Stored 4x4s can suffer from bad gas. Over time gas oxidizes, making it gummy. Eventually, what was once liquid turns to hard varnish. The result is plugged fuel filters, fuel pumps, and carburetors. For mild bad-gas woes, the fix can be as simple as draining the old fuel, changing the filters, and running new gas with aftermarket fuel-system cleaner to de-gunk the lines, pump, and carburetor (the factory Jeep Universal Service Manual also recommends acetone as a fuel-system solvent).
Most degraded-fuel problems aren't that easy to solve, though. We had a worst-case scenario in a 1969 CJ-5, which got parked and stored without having the tank drained and engine run till the carb was empty. The Jeep was converted to an electric fuel pump, which became inoperable from varnish. Carburetor dissection showed both severe funk and rust. Instead of attempting a Band-Aid fix, we decided to start from scratch. Rusty gas tanks can be almost as expensive to fix as to replace. Some radiator shops chemically clean and de-scale gas tanks, but this service is becoming a lost art in many heavily environmentally-regulated areas. Do-it-yourself tank-sealing chemicals might be an option for owners of oddball 4x4s that don't have readily available replacement tanks.
For Jeep owners, Omix-Ada is a one-stop source for new fuel-system parts. In addition to OE and reproduction parts, the company also stocks polyethylene gas tanks for most popular Jeeps. Since our CJ's underseat tank had a rust problem, we decided to go the poly route to avoid additional problems down the road. However, using an OE-spec tank makes for a more straightforward installation.
The first step is determining what you actually have before finalizing what you need. We assumed our underseat tank was stock but discovered that a previous owner had apparently swapped a four-banger fuel tank into a factory-V-6 CJ (either that or Kaiser only had four-cylinder tanks in stock the day this particular Jeep was assembled). The shells are the same, but the sending units and tank bungs are different. Also, our previous electric fuel-pump conversion eliminated the OE fuel-return line.
Gas cap configurations also vary depending on vehicle. To keep the gas tank from distorting, the system needs a way to relieve excess pressure. Some vehicles use a vented gas cap for this purpose; others have a sealed cap with a pressure-relief valve somewhere in the system. Omix-Ada has both cap styles and can help Jeep owners determine what they need.
Tank replacement is fairly brainless. Obviously, the sending unit must be ground to metal when using a poly tank in order for the float level to be transmitted to the gauge. Also, this particular tank setup uses screws and clamps to secure the sending unit to the poly tank. Overtorquing the screws will strip the plastic holes.
Final Notes: We plumbed in Proform two-stage glass-bodied fuel filters to prevent against any further particulate mayhem. The Proform system filters down to 30 microns, ensuring that the carburetor won't get any chunks larger than it can pass. Also, our Jeep's OE Rochester 2-Jet carburetor uses an internal sintered bronze fuel filter. We tried to replace the old one with a modern paper model, which didn't fit. Amazingly, our local Napa store had the correct sintered filter in stock.
The photos here show highlights of what it takes to revive a Jeep that has fallen victim to bad gas. We also added fuel stabilizer to its first fresh fill-up.