Checking the engine with a vacuum gauge (Audi A4 B7)

1. Using a vacuum gauge, you can get information about the condition of the engine and determine if the cylinder head gasket is blown, the fuel system, piston rings or valves are damaged, the fuel injection system is incorrectly adjusted, etc. Unfortunately, the readings of the vacuum gauge can be misinterpreted, so these measurements should be used in combination with other methods of checking the engine. For a correct determination, both the absolute value of the vacuum gauge readings and the rate of their change are important (see illustration).



2. Connect a vacuum gauge to the intake manifold. Start and warm up the engine to normal operating temperature.

3. Read the vacuum gauge. If the engine is in normal condition, the vacuum gauge readings should be constant and be 430÷560 mm Hg.

Low constant vacuum readings indicate a damaged gasket between the intake manifold and the throttle body, a damaged vacuum hose, incorrectly set ignition timing, or incorrectly set valve timing.

If the vacuum gauge readings are 80÷200 mm Hg below normal and fluctuate, then the intake manifold gasket near the inlet may be damaged or the injector may be faulty.



If the readings constantly drop by 50÷100 mm Hg relative to the stable value, then the valves may be damaged. In this case, you should measure the compression in the engine cylinders (see Section 2).

If the readings drop, but not regularly, then perhaps a valve is jammed or there is an ignition failure. If at constant idle speed the readings quickly fluctuate with an amplitude of about 100 mm Hg, and smoke comes out of the exhaust pipe, then the valve guides may be damaged.

If the reading fluctuates rapidly as the idle speed increases, the intake manifold gasket or cylinder head gasket may be damaged, the valve springs may be weak, the valves may be burnt, or the ignition may be malfunctioning.

Small fluctuations of about 25 mm Hg are usually associated with problems in the ignition system.

If the readings fluctuate greatly, the cylinder head gasket or cylinder may be damaged.

If the needle moves slowly over a wide range of values, the crankcase ventilation system may be clogged, the intake manifold gasket or the gasket between the throttle body and the manifold may be damaged.

4. Check how quickly the vacuum gauge readings are restored after the throttle valve is opened sharply and fully and then returned to its original position. If the engine is in normal condition, the readings will drop to almost zero, then increase by about 130 mm Hg above normal and then decrease again to the previous values at constant idle speed. If the readings are restored slowly and pass through the maximum after closing the throttle valve, then the piston rings may be damaged. If there is a long delay, then the exhaust system may be clogged.