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Engine Performance Auto Service Tech Tips
These Tech Tips are provided courtesy of autoclassroom.com, an online training resource for automotive instructors, students, and technicians.
Use a hydrocarbon detector
One of the many items that should be inspected while underneath the vehicle is the condition of all the fuel lines. Many gasoline (fuel) leaks do not show as wet areas. A hydrocarbon detector available from automotive test equipment suppliers is an excellent tool to use to locate small gasoline leaks that may not be visible.
Engine temperature and exhaust emissions
Many areas of the United States and Canada have exhaust emission testing. Hydrocarbon (HC) emissions are simply unburned gasoline. To help reduce HC emissions and be able to pass emission tests, be sure that the engine is at normal operating temperature. Vehicle manufacturers' definition of "normal operating temperature" includes the following:
Upper radiator hose is hot and pressurized.
Electric cooling fan(s) cycles twice.
Be sure that the engine is operating at normal operating temperature before testing for exhaust emissions. For best results, the vehicle should be driven about 20 miles (32 kilometers) to be certain that the catalytic converter and engine oil, as well as the coolant, are at normal temperature.
Check the hose
A defective vacuum hose to a MAP sensor can cause a variety of driveability problems including poor fuel economy, hesitation, stalling, and rough idle. A small air leak (vacuum leak) around the hose can cause these symptoms and often set a trouble code in the vehicle computer. When working on a vehicle that uses a MAP sensor, make certain that the vacuum hose travels consistently downward on its route from the sensor to the source of manifold vacuum. Inspect the hose, especially if another technician has previously replaced the factory-original hose. It should not be so long that it sags down at any point. Condensed fuel and/or moisture can become trapped in this low spot in the hose and cause all types of driveability problems and MAP sensor codes. Remove the hose from the MAP sensor. If anything comes out of the MAP sensor such as water, or goo, replace the sensor.
Check power and ground before condemning a sensor
Most engine sensors use a 5 volt reference and a ground. If the 5 volts to the sensor are too high (shorted to voltage) or too low (high resistance), the sensor output will be skewed or out of range. Before replacing the sensor that did not read correctly, unplug the sensor and measure both the 5 volt reference and ground. To measure the ground, simply turn the ignition on (engine off) and touch one test lead of a DMM set to read DC volts to the sensor ground and the other to the negative terminal of the battery. Any reading higher than 0.6 volt (600 mV) represents a poor ground.
The tap test
With the engine running at idle speed, gently tap the MAP sensor with the fingers of an open hand. If the engine stumbles or stalls, the MAP sensor is defective. This test is commonly called the tap test. |
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