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How To Detect The Vacuum Degree Of The Intake Manifold ?
- Sep 21, 2018 -

The essence of diagnosing EFI engine with vacuum meter 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 back door of the throttle valve, run the gasoline engine without load according to the specified idle speed under normal conditions, remove the air cleaner, and check the reading and indication status of the vacuum gauge.  The application status of vacuum meter in actual detection is as follows ( P is cylinder pressure, PX is intake pipe vacuum degree ):

1 ) When the engine is idling under normal conditions, the vacuum gauge needle should be stable between 64 and 71 kPa ( its swing amplitude and swing speed are related to the engine's tightness, air-fuel ratio and ignition performance ).  If it is suspected that a certain cylinder is not working well, a single-cylinder misfire method can be used. After a single-cylinder misfire, the falling value of PX should be as large as possible, which is an indicator to judge whether each cylinder is working well or not ( ignition, oil injection and sealing ).

 The rapid opening and closing of the throttle valve, if the watch needle swings sensitively between 6.7 and 84.6 kPa, indicates that PX has a good follow-up to the change of throttle opening, which means that all parts have a good sealing performance under all working conditions.  If the sealing is not good, PX is lower than normal at idle speed and is obviously unstable.  When the throttle is quickly opened, the gauge pin will drop to zero and the gauge pin will not return to 84.6 kPa after the throttle is closed.

 In order to verify the sealing performance of each cylinder, the vacuum gauge should be replaced at the hole where the oil dipstick is inserted, and the measured pressure in the crankcase should be negative pressure.  If it is positive, it means that the sealing is not good or the PVC ventilation is blocked.

( 2 ) What to do when the engine ignition timing is incorrect, the valve timing is incorrect, and the spark is in a bad state?

 When the ignition timing of the engine is incorrect, the valve timing is incorrect, and the spark is not good, the combustion condition becomes bad, the power loss and the speed fluctuation are large, the high vacuum degree cannot be formed, and the idling speed is unstable and the acceleration is weak.

 When idling, the watch needle swings between 46.7 and 57 kPa.  If the ignition is too early, the amplitude of the needle swing is larger.  On the contrary, the swing is small.  If the valve timing is incorrect, the phenomenon is similar to that of ignition timing, and should be handled separately according to the cause.

( 3 ) What to do when the engine exhaust system is blocked?

 There will be a large back pressure when the engine exhaust system is blocked.  At idle speed, PX can sometimes reach 53K Pa, but it quickly drops to zero or very low, and the gasoline engine can only barely run when congestion is serious.  At this time, it can be verified by observing the exhaust status of the exhaust pipe or removing the exhaust pipe.


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