by MOU YONG HONG on October 07, 2021

Engine start difficulty means that the starter can drive the engine to rotate at the normal speed, and there are obvious signs of starting, but it is difficult to start, or it needs to be started several times in a row, or the starter is turned for a long time to start the engine.

Common causes of failure:

1. Air intake system leaks
2. The fuel pressure is too low or the maintenance pressure is abnormal.
3. There is a large amount of air in the oil circuit, which causes an airlock.
4. The oil return pipe or the fuel filter element is blocked.
5. The air filter element is clogged.
6. The cooling temperature sensor is faulty.
7. MAF/MAP sensor failure.
8. Idle speed control valve or additional air valve failure.
9. The exhaust gas recirculation valve does not work well.
10. The injector works poorly (oil leakage, carbon deposits, gum blockage, etc.).
11. The ignition timing is not accurate (the timing belt wheel connection key is worn out).
12. The wiring of the start switch computer is broken.
13. The cylinder compression pressure is too low.
14. Excessive carbon deposits in the intake pipe and intake and exhaust valves (the valve is not closed tightly).
15. Ignition coils, spark plugs are not working properly, or high-voltage lines are damaged.
16. Cold start injector does not work.

General procedure for fault diagnosis:

1. Perform fault self-diagnosis. If there is a fault code, find the cause of the fault according to the fault code. The instrument can be used to read the data flow at startup and analyze the cause of the failure based on the data flow.

2. Check the vacuum of the intake pipe at idle. If the vacuum is less than 66.7KPa, or there is a “hissing” air leak near the intake pipe during idling operation, it means that there is air leakage in the intake system. Check the various pipe joints, gaskets, vacuum hoses, etc. of the intake pipe. And exhaust gas recirculation system, fuel evaporation recovery system.

3. Check the air filter. If the filter element is clogged, it should be cleaned or replaced.

4. If the engine can start normally when the throttle valve is about 1/4 opening, but it is difficult to start when the throttle valve is fully closed, check whether the idle speed control valve and the additional air valve are working properly. Unplug the harness connector of the idle speed control valve during idling operation of the cold car, or clamp the intake hose of the additional air valve with pliers during idling operation of the cold car. If the engine speed does not drop, it means that the idle speed control valve is not working properly, and the idle speed control valve and control circuit should be checked.

5. Check fuel pressure

6. Check the cooling temperature sensor and intake air temperature sensor. If the resistance value does not meet the standard, it should be replaced. Check the signal voltage online to see if it is the same as the standard, otherwise check the circuit.

7. Check the airflow meter or the absolute pressure sensor of the intake manifold. Use a multimeter or special instrument to detect the signal voltage of the airflow meter or the absolute pressure sensor of the intake manifold. If it does not meet the requirements, it should be replaced.

8. If it is not easy to start when the car is cold, but it starts normally when the car is hot, check whether the cold start injector is working normally.

9. If the cold car starts normally but the hot car is difficult to start, the ignition coil and igniter should be checked. If there is no abnormality, the fuel pressure should be checked. If the fuel pressure is too high, it means that the oil return pipe of the oil pressure regulator is blocked, causing the fuel in the fuel pipe to have an airlock due to the high temperature after the hot car is turned off. When troubleshooting the difficulty of starting a hot car, check the working condition of the solenoid valve of the activated carbon tank. If there is a jam inside the solenoid valve, the spool will not return normally when the power is cut off, causing the mixed gas to be too thick.

10. Check the ignition timing when idling. If it does not meet the standard value, it should be adjusted.

11. Check whether the start signal from the start switch to the computer is normal. If the computer cannot receive the start signal of the start switch, it will not be able to start the enrichment control, which will also cause difficulty in starting. In this regard, you should use a computer tester to analyze the engine computer data or check from the computer wiring harness plug to see if there is a start switch signal transmitted to the computer when starting. If there is no signal, check the start switch and wiring.

12. Disassemble and inspect the fuel injector, check whether the fuel injector is dirty, leaking, spray atomization, etc. After cleaning, it does not meet the requirements and should be replaced.

13. Check the intake pipe and intake and exhaust valves for carbon deposits. If the carbon deposit is serious, it should be removed.

14. Check the cylinder compression pressure. If the cylinder pressure is too low, the engine should be disassembled and inspected.

15. If the above checks are normal, you can change to a new computer for testing. If it improves, the original computer is faulty and the computer should be replaced.

Related points of fault diagnosis:

1. Analyze the control principle of air intake and fuel injection when electronically controlled fuel starts

The opening state of the additional air valve and the idle valve is directly controlled by the cooling water temperature or indirectly controlled by the engine computer by the cooling water temperature and the start signal. The intake air volume and the fuel injection volume cannot be matched reasonably, which will cause the mixture to be too thick or too lean, which may cause difficulty in starting.

(1) The idle speed control valve is stuck due to carbon deposits and the air passage is very small. At this time, the injector still sprays the same amount of oil, which will cause difficulty in starting the mixture if the mixture is too thick.

(2) If additional intake air is generated for some reason, the mixture will be too lean and it will be difficult to start. Such as exhaust gas recirculation stuck in the open position, leakage of the intake pipe, etc.

(3) Due to the low temperature and low speed of the engine during cold start, the injected fuel is not easy to vaporize, which easily causes the mixture to be too lean. Therefore, when the engine is started, the computer can determine the starting conditions based on the switch signal and the engine speed signal.

The specific controls are as follows: ①The fuel injection increment is obtained through the cold start injector.

②The computer-controlled fuel injector directly obtains the fuel injection increment. (The computer calculates the basic fuel injection pulse width according to the water temperature signal, and then corrects it according to the intake air temperature signal and the battery voltage signal to obtain the fuel injection pulse width at startup)

2, key inspection items

① Carefully check the fuel pressure, including how fast the fuel pressure is built up, the pressure value, and the pressure maintained. ② The strength of the ignition system’s high-voltage sparks during start-up also affects the start-up time. At the same time, don’t forget to measure the battery voltage, the ignition voltage of the computer, and the igniter when starting.

3, the difference between cold start difficulty and hot start difficulty

A cold car hard to start means that when a cold car starts, it must be started several times before it can start, but when a hot start starts, the car can start immediately. The reason for the difficulty of cold start is that the mixture is too thin or too thick. The reasons for the difficulty of starting a cold car include cold start injector not injecting fuel, coolant temperature sensor failure, intake sensor failure, poor injector atomization, carbon deposits in the intake pipe, insufficient ignition energy, spark plug failure, idle speed control valve Failure, etc.

Hot car hard to start means that the cold start is normal, the hot start is difficult, or even cannot be started. The reasons for the difficulty of hot start include coolant temperature sensor failure, intake air temperature sensor failure, several injectors leaking or severely poorly atomized, cold start injector failure, idling valve, high oil pressure ignition failure, etc. The fundamental reasons are The mixture is too thick.

4. The influence of carbon deposits on engine starting performance

Among the carbon deposits formed in various parts of the engine, the carbon deposits formed at the rear of the intake valve have the greatest impact on the engine.

1. The amount of intake air and instantaneous intake speed can cause poor acceleration and a decrease in the maximum speed;

2. The carbon deposit is porous. When the engine is a cold start, the fuel is absorbed by the carbon deposit on the intake valve, resulting in less fuel actually entering the cylinder, resulting in a low air-fuel ratio.


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