Five free practice questions on fuel and air induction diagnosis: content area D of the ASE L1 Advanced Engine Performance Specialist test, 8 of its 50 scored questions (16%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.
01The questions
-
A sedan stored for 18 months will not start. The in-tank fuel pump is seized, and two port injectors are stuck shut. The fuel drained from the tank is dark amber and smells sour, like varnish, and when it settles in a clear jar it shows no water layer and no sediment. Which of these is the root cause of the failures?
- Gasoline that oxidized into gum
- Water separated out of the gasoline
- Rust flakes from inside the tank
- A fuel pump relay stuck open
Show the answer
A. Correct. Gasoline left standing for many months oxidizes, darkens and loses its lighter parts, leaving gum and varnish that glue pump armatures and injector pintles in place. Draining and flushing the system with fresh fuel, along with replacing the varnished parts, keeps the new pump and injectors from sticking the same way.
Why not the others
B. Water that separates from gasoline settles to the bottom of the jar as its own layer, and none appeared. The dark, sour fuel shows gasoline that has oxidized into gum and varnish.
C. Rust from the tank would show up as sediment in the jar and in the strainer, and none settled out. Gum and varnish from stale gasoline stuck the pump and injectors.
D. A relay stuck open would keep the pump from running, but it would not seize the pump or stick two injectors shut. Both failures come from varnish left by stale gasoline.
-
Compared with port fuel injection, why does gasoline direct injection (GDI) need less extra fuel when a cold engine is accelerated?
- No fuel film forms on the port walls
- High rail pressure keeps the fuel warm
- It runs closed loop from the moment it starts
- Cold fuel atomizes more finely in the port
Show the answer
A. Correct. With port injection, part of the fuel sprayed at a cold port and valve sticks to the walls as a film, so the PCM adds fuel on acceleration until that film builds up. Direct injection sprays straight into the cylinder, so no port film forms and much less extra fuel is needed.
Why not the others
B. Squeezing fuel to rail pressure warms it very little, and that is not why direct injection needs less enrichment. The reason is that fuel sprayed into the cylinder never wets the port walls.
C. A GDI engine still runs open loop until its air/fuel ratio sensors are ready, like any other engine. It needs less acceleration enrichment because no fuel film forms on the port walls.
D. Cold fuel atomizes less well, not better, and a GDI engine does not inject into the port at all. Less enrichment is needed because the fuel goes straight into the cylinder without wetting the port walls.
-
A flex-fuel vehicle's ethanol content sensor, the manufacturer says, outputs 50 Hz at 0% ethanol and 150 Hz at 100% ethanol, changing in a straight line between them. With fuel from the tank flowing through it, a lab scope shows 135 Hz. What ethanol content does this indicate?
- 57%
- 85%
- 90%
- 135%
Show the answer
B. Correct. The sensor's span is 150 - 50 = 100 Hz for 0 to 100% ethanol, so each 1 Hz is 1%. The reading is 135 - 50 = 85 Hz above the 0% point, so the fuel is 85% ethanol. The PCM uses this figure to scale injector pulse width and timing for the blend.
Why not the others
A. This divides the reading above 50 Hz by the full 150 Hz: (135 - 50) / 150 = 57%. The span is only 100 Hz (150 - 50), so the ethanol content is 85 / 100 = 85%.
C. This divides the reading straight by the 100% frequency: 135 / 150 = 90%. It ignores the 50 Hz the sensor puts out at 0% ethanol; the correct figure is (135 - 50) / (150 - 50) = 85%.
D. This divides the raw reading by the 100 Hz span: 135 / 100 = 135%, which is impossible. The 50 Hz the sensor puts out at 0% must be subtracted first: (135 - 50) / 100 = 85%.
-
Any of these could keep an electronic throttle plate from following the ECM's commanded position EXCEPT:
- a vacuum leak below the throttle plate.
- worn teeth in the throttle body gear train.
- a TAC motor with worn brushes.
- high resistance in the TAC motor circuit.
