Piston travel and throttle...
Piston travel and throttle position determine the amount of fuel consumed off-road. Using low range and lower gears, consider how many feet - or miles - the pistons travel up and down in their cylinders. This determines the amount of fuel burned, and running time becomes more important than miles traveled. Winch loads and fast idling demand fuel as well.
So, what is a trail "tune-up" for the modern fuel-injected engine? After a fresh oil filter and oil change, a tune-up, much like carbureted engines with conventional ignitions, is restorative. Despite the greatly extended service intervals on items like spark plugs, plug cables and the latest coil-on-plug distributor-less ignitions, parts wear and fatigue are still an issue. Spark plugs, even platinum, palladium and nickel types, do have a lifespan. Spark plug cables face deterioration along with the distributor cap and rotor. True, that interval may be longer (our '99 XJ Cherokee had what looked like the original spark plugs, cables, cap and rotor when we bought the vehicle at 94,000 miles!). An improperly maintained engine will exhibit poor fuel efficiency, higher operating temperatures, and poor performance under load.
The milestone breakthrough...
The milestone breakthrough for altitude and fuel metering was fuel injection with electronic fuel-and-spark management. Jeep was among the earliest 4WD platforms to use EFI. Here is the 1987 YJ Wrangler 2.5L four-cylinder with TBI. The feedback sensors, including an oxygen sensor and manifold pressure sensor, enabled the electronic control module to instantaneously adjust fuel flow and spark timing to meet various engine loads and atmospheric conditions.
Spark plugs are relatively cheap-even the highest quality plugs run no more than the current cost of 3-4 gallons of unleaded regular fuel. New spark plugs can restore power and fuel efficiency, preserving an engine while earning back the parts cost through fuel savings. As a rule, replace spark plugs at no more than 30,000 miles on an EFI engine, 15,000 or less on a carbureted engine. Unless your vehicle uses extended life (100,000-mile) type spark plugs, the 30K figure should work for EFI inline sixes, V-8s and all fours. V-6s like the Liberty's 3.7L - and other engines that require gymnastics to change the plugs - can follow factory-recommended service intervals.
If your 4x4 has a carburetor, this is a periodically serviced item. The legendary woes of a Carter BBD two-barrel on 258 Jeep inline sixes can often be eliminated by a blueprint rebuild of the carburetor. Expect to rebuild the carburetor each 30,000 to 40,000 miles, more often if you operate the vehicle in dusty or rock crawling conditions - or if your fuel tank is aging...If the fuel tank condition is questionable, service or replace the tank!
EFI/MPI can keep the fuel...
EFI/MPI can keep the fuel mix stable at all altitudes by constantly reading the oxygen content of the exhaust and setting the injector fuel flow accordingly. Note that while the fuel mix is correct, there is still a loss of horsepower, approximately 3- to 5-percent per 1,000 feet of elevation above sea level - another reason to use low range at higher elevations!
This raises the issue of filtration, and adequate fuel and air filtration is crucial for 4x4s. If you have a model with inline fuel filter(s), replace them according to the factory service interval. Frequency for changing an air filter element varies. The factory recommended interval is the longest you should leave the filter in place. For any trail-driven 4x4, the air filter's condition depends upon the dust level of the trail or road you've just traveled! There are filters that clogged in a dozen miles of the Rubicon Trail or one trip down an alkaline, corrugated dirt road in the desert. Always carry a spare air filter, and when traveling to a remote area, consider carrying a spare fuel filter, too. On any model, be certain that the fuel cap seals properly and keeps dust from entering the fuel tank.
Conserving Fuel at All Cost
There are instances when off-road fuel pinching is crucial. Fuel may be running low, or the trail time is ticking away faster than anticipated. For the two Geo Trackers, strap towing and long idling used up all the fuel in the support vehicle and just a bit more than half a tank in the other! Beyond selecting the correct transmission gear and monitoring throttle use, there are other ways to conserve fuel off-pavement.
In some instances, it is wise to winch instead of drive through a stretch. Examples would be deep sand, mud or snow - or maybe a swift creek with a questionable bed. Spending hours spinning wheels churning through muck or inching out of a snow pack could be a colossal waste of fuel. Use of a winch, a Pull-Pal anchor, a Hi-Lift jack, and other accessories could save time, fuel, and energy. A winch uses current from the battery, and the load on your engine becomes nothing more than the speed to keep the alternator charging. If you have dual batteries and an isolator, you may be able to winch through an obstacle without running the engine.