There are many reasons why a car engine might start but not run: a faulty crankshaft position sensor, insufficient fuel, a partially blocked or clogged fuel injector nozzle, or a faulty solenoid valve. It is necessary to find out the specific cause in order to "treat the symptoms."
Engine starts but does not run
Possible causes of failure
◆ Crankshaft position sensor malfunction
◆Insufficient fuel
◆The fuel injector nozzle is partially blocked or clogged, or the solenoid valve is malfunctioning.
◆The engine's electrical grounding is loose and unreliable.
◆Water or foreign matter in the fuel may prevent the engine from starting or causing it to stop running.
◆Ignition system affected by moisture
Fault diagnosis methods
The intelligent diagnostic tool reads snapshot data. When storing DTCs, the ECM records vehicle and driving condition information as snapshot data. During troubleshooting, snapshot data helps determine whether the vehicle was running or stopped, whether the engine was warming up or cold, whether the air-fuel ratio was lean or rich, and other data when the fault occurred.
Diagnosis: Check for any of the following conditions:
1. If equipped with a key-type ignition system, is the ignition key partially retracted or folded? When attempting to start the engine, the mechanical part of the vehicle key must be locked in the fully extended position, with the remote door lock transmitter parallel to the key. Retracting or folding the key partially during startup may interrupt radio frequency transceiver verification and cause intermittent starting failures. In this case, DTCB3055 may also be triggered. If a repair request for intermittent starting failure is received, it is necessary to discuss the customer's starting habits and confirm that the key is locked to the fully extended position and the remote door lock transmitter is parallel to the key before starting.
2. Crankshaft position sensor malfunction. Use a diagnostic tool to check the crankshaft position sensor engine reference signal. While starting the engine, observe the "EngineSpeed" parameter. During engine start-up, the diagnostic tool should indicate a stable engine speed of 200–300 rpm. If the speed value is abnormal, such as a sudden increase in engine speed, the engine reference signal is not stable enough, preventing the engine from starting and running correctly.
3. Insufficient fuel. Thoroughly check the fuel delivery system to ensure it is supplying sufficient fuel to the fuel injectors. Check for any blockages or obstructions in the fuel supply components.
4. The fuel injector nozzle is partially blocked or clogged, or the solenoid valve is malfunctioning.
5. Insufficient fuel injector energization time. When this type of fault occurs, even if the fuel injectors are injecting fuel and the indicated fuel pressure is correct, there may not be enough fuel to start the engine. If the engine control module (ECM) receives incorrect input from various information sensors, the amount of fuel delivered by the fuel injectors may be insufficient to start the engine. Use a diagnostic tool to check all engine data parameters and compare them with expected values or values from known good vehicles.
6. Loose or unreliable engine electrical grounding. Check that the engine has a good, reliable electrical grounding.
7. Water or foreign matter in the fuel may cause the engine to fail to start or stop running. In cold weather, water may freeze inside the fuel system. The engine may start after being left in a heated repair shop for 30 minutes. This problem may not recur as long as the vehicle is not left parked overnight at freezing temperatures. Contaminated fuel may also prevent the vehicle from starting in extreme weather conditions.
8. Ignition systems are susceptible to moisture. If the engine is started and run in a warm, dry repair shop, the ignition system may be affected by moisture. When starting the engine, spray water onto the ignition system components and wiring to check if the engine starts or fails to run continuously.
9. If stalling occurs just before engine start, check if the ground circuit of the crankshaft position sensor, intake camshaft position sensor, or exhaust camshaft position sensor is open.
Three-way catalytic converter fault analysis
Fault phenomenon
Three-way catalytic converter blockage:
◆ Poor engine acceleration, lack of power at high speeds
◆ During rapid acceleration in neutral, the engine speed should not exceed 3000 rpm.
◆When completely blocked, the engine has fuel and sparks, but cannot start.
Cause of the fault
1. Some cars jerk at low speeds, and the jerking becomes more pronounced after deceleration and acceleration. Replacing the ignition coil high-voltage wire, spark plugs, and electronic control unit does not improve the situation. In this case, excessive exhaust back pressure should be considered. This situation differs from poor acceleration, difficulty in increasing speed, backfiring or even stalling during rapid acceleration; the only difference is the degree of blockage in the three-way catalytic converter. In short, if the three-way catalytic converter is blocked and the exhaust back pressure is too high, it will cause engine starting difficulties, poor idling, weak acceleration, unstable engine speed, and ignition regulation malfunctions.
2. Cars that have driven more than 120,000 kilometers will generally experience varying degrees of blockage in their three-way catalytic converters. The causes of this blockage are multifaceted, with one significant factor being the poor quality of fuel and lubricating oil. Unleaded gasoline not only has a high octane rating and good anti-knock properties, but also extremely low sulfur and phosphorus content. Furthermore, it avoids prolonged traffic jams, resulting in less pollution to the oxygen sensor and three-way catalytic converter.
3. The breakage and peeling of the catalyst carrier and the accumulation of oil stains can also block the air passages in the three-way catalytic converter, increasing exhaust resistance and causing a significant pressure loss.
Fault diagnosis methods
1. Use vacuum measurement to check for blockages:
When the three-way catalytic converter is blocked, the vacuum gauge reading should be as follows (the same applies when other parts of the exhaust passage are blocked): When the engine is idling, the vacuum gauge reading starts at a certain value (some may be the normal value), and then quickly drops to 10 kPa or a very small value; when the engine speed is rapidly increased to 2000 r/min and then the throttle is suddenly closed, if the vacuum gauge reading rises to about 80 kPa and then quickly drops to below 7 kPa, and then rises back to the value when the engine is idling, then the three-way catalytic converter is blocked.
When the three-way catalytic converter is severely blocked, the vacuum gauge reading should be as follows (the same applies when other parts of the exhaust passage are severely blocked): When the engine is idling, the vacuum gauge reading is very small; when the engine speed gradually increases from the idle speed to 2500 r/min and then remains at 2500 r/min, the vacuum gauge reading continues to drop rapidly.
2. Use the exhaust back pressure measurement method to check for blockage:
Because the engine exhaust system has relatively long wiring and includes components such as a three-way catalytic converter and muffler, it naturally experiences a certain amount of pressure (exhaust back pressure) during operation, although this pressure is very small. If the exhaust system becomes blocked, the system pressure will increase due to the higher engine exhaust pressure, affecting engine operation. Therefore, measuring the exhaust back pressure can help determine if the exhaust system is blocked.
There are several ways to connect a pressure gauge to the exhaust pipe at the front end of the three-way catalytic converter: If the vehicle has a secondary air pump, disconnect the air pump check valve from the secondary air injection line and then connect the pressure gauge to the exhaust pipe end of the secondary air injection line; alternatively, remove the front oxygen sensor and connect the pressure gauge to its interface. Some vehicles have a pre-installed interface on the exhaust pipe at the front end of the three-way catalytic converter; simply remove the plug. The pressure gauge's range is 0–50 kPa. Start the engine and allow it to reach a temperature above 85°C. Read the pressure gauge reading at an engine speed of 2500 rpm. This reading is the exhaust back pressure, and it should be less than 13.8 kPa. If the exhaust back pressure does not exceed the engine's specified limit, it indicates that the three-way catalytic converter is not blocked.
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