418 Section 4 Computer Control Technology Copyright Goodheart-Willcox Co., Inc. Vacuum Leaks Vacuum leaks are frequently caused by deterio- rated, broken, or loose vacuum hoses. Vacuum leaks often make a hissing sound. Some vacuum leaks can upset the operation of a computer system and cause a wide range of symptoms. Also, some engine sensors and actuators rely on engine vacuum for operation. Always check for vacuum leaks when they could be causing a performance problem, Figure 25-3. For example, if the trouble code indicates a problem with the MAP (manifold absolute pressure) sensor, check the vacuum lines leading to the sensor. If there is a vacuum leak, the sensor cannot function normally. Also check the intake manifold gasket area, as this is a common location for vacuum leaks. Air leaks after a mass airflow sensor can also cause problems. The sensor cannot measure the A B C D E F Snap-on Tool Corp. Figure 25-2. The scan tool will usually direct you to the area of the vehicle causing the problem. A—“Read DTC Options” allows you to select stored codes, pending codes (codes that are almost out of normal operating parameters), and manufacturer data. B— The scan tool denotes the vehicle has tripped two trouble codes. C—The scan tool readout explains that the mass or volume airflow sensor circuit is showing low voltage or current input to the computer. D—The scan tool readout shows there is no engine misfire problem, the fuel injection system is working normally, and the catalytic converter is working. E—No signal from the heated catalyst may be due to the engine not warming up completely. The evaporative emission control system is working normally. Air conditioning has no signal because it might not be turned on. F—The scan tool indicates both O2 sensors are working normally. The EGR system may not have a signal because the engine has not warmed fully. Goodheart-Willcox Publisher Figure 25-3. Always inspect the engine compartment for signs of trouble. A loose or corroded connection or a vacuum leak may be setting trouble codes or tricking the computer system.