Many causes of low power 1) Lack of fuel: Filter restriction, pump pressure, skewed data from MAF, MAP, IAT, 02 sensors. 2) restricted intake/exhaust: Plugged air filter/cat converter. 3) ignition problem: Improper timing, weak coil, spark plugs 4) The unexpected: Dragging brakes, Binding transmission/rear differential, muffler plugged/broken baffle
5) If the engine is old; Loss of compression due to cylinder and ring wear.