How is the automobile vacuum pressure gauge connected when measuring the pressure of engine intake manifold?

The essence of diagnosing EFI engine with vacuum gauge is to detect the vacuum degree of engine intake pipe.

When detecting the vacuum degree of the engine intake pipe, connect the vacuum gauge to the throttle back door. When the gasoline engine is in a normal state, it runs at no-load according to the specified idle value. Remove the air filter and check the reading and indicating status of the vacuum gauge. The application of vacuum gauge in actual detection is as follows (P is cylinder pressure, Px is intake pipe vacuum):

(1) The sealing performance of the engine is in a normal state. When the engine is idling, the pointer of the vacuum gauge should be kept between 64-7 1kPa (its swing amplitude and swing speed are related to the sealing performance, air-fuel ratio and ignition performance of the engine). If it is suspected that a cylinder is not working properly, the diagnosis method of single cylinder flameout can be adopted. After the single cylinder is turned off, the greater the drop value of Px, the better. This is an index to judge the working quality of each cylinder (ignition, fuel injection and sealing).

Quickly open and close the throttle. If the pointer swings sensitively between 6.7-84.6 kPa, it shows that Px has good following performance to the change of throttle opening, that is to say, the sealing performance of all parts is good under all working conditions. If the seal is not good, Px is lower than normal at idle speed, which is obviously unstable. When the throttle is opened quickly, the pointer will drop to zero, and the pointer will not return to 84.6kPa after the throttle is closed.

In order to verify the tightness of each cylinder, the vacuum gauge should be replaced at the hole of the dipstick, and the pressure measured in the crankcase should be negative pressure; If it is positive, it means that the sealing is not good, or the PVC is not well ventilated.

(2) When the ignition timing of the engine is incorrect, the valve timing is inaccurate and the spark state is not good.

When the ignition timing of the engine is incorrect, the valve timing is inaccurate, and the electric spark is poor, the combustion conditions become worse, the power loss and speed fluctuation are large, the high vacuum degree cannot be formed, the idle speed is unstable, and the acceleration is weak.

When idling, the pointer swings between 46.7 and 57 kPa. If the ignition is too early, the hands will swing greatly; On the contrary, the swing amplitude is small. When the valve timing is wrong, the phenomenon is similar to the ignition timing, and it should be handled according to the cause.

(3) When the engine exhaust system is blocked.

When the engine exhaust system is blocked, it will produce a large back pressure. In the idle state, Px sometimes reaches 53kPa, but it soon drops to zero or very low, and the gasoline engine can only barely run when the congestion is serious. At this time, it can be verified by observing the exhaust state of the exhaust pipe or removing the exhaust pipe for operation.