Sample Review Questions:
1. Counterman A says a “speed-density” fuel injection system uses a mass airflow sensor to monitor the volume of air entering the engine. Counterman B says the signal from the mass airflow sensor is used to control the air/fuel mixture. Who’s right?
a. Counterman A only
b. Counterman B only
c. Both Counterman A and B
d. Neither one
2. A fuel filter plugged with rust probably indicates what?
a. Dirty fuel injectors
b. Dirty fuel pressure regulator
c. Bad gas contaminated with water
d. Corrosion inside a steel fuel tank
3. Which of the following would NOT prevent an electric fuel pump from running?
a. Faulty fuel pressure regulator
b. Bad fuel pump fuse or relay
c. Wiring problem in fuel pump circuit
d. Open inertia safety switch
4. Counterman A says dirty fuel injectors can cause poor fuel economy, poor performance and engine misfires. Counterman B says a defective idle speed control (ISC) motor can upset the idle fuel mixture. Who’s right?
a. Counterman A only
b. Counterman B only
c. Both Counterman A and B
d. Neither one
ANSWER KEY: 1B, 2D, 3A, 4A
EXPLANATIONS:
1. A mass airflow (MAF) sensor is used on many-fuel injected engines to monitor the volume of air entering the engine. The MAF sensor is located between the air filter and throttle, and generates a signal when air flows past a heated wire. The amount of cooling the wire undergoes is proportional to airflow. The PCM uses this information to adjust the air/fuel mixture. But on speed-density systems, there is no MAF sensor.
2. The fuel filter is usually located between the fuel tank and injector fuel rail on the engine. In some applications, the filter may be located inside the tank. If the filter is plugged with rust, the rust came from corrosion inside the fuel tank. Replacing the fuel filter may restore the flow of fuel, but the filter will likely plug again unless the fuel tank is cleaned or replaced. Bad gas contaminated with water can be a contributing factor to rust inside the tank, but by itself won’t plug the filter.
3. Electric fuel pumps are often replaced unnecessarily because of misdiagnosis. A dead fuel pump obviously won’t produce any fuel pressure. But the problem may not be the pump but the pump’s voltage supply. Possible causes include a blown fuse, bad fuel pump relay, wiring problem, bad driver circuit in the PCM, or anti-theft system issue. The fuel pressure regulator is a mechanical device that maintains a certain level of fuel pressure in the system. The regulator does not affect the operation of the pump.
4. Fuel injectors can become clogged with fuel varnish deposits if low-quality gasoline is used that does not contain adequate levels of detergents and cleaners. Dirty injectors do not deliver a normal quantity of fuel, causing the engine to run lean and misfire.
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