NAPA - National Automotive Parts Association Certified Mechanic Engine Repair and Diagnosis Questions and Answers — Questions and Answers
Question 1: A customer's vehicle is brought in with the chief complaint of a rattling noise from the front of the engine, particularly noticeable on a cold start-up. The check engine light is also illuminated. Which of the following is the MOST likely cause?
- A worn or failing timing chain and tensioner (Correct answer)
- A loose serpentine belt
- A failing water pump bearing
- A cracked exhaust manifold
Correct answer: A worn or failing timing chain and tensioner
A rattling noise from the front of the engine, especially on cold starts, is a classic symptom of a worn timing chain or a failing tensioner. The slack in the chain causes it to slap against the timing cover, creating the rattle. The check engine light is often triggered by the camshaft position sensor detecting that the timing is out of sync due to the loose chain.
Question 2: A technician performing a cylinder compression test on a 4-cylinder engine obtains the following readings: Cylinder 1: 150 psi, Cylinder 2: 145 psi, Cylinder 3: 85 psi, Cylinder 4: 155 psi. What is the MOST likely diagnostic conclusion?
- Normal wear on all cylinders
- A blown head gasket between cylinders 1 and 2
- A significant compression leak in Cylinder 3 (Correct answer)
- Incorrect valve timing affecting all cylinders
Correct answer: A significant compression leak in Cylinder 3
The readings for cylinders 1, 2, and 4 are within an acceptable range of each other. However, the reading for Cylinder 3 (85 psi) is significantly lower than the others, indicating a substantial leak. This could be caused by worn piston rings, a burnt valve, or a blown head gasket affecting only that cylinder. A blown head gasket between cylinders would typically cause low compression in both adjacent cylinders.
Question 3: A vehicle presents with a low oil pressure warning light that illuminates only at idle when the engine is warm. The oil level is correct. Which of the following is the MOST likely cause?
- A faulty oil pressure sending unit
- Worn main or connecting rod bearings (Correct answer)
- A clogged oil filter
- An overfilled crankcase
Correct answer: Worn main or connecting rod bearings
Worn engine bearings create excessive clearance, which the oil pump cannot overcome at low RPM (idle speed), resulting in a drop in oil pressure. As engine RPM increases, the pump spins faster and can generate enough flow to compensate for the wear, causing the light to go out. While a faulty sending unit is possible, it would typically behave more erratically or fail completely, not just at warm idle.
Question 4: Which of the following symptoms is LEAST likely to be caused by a failing oxygen (O2) sensor?
- Decreased fuel economy
- Engine overheating (Correct answer)
- Illuminated Check Engine Light
- Rough idling or engine misfires
Correct answer: Engine overheating
A failing oxygen sensor primarily affects the air-fuel ratio calculation by the engine control module (ECM). This can lead to poor fuel economy, rough idling, misfires, and will almost always trigger the Check Engine Light. However, it does not directly control the cooling system, making engine overheating an unlikely symptom of a bad O2 sensor.
Question 5: A customer complains of white smoke with a sweet smell coming from the exhaust, along with a gradual loss of coolant without any visible external leaks. The engine has also been running hotter than normal. These symptoms are most indicative of a:
- Failing catalytic converter
- Cracked radiator
- Blown head gasket (Correct answer)
- Stuck open EGR valve
Correct answer: Blown head gasket
The combination of white smoke from the exhaust (coolant being burned in the combustion chamber), a sweet smell (antifreeze), coolant loss without external leaks, and engine overheating are classic signs of a blown head gasket. The gasket has likely failed between a coolant passage and a cylinder, allowing coolant to enter the combustion chamber.
Question 6: During a vehicle inspection, a technician finds metal shavings in the engine oil during a routine oil change. This finding, combined with a customer complaint of a rough idle, most strongly suggests a problem with the:
- Oil pump
- Piston rings
- Valve seals
- Timing chain (Correct answer)
Correct answer: Timing chain
Finding metal shavings in the engine oil is a significant red flag for internal engine wear. When a timing chain wears, the metal-on-metal contact between the chain and its guides or sprockets can shed metal particles into the oil. This wear can also cause the timing to become retarded, leading to a rough idle and misfires.
NAPA - National Automotive Parts Association Certified Mechanic Engine Repair and Diagnosis Questions and Answers
A customer's vehicle is brought in with the chief complaint of a rattling noise from the front of the engine, particularly noticeable on a cold start-up.
The check engine light is also illuminated.
Which of the following is the MOST likely cause?