Power loss has always been a sign of something awry in a car engine and that some part of the system is operating at less than its full potential. Modern computer controls complicate things even more -- the ghost in your machine is ever watchful, ever paranoid and always willing to kill your fun if it sees something it doesn't like.
Too much or too little air will cause your engine to run either rich or lean and lose power when you're trying to accelerate. Vacuum leaks are probably the most common source of excess air entering the engine, followed by idle air-control valves stuck open. A malfunctioning exhaust gas recirculation valve will have much the same effect. Air reductions are a bit rarer, since a deficit of air means something is either severely clogged or not opening. Clogged, wet or oil-soaked air filters and malfunctioning choke mechanisms are the likeliest suspects. Other engines, such as those using a dual-intake-runner design and intake manifold runner control -- like the Ford DOHC Modular motor -- may have experienced a failure in the intake control mechanism.
Too little fuel will cause your engine to run rich, which slows combustion and reduces power output once the engine gets warm. Excess fuel is unlikely unless you've got a fuel injector or carburetor needle valve stuck open, or a malfunctioning fuel injection pump or pressure regulator. A reduction in fuel volume typically comes from clogged filters, a malfunctioning pump or clogged fuel-injector screens. A malfunctioning injector -- one that fails to open or open all the way -- will typically cause a single-cylinder misfire. This will cause a reduction in power but will also result in noticeable engine vibration at idle.
While the average ignition system can malfunction in hundreds of different ways, most will result in a single- or multiple-cylinder misfire rather than a net loss in power. A faulty or weak ignition coil will emit a weak or intermittent spark to all cylinders, and a frayed, loose or corroded ground strap will deny power to the coil. Fouled plugs or broken ceramic insulators on the plugs will cause a misfire and loss of power, but this is unlikely if you haven't noticed the telltale vibration that typically accompanies a misfire.
Restrictions in the exhaust will almost always manifest as a loss in power, and there are really only two places such restrictions can randomly occur. Collapsed or melted catalytic converter innards will act like a cork on your exhaust, as will a muffler that has internally corroded enough to collapse. Converter meltdown will usually follow a severe rich condition, where excess fuel flowing through the motor ended up burning in the catalytic converter. If your vehicle rolled off the assembly line later than 1995, such converter failure will always trigger a check engine light and throw your computer into the dreaded "limp home" mode.
Any kind of sensor failure, knock, ping, emissions component failure or consistent deviation in air/fuel ratio will trigger your computer's default "safe" mode. Most computers have two safe modes: "open-loop" mode is essentially a de-tuning program that uses a less aggressive fuel and ignition advance settings to keep the engine from self-destructing after a sensor failure. The computer, deprived of the appropriate input, uses its best guess to compensate for the malfunctioning sensor, and loss of power is the inevitable result.
Limp-home mode is an emissions system failsafe designed to make your car practically undriveable if something goes wrong with the converter or anything else that affects emissions. Power will drop severely, the transmission may refuse to go into top gear, and the computer may drop the engine's rpm limit and the vehicle's permissible top speed to under 55 mph.
