Podcast
Questions and Answers
What does a defective fuel pump indicate when the long-term fuel trim is negative at -30%?
fuel being reduced to a rich condition
In the case of a vehicle that stalls occasionally, Technician A says the EGR valve is sticking. Technician B says the EVAP canister purge valve is stuck open. Who is right?
When a vehicle's MIL is on with DTC P0010, Technician A suggests a loss of one CMP sensor could cause this. Technician B suggests the TP and APP sensors are frozen. Who is right?
In the case of DTC P0456, which values could indicate the issue?
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What is one possible cause for a P0455 DTC?
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Which technician is correct regarding a test of the HO2S on bank 1?
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Which technician is right about the PCV hose effect on idle?
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What is a symptom of a closed EGR valve?
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Who is correct regarding excessive back pressure on sensors?
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What is the best parameter to monitor on a vacuum-type EGR system during evaluation?
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What is a prime source of problems in secondary air systems?
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What happens when the rear O2 sensor signal reaches approximately 75% of the front O2 sensor signal?
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What could cause a canister purge valve control solenoid to be always open?
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Which technician is correct about OBD systems?
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Regarding DTC erasure in OBD II, which technician is right?
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Which of the following is the LEAST likely cause of spark knock or detonation during rapid acceleration or driving up a hill?
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Who is right regarding the fuel pressure regulator hose when removed?
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What should be done first when checking the fuel pressure on a sequential fuel injected engine?
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What happens when both APP sensors have failed on the composite vehicle?
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What should be checked if the injector has failed but is still receiving system voltage?
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What is the MOST likely cause of the VVT trouble code P1349 and a no-start condition?
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What do the mass air flow sensor voltage readings indicate with the ignition key on and engine off?
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What do the manifold absolute pressure sensor voltage readings indicate with the ignition key on and engine off?
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Who is right about electronic throttle control on the Composite Vehicle Type 4?
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Which of the following components is NOT malfunctioning if it shows a correct voltage value?
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Who is right regarding the open loop condition on the composite vehicle?
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If the scan tool cannot hear the fuel pump running, what should the technician do next?
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Who is right regarding ignition misfires and emissions?
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Which of the following should NOT be checked if a return-less fuel system pressure gauge reading is out of specification?
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Which condition is MOST likely indicated by high O2 readings and normal HC and CO readings?
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What happens if the fuel filter is installed backwards?
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Who is right regarding the vacuum hose and fuel pressure regulator?
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Who is right about the effects of a cracked exhaust manifold on oxygen sensor operation?
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Who is right regarding a vehicle’s poor fuel economy based on scan tool data?
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Who is right regarding rough and unstable engine idle based on scan tool data?
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Study Notes
Fuel and Air Induction Systems Diagnosis
- In cases of engine spark knock or detonation, a pinched or restricted fuel return line is the least likely cause among common issues, which include carbon buildup and lack of EGR flow.
- If gasoline is present at the vacuum hose from the fuel-pressure regulator, it indicates a leaking diaphragm, while Technician A's claim of normal behavior is incorrect.
- The first step in checking fuel pressure on a sequentially fuel-injected engine is to look up the manufacturer's procedure to ensure proper diagnostics.
- When both APP sensors fail, the ECM limits throttle opening and sets the fast idle speed between 1400-1500 RPM rather than the normal idle speed.
- In diagnosing a misfire on cylinder #3, checking the voltage drop of the injector at ECM pin 122 is crucial as it indicates whether the ECM can ground the injector.
- A diagnostic trouble code P1349 for a VVT system malfunction is most likely caused by both CMP sensors failing rather than one sensor.
- Bad ground conditions are indicated when a mass airflow sensor voltage reading shows unexpected results.
- For a manifold absolute pressure (MAP) sensor reading of 4.5 volts at closed throttle engine off, this indicates a defective MAP sensor.
- If one APP or TP sensor fails, the ECM limits throttle opening to 35%, whereas two or more failures will not completely shut the engine down but set it into a limp-in mode.
- In diagnostics of malfunctioning components, the EGR system stands out as functioning correctly if its voltage readings are normal.
- In open-loop fuel conditions, the injector pulse width is calculated from engine temperature and MAF sensor data, not AFRS readings until conditions are met.
- If a fuel pump relay is not functioning, check downstream components before assuming a faulty relay.
- An exhaust leak may cause an abnormally high O2 reading while maintaining normal HC and CO emissions.
- If a fuel filter is installed backwards, expect lower power under load due to flow restrictions.
- A vacuum gauge connected to the intake manifold will drop gradually under constant speeds if exhaust systems are restricted.
- Removing a vacuum hose from a fuel pressure regulator should ideally increase fuel pressure, not decrease it.
- A cracked exhaust manifold affects O2 sensor operation by allowing excessive oxygen into the exhaust stream.
- Conditions leading to poor fuel economy could be attributed to a leaking canister purge diaphragm rather than a stuck open EGR valve.
- Rough engine idle could be caused by a partially stuck open EGR valve, while a defective EVAP purge valve would not present issues in this instance.
- Low HO2S voltage coupled with a rich mixture suggests a defective fuel pump, especially if long-term fuel trim readings are negative.
- Stalling issues in a vehicle may be caused by a sticking EGR valve, while a non-moving EVAP canister purge valve indicates that it's functioning correctly.
- Loss of a CMP sensor signal leads to a DTC and a MIL; frozen TP and APP sensors do not directly relate to this issue.### Diagnostic Trouble Codes (DTC) and Engine Systems
- DTC P0456 indicates a small leak in the EVAP canister system; both technician A (defective solenoid or leak) and technician B (defective or wrong gas cap) are correct.
- DTC P0455 signifies a small leak; a defective purge (vent) solenoid is one possible cause among several others.
Oxygen Sensor Testing
- Testing a heated oxygen sensor shows variable readings; a change in output suggests potential defects in the sensor rather than issues downstream in the exhaust system.
- A faulty PCV hose can lead to engine idle issues and possible oil leaks; at 2500 RPM, the PCV system should create a slight vacuum, not pressure.
Exhaust Gas Recirculation (EGR) System
- A closed EGR valve can cause spark knock, which is a distinct indicator.
- Excessive back pressure from a clogged exhaust system can melt plastic connectors on a position sensor; verifying exhaust restrictions is important during EGR sensor replacement.
- In vacuum-type EGR systems, monitoring EGR temperature sensor voltage during part-throttle conditions is crucial.
Secondary Air Systems
- Water is a prime source of issues in secondary air systems, often due to reed valve failures.
Catalytic Converter Functionality
- Decreased catalytic converter efficiency results in the rear O2 sensor signal mimicking the front O2 sensor; at 75% similarity, a DTC is stored and the MIL activates.
Purge Valve Control and On-Board Diagnostics (OBD)
- A consistently open canister purge valve control solenoid may indicate a defective Engine Control Module (ECM).
- OBD II systems necessitate misfire detection with a flashing MIL to protect the catalytic converter; OBD I does not detect nitrogen oxides (NOx).
- Engine warm-up cycles will erase DTCs and freeze frame data after 50 cycles; OBD II monitors emission-related components comprehensively.
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Description
Test your knowledge on fuel and air induction systems with this practice exam. This set of flashcards challenges you to identify the least likely causes for engine issues like spark knock and detonation. Enhance your diagnostic skills and prepare effectively!
