Key points of engine fault diagnosis

When introducing fault diagnosis to the electronic control system, don’t forget to carry out basic inspection first.

For example, before trying to diagnose the fault of fuel injection system controlled by ECU, make sure that there is no leakage in intake pipe, valve timing and ignition timing.

If there are these adverse phenomena, the engine’s anti load alternating ability is poor, and it may be flameout in the case of sudden change of working conditions, such as accelerating flameout, braking flameout, turning on the air conditioner flameout, and shifting flameout.

1. It is difficult to diagnose the intermittent fault of some cars, unless the fault appears when checking the car.
2. Sometimes it is necessary to change the spark plug when checking the intermittent idle flameout fault.
3. When it is suspected that the air flow meter is defective (e.g. the hot wire of the air flow meter is too dirty); When the internal circuit connection solder joint falls off, poor contact, etc.), the oscilloscope can be used to check the signal voltage waveform of the air flow meter. When the engine runs stably, tap the air flow meter housing gently with a screw bit handle and observe the oscilloscope. If the waveform changes greatly or the engine stops, replace the air flow meter. Some air flow meters have loose internal connection, which will lead to sudden change of voltage signal, resulting in flameout. In this test, the vibration method should be used to make sure that the harness connector is in good contact.
4. When it is suspected that the intake pressure sensor is defective, the vacuum hose of the sensor should be checked first to see if it is broken, bent, sometimes air leakage, sometimes no air leakage, so that the signal of the intake pressure sensor is normal and sometimes abnormal, resulting in the engine flameout when the accelerator pedal is closed.
5. Also check the sensors that have a great influence on the injection quantity, such as the coolant temperature sensor and the oxygen sensor. The coolant temperature sensor not only affects the fuel injection quantity, but also one of the signals to correct the ignition advance angle, so it should also be paid attention to. Sometimes, the signal voltage of the oxygen sensor of some models does not change, which is easy to cause the engine to shut down during acceleration.
6. In case of poor acceleration first and then flameout when driving at a higher speed, it is necessary to check the oil circuit; If the engine stops suddenly in the process of driving at a higher speed, it is necessary to find out the cause of the circuit. Whether the high-voltage spark is too weak is one of the items that must be checked.
7. In case of sudden and intermittent flameout, the main sensors controlling ignition, engine speed and crankshaft position sensors should also be checked.
8. Fault symptom simulation test method. The most difficult situation in fault diagnosis is that there is a fault, but there is no obvious fault sign. In this case, a thorough fault analysis must be carried out, and then the same or similar conditions and environment as when the user’s vehicle fails must be simulated. No matter how experienced the maintenance personnel are, and no matter how skilled they are, if they diagnose the fault symptoms without verification, they will ignore some important things in the maintenance work, which will lead to the operation failure of the vehicle. For example, for problems that only occur when the engine is cold, or problems caused by vibration when the vehicle is running, these problems must not be diagnosed only by the verification of fault symptoms when the engine is hot and the vehicle is stopped. Therefore, vibration, high temperature and water seepage (dampness) may cause failures that are difficult to reproduce. At this time, the simulation test of fault symptom will be a useful method.

Effective measures. It can exert external effects on the vehicle in parking conditions.

In the fault symptom simulation test, the fault symptom must be verified, and the fault location or parts must also be found out.

In order to achieve this, before the pre connection test and the start of the test, the scope of the possible fault circuit must be reduced, and then the fault symptom simulation test must be carried out to judge whether the tested circuit is normal and verify the fault symptom.

(1) Vibration method. When the vibration may be the cause of the failure, the vibration method can be used for the test. The basic test methods are as follows:

① Connector. Gently shake the connector in the vertical and horizontal directions,

② Wiring. Gently swing the wiring vertically and horizontally. The connector joint, vibration support and connector body passing through the opening shall be carefully inspected.

③ Parts and sensors. Tap the parts with sensors with fingers to check whether the engine is off, as shown in figure 7-1c. Remember not to beat the relay with force, otherwise it may open the relay.

(2) Heating method. Some faults only occur when the car is hot, which may be caused by the heating of relevant parts or sensors.

Use an electric hair dryer or similar heating tools to heat the parts or sensors that may cause failure, and check whether there is a failure.

However, it must be noted that the heating temperature shall not be higher than 60 ℃ (the temperature is limited to the range that will not damage electronic components); Do not heat the parts in the computer directly.

(3) Water drenching method. When some faults occur in rainy days or high humidity environment, water can be sprayed on the vehicle to check whether the fault occurs.

However, it should be noted that water should not be sprayed directly on the electronic control parts of the engine, but should be sprayed in front of the radiator to indirectly change the humidity and temperature; Do not spray water directly on electronic devices; In particular, it is necessary to prevent water from leaking into the computer (if the vehicle leaks, the leaking water may invade into the computer, so special attention must be paid when the test vehicle leaks).

(4) Full connection method of electrical appliances. When it is suspected that the fault may be caused by excessive power load, all electrical equipment on the vehicle can be connected to check whether the fault occurs.