NAPA - National Automotive Parts Association Certified Mechanic Engine Performance Questions and Answers — Questions and Answers
Question 1: A vehicle with a P0171 (System Too Lean Bank 1) trouble code shows a Long Term Fuel Trim (LTFT) of +25% at idle. When the engine is held at 2,500 RPM, the LTFT value drops to +5%. What is the MOST likely cause of this condition?
- A faulty fuel pump.
- A clogged fuel injector on Bank 1.
- A vacuum leak on the intake manifold after the MAF sensor. (Correct answer)
- A malfunctioning oxygen sensor on Bank 1.
Correct answer: A vacuum leak on the intake manifold after the MAF sensor.
A vacuum leak introduces unmetered air into the engine. At idle, the volume of this leak is significant compared to the small volume of air entering through the throttle body, forcing the PCM to add a large amount of fuel (+25% LTFT) to compensate. At higher RPMs, the volume of the leak becomes a much smaller percentage of the total air intake, so its effect is diminished and the fuel trim returns closer to normal.
Question 2: The driver of a vehicle reports a noticeable pinging or knocking sound from the engine during moderate acceleration, particularly when going up hills. No check engine light is on. Which of the following is the most probable cause?
- A stuck-open EGR valve.
- A clogged or inoperative EGR valve. (Correct answer)
- An overly rich fuel condition.
- Excessively retarded ignition timing.
Correct answer: A clogged or inoperative EGR valve.
The Exhaust Gas Recirculation (EGR) system introduces a small amount of inert exhaust gas into the combustion chamber to lower temperatures. This is done to prevent the formation of NOx and to control detonation (pinging). If the EGR valve is clogged or stuck closed, it cannot introduce exhaust gas, leading to higher combustion temperatures and detonation under load.
Question 3: Which of the following symptoms is MOST likely to be caused by a Variable Valve Timing (VVT) solenoid that is stuck in the fully advanced position?
- An extremely rough idle or stalling when coming to a stop. (Correct answer)
- A lack of power at high engine RPM.
- A no-start condition where the engine cranks but does not fire.
- Black smoke from the exhaust during acceleration.
Correct answer: An extremely rough idle or stalling when coming to a stop.
Advancing the camshaft timing increases valve overlap, which is beneficial for engine breathing and power at higher RPMs. However, at idle, this increased overlap allows exhaust gas to dilute the incoming air/fuel mixture, leading to an unstable, rough idle or causing the engine to stall completely. It mimics the effect of a large EGR leak at idle.
Question 4: A technician is diagnosing a P0304 code (Cylinder 4 Misfire) on an engine with a Coil-On-Plug (COP) ignition system. The technician swaps the ignition coil from cylinder 4 with the coil from cylinder 1. After clearing the codes and test-driving the vehicle, a new code, P0301, is stored. What is the next logical step?
- Replace the fuel injector for cylinder 4.
- Perform a cylinder compression test on cylinder 4.
- Check the PCM driver circuit for cylinder 1.
- Replace the ignition coil that was originally on cylinder 4. (Correct answer)
Correct answer: Replace the ignition coil that was originally on cylinder 4.
The diagnostic process of swapping components is used to isolate a fault. Since the misfire code moved from cylinder 4 (P0304) to cylinder 1 (P0301) immediately after the ignition coil was moved from cylinder 4 to cylinder 1, it confirms that the ignition coil itself is the source of the misfire.
Question 5: A vehicle is brought to the shop with a 'crank, no-start' condition. Using a scan tool, the technician confirms that the engine RPM is displaying a steady 200 RPM while cranking. This observation makes which of the following components the LEAST likely cause of the no-start condition?
- The fuel pump.
- The crankshaft position (CKP) sensor. (Correct answer)
- The ignition control module.
- A blown fuse for the fuel injectors.
Correct answer: The crankshaft position (CKP) sensor.
The Powertrain Control Module (PCM) uses the signal from the crankshaft position (CKP) sensor to determine engine speed (RPM) and position. Since the scan tool is displaying a stable RPM signal during cranking, it indicates the CKP sensor is functioning correctly and providing a signal to the PCM. The cause of the no-start lies elsewhere (e.g., fuel or spark delivery).
Question 6: A technician measures fuel pressure on a port-injected engine. With the key on, engine off (KOEO), the pressure is 58 psi (within spec). As soon as the engine is started, the pressure drops and remains at 25 psi, causing a lean condition and poor performance. What is the MOST likely cause?
- A restricted fuel filter or a weak fuel pump. (Correct answer)
- A leaking fuel pressure regulator diaphragm.
- One or more leaking fuel injectors.
- An open in the power supply to the fuel pump.
Correct answer: A restricted fuel filter or a weak fuel pump.
This symptom points to a fuel delivery volume problem. A weak pump or a clogged filter can often build up sufficient static pressure when there is no demand (KOEO). However, once the engine starts and the injectors begin firing, the demand for fuel volume increases. A restriction or weak pump cannot supply the required volume, causing the pressure in the fuel rail to drop significantly.
NAPA - National Automotive Parts Association Certified Mechanic Engine Performance Questions and Answers
A vehicle with a P0171 (System Too Lean Bank 1) trouble code shows a Long Term Fuel Trim (LTFT) of +25% at idle.
When the engine is held at 2,500 RPM, the LTFT value drops to +5%.
What is the MOST likely cause of this condition?