, I'm not a mechanic or a car expert, but perhaps my experiences will be helpful to someone who ends up trying to figure out the same thing.