I bought this truck with a bad engine, and spent a weekend installing a junkyard engine.
The fuel gauge sender needed to be replaced. I bought this on eBay for $35, but there was a change on the fuel pump plug on my model year so I had to swap out on of the pigtails on the sensor. Also the fuel pump is held to the tank with a twist-lock that you have to turn by setting a brass or copper pipe against and hit this with a piece of wood to work it all loose. (Don't use steel tools for this step, it could be a serious fire hazard)
Lastly on this subject, on my 4wd rodeo, there was a total of 6 fasteners holding the tank to the car.
I had to replace an oxygen sensor (which is due around 100k miles)
Also, my ABS light is on. You can retrieve the codes from the ABS computer by putting a small jumper from pin 4 to pin 12 on the OBDII port and turning the ignition on. If you Google search "01 Isuzu rodeo ABS light" or something similar, one of the results is a PDF you can download that gives all of the error codes. Be warned however, it was my impression that the Pin numbers given in that PDF are backwards and I recommend you go by the label on the OBD port. Pin 1 is marked with a very small number in the plastic casting, and you just count down the row until you count to pi 4. Pin 12 is the pin immediately next to pin 4 on the opposite row.
Lastly, on the subject of oil consumption, I have a theory. It seems to me that the routing of the Positive Crankcase Ventilation (PCV) system is the source for this. The hose coming off the PCV valve enters the intake manifold after the throttle body. This means that every time you decelerate by engine braking, the oil-vapor-laden air in the valve cover gets sucked into the engine at upwards of 30 inches of vacuum. I have put an open breather filter on the inlet side of the PCV system (passenger side valve cover) and rerouted the hose from the PCV valve to the air filter side of the throttle body. I also put a cap over the nipple on the intake where the PCV used to route to. (as for the other nipple on the plastic intake hose, this is where I routed the PCV valve into. While this method allows some un-metered air into the engine, it has made a significant difference in the oil consumption.
Furthermore It is my opinion that the poor fuel mileage is at least somewhat attributable to the fact that the engine is burning so much oil (in some cases) that the oxygen sensor and the catalytic converters become coked up with oil, and are unable to work properly.