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3.
INTRODUCTION
The use of a vacuum gauge is often overlooked when determining mechanical condition and carrying out fault diagnosis on internal combustion engines.
Monitoring actual manifold vacuum is invaluable when troubleshooting engine faults. This can only be carried out by using a good quality vacuum gauge and this,
coupled with a hand-operated vacuum pump, also allows static testing of all types of vacuum operated systems.
Set out on the following pages are applications that the VS401 vacuum tester kit can be used for, but it must always be remembered that these are examples only
and reference to manufacturer's service manuals should always be made for correct testing procedures and specifications.
In addition to this, it is always recommended that additional tests, ie. compression tests, cylinder leakage tests, ignition timing checks etc. be carried out to confirm
indications of vacuum gauge readings.
4.
OPERATING INSTRUCTIONS
Analysing Engine Mechanical Condition via Manifold Vacuum
Readings
The readings shown below are only examples of what may be observed. It is important to
remember that it is the action of the needle rather than the actual reading that is more
important. Varying engine types will run different manifold vacuum pressures, depending on
camshaft profile, valve overlap, timing etc, so an exact vacuum reading cannot be
specified. The main criteria is that the needle reading is between 16 to 21 inHg and steady.
Manifold vacuum is also affected by altitude and it will drop approximately 1 inHg for every
1000 feet above sea level so this must also be considered when assessing manifold
vacuum actual readings.
Step 1. Run engine until normal operating temperature is reached. Switch engine off.
Step 2. Locate and connect the vacuum gauge to a port directly on the manifold (fig.1) or
on the carburettor/throttle body below the throttle butterfly.
Step 3. Start and run the engine at idle, observing the gauge reading.
Refer to the following table to indicate any problems and their possible causes.
fig.1
VS401 - 1 - 061006
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