ACS Engine Testing & Diagnostics 3 — Questions and Answers
Question 1: When checking ignition timing using a timing light on a running engine, the light should flash at what point relative to the crankshaft?
- At TDC on the compression stroke
- At the specified degrees before TDC (Correct answer)
- At BDC on the power stroke
- At the intake valve opening point
Correct answer: At the specified degrees before TDC
Ignition timing is set to fire the spark plug at a specified number of degrees before TDC to allow the fuel-air mixture to reach peak pressure at the optimal piston position.
Question 2: A turbocharged engine's manifold pressure continues to rise after the throttle is advanced to a set position. What component has likely failed?
- Wastegate controller (Correct answer)
- Throttle body seal
- Intercooler bypass valve
- Fuel control unit
Correct answer: Wastegate controller
A failed wastegate that sticks closed prevents exhaust from bypassing the turbine, causing uncontrolled manifold pressure rise (overboosting).
Question 3: Fuel flow is within normal limits but EGT is abnormally high on one cylinder. What is the most likely cause?
- Clogged fuel injector on that cylinder (Correct answer)
- Intake valve sticking open
- Cracked exhaust valve
- Incorrect spark plug gap
Correct answer: Clogged fuel injector on that cylinder
A clogged fuel injector delivers insufficient fuel to one cylinder, creating a lean mixture that raises EGT on that cylinder.
Question 4: What is the primary purpose of performing a cold cylinder check during engine diagnostics?
- To identify cylinders not firing or producing power (Correct answer)
- To measure cylinder wall wear
- To check valve spring tension
- To verify compression ratio
Correct answer: To identify cylinders not firing or producing power
A cold cylinder check identifies cylinders that are not contributing power, indicating ignition failure, fuel starvation, or mechanical issues.
Question 5: An engine oil analysis report shows elevated iron and chrome levels. What engine components are most likely experiencing abnormal wear?
- Piston rings and cylinder walls (Correct answer)
- Camshaft and lifters
- Crankshaft main bearings
- Oil pump gears
Correct answer: Piston rings and cylinder walls
Chrome (from chrome-plated rings) and iron (from cylinder walls) appearing together in oil analysis are classic indicators of ring and cylinder bore wear.
Question 6: During engine run-up, the prop governor check is performed by increasing RPM and pulling the prop control back. What should happen to RPM?
- RPM should drop and stabilize at a lower value (Correct answer)
- RPM should continue increasing
- RPM should remain unchanged
- RPM should fluctuate rapidly
Correct answer: RPM should drop and stabilize at a lower value
Pulling the prop control back commands a lower RPM setting; the governor responds by coarsening the blade pitch, reducing RPM to the selected value.
Question 7: What reading on a differential compression tester indicates a cylinder is in acceptable condition?
- Differential pressure greater than 80/80
- Less than 25 psi differential between reference and cylinder pressure (Correct answer)
- Greater than 50% pressure drop
- Equal pressure on both gauges only at TDC
Correct answer: Less than 25 psi differential between reference and cylinder pressure
FAA guidance generally accepts cylinders with no more than a 25% (about 20 psi on an 80 psi tester) differential pressure drop as serviceable.
ACS Engine Testing & Diagnostics 3
When checking ignition timing using a timing light on a running engine, the light should flash at what point relative to the crankshaft?