A vacuum gauge is used to measure the manifold pressure. In naturally aspirated engines, this pressure is always below atmospheric pressure. It is referred to as a vacuum. The objective of this procedure is to show you how to connect and use a vacuum gauge to check engine manifold vacuum.
Part 1. Preparation and safety
- Connect and use a vacuum gauge to check engine manifold vacuum.
Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:
- Work clothing - such as coveralls and steel-capped footwear
- Eye protection - such as safety glasses and face masks
- Ear protection - such as earmuffs and earplugs
- Hand protection - such as rubber gloves and barrier cream
- Respiratory equipment - such as face masks and valved respirators
If you are not certain what is appropriate or required, ask your supervisor.
- Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.
Points to note
- Vacuum gauges are possibly the most useful diagnostic tool in engine diagnosis. They are often forgotten by technicians who may prefer to use modern electronic diagnostic equipment.
- A vacuum gauge is used to measure the manifold pressure. In naturally aspirated engines, this pressure is always below atmospheric pressure. It is referred to as a vacuum.
- The vacuum gauge reads in either millimeters of mercury (mm Hg) or inches of mercury (in Hg). Those measurements mean the vacuum is strong enough to support a column of mercury in a tube to a height of so many millimeters or inches.
- 0 in Hg is equal to atmospheric pressure. A fully loaded engine at wide-open throttle will reach close to this reading.
- 30 in Hg is referred to as a perfect vacuum. It is a point where there is no pressure at all. It is a theoretical number because it is impossible to achieve. An engine decelerating on closed throttle will reach close to this reading.
- Healthy engines will create more vacuum in their inlet manifolds than worn engines. So a pressure comparison between a known healthy reading and the engine you are testing can assist in determining the state of wear of the engine.
- Refer to the chart below for various vacuum gauge readings.
||16" - 22" steady
||14" - 20" steady
||Fair but worn
||Jumps to 2" then on decel moves to 25"
||Jumps to 1" then on decel moves to 22"
||Fair but worn
||8" or less
||Vacuum leak, loose intake manifold
||Fluctuated between 14" - 19"
||Worn valve guides or head gasket blown between 2 cylinders
||Reading drops from normal
||Burnt valve, valve stuck open, misfiring spark plug
||8" - 14"
||Valve timing incorrect or large camshaft overlap
||14" - 16"
|| Ignition timing wrong
|| Moves between 12" - 16"
||Idle mixture incorrect
|Slow engine speed rise
||Needle falls then rises suddenly
||Blocked exhaustWeak valve springs
||Needle fluctuates and worsens with higher rpm
||Weak valve springs
Part 2: Step-by-step instruction
- Examine the gauge
The vacuum gauge has many uses in vehicle maintenance but its importance in detecting leaks or diagnosing problems is often overlooked. Examine the vacuum gauge that’s used in your workshop. Determine the units of measurement. And be sure to read the printed instructions that accompany the gauge.
- Fit the gauge to the manifold
In this case, the vacuum gauge will be used to measure manifold vacuum. Fit the vacuum gauge onto the engine intake manifold. You may need to fit a “Tee” piece to an existing vacuum connection.
- Start the engine
With the vehicle in neutral or park, and the emergency brake on, start the vehicle’s engine and let it settle into a uniform idle.
- Check the reading
Check the reading on your vacuum gauge. If the engine has no problems, the reading should be within the range fourteen to twenty- two inches of mercury and the needle steady.
- Change the load on the engine
Snap the throttle open and let it close. You will see the vacuum gauge needle quickly move to near zero then move all the way to almost thirty inches then back to the idle position. Open the throttle slowly. Notice the reaction of the needle.
- Record your observations
Make sure you keep a record of the readings and needle movements you experience when working with different engine components.