Detailed explanation of maintenance method for abnormal operation of BMW 740 engine

Recently, some netizens often ask BMW 740' s engine often has problems, and I don't know how to repair it. Next, I, Wang Chang, will introduce the maintenance method of BMW 740 engine fault to the owner, hoping to help the owner. Fault symptom: A BMW 740 car is equipped with a straight-cylinder engine, but the engine is unstable, so it is impossible to refuel whether it is accelerated rapidly or slowly. Tempering when refueling, the exhaust pipe emits black smoke. After adjusting and cleaning the fuel injector and gasoline filter, the fault still exists. Cause: The engine is unstable, and the exhaust pipe emits black smoke, indicating that the mixture is not fully burned in the engine cylinder. This phenomenon is caused by many reasons. When the engine works normally, the mixture is fully combusted, which generates a lot of heat energy to push the piston to do work. The emissions are mainly water and carbon dioxide, so the tail gas is basically colorless and odorless. When the engine works abnormally, the exhaust gas contains a lot of carbon monoxide, hydrocarbons and various particles, which are black, especially when accelerating rapidly. Troubleshooting: Under the above circumstances, check the spark plug for abnormal phenomena such as excessive carbon deposition and electrode ablation. If not, check the working pressure of each cylinder, which should generally be greater than 1.2 ~ 10pa. If the cylinder pressure is too low, the mixture will not be completely compressed, resulting in poor combustion. At this time, check the cylinder gasket, valve and piston ring, and analyze and judge the cause of air leakage. If the above steps are normal, check the engine control system. The theoretical value of engine air-fuel ratio "a: f" is 14.7: 1. When cold start, rapid acceleration or high power output are needed, the mixture should be enriched. Under other working conditions, the air-fuel ratio should be kept at the theoretical level. The oxygen sensor can measure the oxygen content in the exhaust gas in time, and correct the fuel injection quantity in a certain range, thus forming a closed-loop control of fuel injection. Generally speaking, check the components that play a major role in fuel injection, such as throttle position sensor (TPS), intake manifold absolute pressure sensor (MAP), air-air flowmeter (MAF), water temperature sensor (ECT) and so on. The change rate of TPS signal reflects the acceleration state of the car. At this time, enrichment and mixing should be carried out to increase the output power and reduce the ignition advance angle to prevent deflagration. It can be seen that MAP plays the role of real air advance angle and power control valve in carburetor. When idling, the output voltage of the intake manifold absolute pressure should be between 1V and 1.4V, and the intake manifold absolute pressure plays a decisive role in fuel injection. If there is something wrong with itself, or the rear intake manifold leaks air (for example, the intake manifold interface pad is damaged, etc.). ).), Duff provides an error signal to ECM, which makes the engine work seriously abnormally, and ECF also determines the fuel injection quantity. When the engine working temperature is low, the fuel injection quantity will increase, and with the increase of temperature, the fuel injection quantity will decrease. If the engine temperature is normal, the engine coolant temperature sensor still provides a low temperature signal to the engine control module, and the mixture concentration will be too high and the engine will work abnormally. The above information is compiled by Bian Xiao, hoping to help all car owners.