Show the answer
A. Correct. A vacuum leak adds air the throttle does not control and changes idle speed and fuel trim, but it does nothing to the motor, gears or shaft, so the plate still reaches the commanded angle. The ECM may command the plate further closed to hold idle, and the plate follows that command.
Why not the others
B. Worn or broken gear teeth let the motor turn without moving the shaft the full amount, so the actual position lags the command. A vacuum leak below the plate is the exception.
C. Worn brushes reduce or interrupt motor current, leaving the motor too weak to drive the plate to the commanded angle. A vacuum leak below the plate is the exception.
D. Resistance in the motor circuit cuts the current the motor can draw, so it may not move the plate fast or far enough to match the command. A vacuum leak below the plate is the exception.
-
Any of these could cause a port-injected engine to backfire through the intake EXCEPT:
- a lean mixture from low fuel pressure.
- a rich mixture in every cylinder.
- an intake valve that does not fully seat.
- a restricted injector on one cylinder.
Show the answer
B. Correct. Intake backfire comes from flame reaching the incoming charge: a slow-burning lean charge still alight when the intake valve opens, or a leaking intake valve. A rich mixture does not linger burning into the next intake event the way a lean one does; its excess fuel leaves as black smoke and unburned fuel that can afterfire in the exhaust instead.
Why not the others
A. A lean mixture burns slowly, so flame can still be present when the intake valve opens and ignite the incoming charge. Low fuel pressure leans every cylinder and can do this; a rich mixture is the exception.
C. A valve that does not seat lets burning gases escape into the intake port during combustion and ignite the mixture there, a classic cause of intake backfire. A rich mixture is the exception.
D. A restricted injector leans its own cylinder, and the slow-burning lean charge can still be burning when that intake valve opens, popping back through the intake. A rich mixture is the exception.
02Answer key
| Question | Answer |
|---|---|
| Q1 | A |
| Q2 | A |
| Q3 | B |
| Q4 | A |
| Q5 | B |
03What area D covers
This content area covers fuel delivery and air induction on port and direct injection engines: reading fuel trim with oxygen and air/fuel ratio sensor data to judge fuel control, separating mechanical from electrical faults in the fuel system and the air intake, electronic throttle included, fuel quality, pressure and volume, and the mechanical and electrical testing of injectors and fuel pumps.
The ASE test specification calls this content area “Fuel Systems and Air Induction Systems Diagnosis”. It carries 8 of the 50 scored questions, 16% of the test.
- Read fuel trim with O2 and air/fuel ratio sensor data
- Separate mechanical from electrical fuel system faults
- Diagnose air induction and electronic throttle faults
- Diagnose port and direct injection driveability symptoms
- Test fuel quality, water and alcohol content
- Measure fuel pressure and volume
- Test injectors and fuel pumps, digitally controlled pumps included
- Find root causes and verify the repair
04What the questions turn on
Where the trim error shows
Compare fuel trim across loads. A correction about the same at idle, 2,500 rpm and cruise points to fuel per pulse, such as restricted injectors. One largest at idle points to something small next to airflow, such as unmetered air or a leaking regulator diaphragm.
A DTC names a condition
A rail pressure DTC says pressure fell short, not which part failed. A sensor reading within its range is not proof it is accurate; comparing it with a gauge is. When the gauge agrees with the sensor, the pressure reading is real.
Pressure that bleeds away
Rail pressure that bleeds away after a hot shutdown means fuel is leaving the pressurized side, so a hot restart cranks long. On direct injection, a dripping injector also wets its own plug; a leaking pump relief valve returns fuel to the low side instead.
Pump command and feedback
Composite vehicle fuel pump questions compare the ECM's pump command with the module's feedback duty cycle. Matching feedback shows the module carrying out the command. Half-duty feedback while full speed is commanded points to the module or its power and ground, not a worn pump.
05The other content areas
The L1 test has six content areas. Each has its own page of free questions:
The ASE L1 Advanced Engine Performance Specialist practice test page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.
ToolboxPrep is an independent study site. It is not affiliated with or endorsed by ASE (the National Institute for Automotive Service Excellence). Our questions are original practice questions written to the published ASE test specifications; they are not actual test questions.