One of the more profound mods has been my extended range fuel tank. I purchased an auxiliary tank from Man-a-Fre (before the suspension part fiasco). It wasn't cheap, but the 19 gallon tank doubles the range of the FJ. As the FJ Cruiser has a short range out of the box, once you add on bumpers, winches, etc., you wind up with a portly truck that gets mediocre mileage, especially under acceleration and climbing hills.
The tank is a fairly simple bolt on, although not without complications (of course).
Hours were spent trying to make sense of MAF's install instructions, which were not only misleading, but wrong. The kit consists of the tank, which fits between the frame rails at the back of the truck, an external fuel pump, a sending unit, which delivers fuel level info to a fuel gauge, and the gauge itself. There is also a switch to turn on/off the fuel pump and a modified filler tube.
The tank bolts onto the frame with 5 bolts going into existing threaded holes. That's easy. 1/2" rubber fuel line connects an outflow fitting to the external fuel pump, and runs from the pump to an inlet barb added to the filler tube. Upstream of this new barb is another, which runs from the filler tube to an inlet port in the top of the tank.
The tank is filled by filling the OEM tank, at which point fuel starts to back up in the fill tube until it can flow through the added tubing into the aux tank. As you drive and deplete fuel from the OEM tank, you can then turn on the external pump and transfer fuel from the aux tank, through the modified filler tube, into the OEM tank.
One must also deal with a vapor line that vents the system through the factory vapor control system.
The sending unit is pretty old school, and I intend to research possible newer units. It's clearly not designed for modern vapor-emissions controlled vehicles, and must be carefully sealed with form-a-gasket to prevent leaks. This requires some trial and error and required dropping the tank a few times to minimize "Check Engine" codes.
The 38 gallons of gasoline apparently creates more vapor than the FJ's evap system can handle, as I'll get a CEL most times when either (or both) tanks are partially full. One quickly learns than a Scan Gauge or similar code reader is necessary to clear codes. A by-product of using a ScanGauge is greater awareness of driving behavior that results in lousy mileage. When you can see that rocketing away from a traffic light gives you 4 mpg; you tend to avoid doing so.
The most time-consuming part of the install was deciding on a location for the fuel gauge and fitting the gauge in place. I decided to install the gauge in the silver panel to the left of the dashboard. It's easy to remove however there is a lot of structure behind it, and I had to carefully plan where to remove enough plastic so the 2" deep gauge would fit. After creating a cardboard template and a lot of measuring I carefully used a Dremel tool to carve away enough plastic substructure so the new gauge would fit into the hole in this panel.
Wiring the gauge to the sending unit was straightforward, and wiring the fuel pump to the switch, relay, and power source should have been easy, however the MAF install sheet erroneously called out pin locations on the relay. I eventually drive to AutoZone and bought 4 relays, and through trial and error eventually figured out the correct pin locations; burning out a couple of relays in this process.
The entire install took 8 hours. Today, I could do it in 4.
Carrying 38 gallons of fuel gives one a great sense of security when you are far from a paved road. The tank is tucked far up into the back of the truck and is out of the way. There are two wrinkles with this design. One is the external fuel pump bolts to the back of the rearmost crossmember. It seems a vulnerable place to run fuel line; although if I got rear-ended hard enough for this to be an issue, I decided it would be only one of many problems I'd be facing. Another issue is the exposed fuel line in the several inches between the back of the tank and the front of the frame crossmember. The fuel line, pump and filter are very exposed to gravel. I intend to cover with an aluminum gravel guard at some point.
I've had this second tank for a couple of years. If I ever had to replace my FJ, installing another auxilary tank would be the first thing I'd do to it.