In a simple view, vaccum is produced in the intake manifold if the engine is operating correctly. Thus vaccum at the intake manifold can be a good tool to diagnose engine problems including leaky valves, timing problems and exhaust problems. When the exhaust system is not working properly, then the vaccum level (I will only refer to the intake vaccum) at idle drops dramatically, for e.g. if there is a clog, the vaccum would be much lower than 16 inch Hg. And normally, the vaccum drops some amount as you acclerate, because the throttle opening lets the air into the intake. But if the exhaust is clogged (or if EGR is always open, it should be closed at idle), it would dive down to zero.
The vaccum created at the manifold is also used to control other vaccum driven elements in the car such as your a/c vent control flaps, automatic door locks, brake booster etc.
On a diesel engine, there is normally a seperate vaccum pump because a diesel engine does not produce enough vaccum for anything useful.
And vaccum is one of the easiest way to control mechanical elements that are widely distributed across the car.
In short, vaccum levels at the intake manifold are critical for diagnosis as well as function of the car. If you dont have a mytivac, it is worthwhile to have and it has a little manual with it which lists all the engine functions you can diagnose with it, pretty handy tool.
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Saumil S. Patel
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