We've become spoiled with computerized engine management systems (EMS) and electric fuel injection (EFI). Unlike days past where a clogged fuel filter, faulty thermostat or bad spark plug wires could leave you stranded on the side of the road, software can compensate for component deterioration. Unfortunately, the ability to keep on motoring comes with degraded performance, higher fuel consumption and increased emissions that you don't notice happening since degradation occurs gradually. Thus, the importance of preventive maintenance rather than repair-only-when- broken repairs that may have you got by with older vehicles. Some PM can be done with easy-to-do parts replacement, other items require sophisticated electronic diagnostic equipment.
There is not much to adjust on modern engines with ignition timing, idle speed and fuel mixture being controlled by the EMS computer. However, these parameters plus various emission functions should be checked to assure that they are operating within specifications. Other systems such as battery voltage and the thermostat should also be checked because in electronically controlled vehicles many seemingly unrelated systems are integrated to work together for optimum performance. Electronics and computers require voltages within narrow ranges and a faulty thermostat cannot maintain correct cooling system operating temperatures. A bad thermostat can directly affect a number of engine functions. For instance, the engine management computer retards engine timing when the engine is under load and coolant temperature increases. If there is any significant deviation from the specified operating temperature range, incorrect timing can result in engine knock, decreased power and poorer fuel economy. Coolant temperatures outside the specified range can cause the computer to improperly adjust the air/fuel mixture. Too rich or lean a mixture can adversely affect exhaust emissions and fuel economy.
Deteriorated spark plugs lead to decreased performance, wasted fuel, and increased emissions as well as rough idling and hard starting. By the time you have driven 50,000 miles, the spark plugs have fired from 65 to 90 million times, each time wearing away tiny bits of the electrode's metal. Also the sharp edges of the center electrode become rounded and deposits form on the electrodes requiring more voltage for the spark to jump the gap. While the ignition system compensates, eventually it cannot meet the increased voltage requirements and starts misfiring. Consider installing long-life plugs with platinum or gold-palladium alloy electrodes that can provide decent performance for about 100,000 miles, provided they are not fouled by over rich mixtures or excessive oil consumption.
Spark plug wires deteriorate with time, and especially when subjected to oil, gasoline, water or fluids. Heat is another enemy and insulation can be damaged by rubbing and scraping. Again the ignition compensates, up to a point.. Inspect for signs of cracking, staining, burning, becoming brittle or other damage. Don't forget the spark plug boots. Loose fitting boots can result in ignition misfire. Make sure the cables are properly routed and supported in their looms to avoid crossfire problems and contact with the hot exhaust manifold.
Inspect the PCV (positive crankcase ventilation) valve at every oil change for proper operation. A defective or clogged PCV valve can allow excessive, unburned gasoline fumes into the cylinders, which displace the flow of fresh air past the valve. This can result in the computer commanding an over rich mixture wasting fuel and decreasing performance.
A clogged fuel filter can make the fuel pump work harder and interrupt the smooth flow of fuel. Again sensors and electronics on modern engines can compensate so you are less likely to experience hesitating and stalling like with older carbureted engines, but less efficient engine operation will occur. Fuel filters should be replaced every couple of years. Likewise, air filters should be changed according manufacturer's recommendations, and more often if you drive much in a dusty environment. A clogged air filter can upset the air-fuel mixture.
Fuel varnish deposits in injectors restrict the amount of fuel injected resulting in a too-lean mixture leading to misfiring and degraded engine performance and drivability. Deposits on the backs of intake valves can mean cold hesitation problems. The need for injector cleaning is less today than in the past due to improved fuel additives and better fuel injectors. But injector problems are cropping up again as reformulated gasoline is being required in parts of the country. To be safe, cleaning should be done on engines with performance problems or after about 50,000 miles.
Finally, don't forget your transmission. A slipping, hesitating, sluggish, and rough shifting transmission can mean poor performance and fuel economy, not to mention reduced service life and expensive repairs. While some new automatic transmissions are sealed and manufacturers say they not need fluid changes, most automatics still require regular servicing.
