Tools & Equipment: Hand & Power Tools: Using measuring tools
This Video Sequence is
not available in the free
CDX eTextbook
 
Click this icon for more information about CDX Automotive Resource Kit Plus
This Knowledge Check is
not available in the free
CDX eTextbook
 
Click this icon for more information about CDX Automotive Resource Kit Plus
This Lab Experiment is
not available in the free
CDX eTextbook
 
Click this icon for more information about CDX Automotive Resource Kit Plus
This Workshop Activity Sheet is not available in the free CDX eTextbook
 
Click this icon for more information about CDX Automotive Resource Kit Plus
This Assessment Checklist is not available in the free CDX eTextbook
 
Click this icon for more information about CDX Automotive Resource Kit Plus
This Handout Activity Sheet is not available in the free CDX eTextbook
 
Click this icon for more information about CDX Automotive Resource Kit Plus

Topic IntroductionHelp

Using a vacuum gauge

Summary
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

Objective

Personal safety

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:

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

Points to note

Vacuum gauge

Engine Speed Gauge Reading Engine Condition
Idle 16" - 22" steady Healthy
Idle 14" - 20" steady Fair but worn
Snap throttle Jumps to 2" then on decel moves to 25" Healthy
Snap throttle Jumps to 1" then on decel moves to 22" Fair but worn
Idle 8" or less Vacuum leak, loose intake manifold
Idle Fluctuated between 14" - 19" Worn valve guides or head gasket blown between 2 cylinders
Idle Reading drops from normal Burnt valve, valve stuck open, misfiring spark plug
Idle 8" - 14" Valve timing incorrect or large camshaft overlap
Idle 14" - 16" Ignition timing wrong
Idle Moves between 12" - 16" Idle mixture incorrect
Slow engine speed rise Needle falls then rises suddenly Blocked exhaustWeak valve springs
3000 rpm Needle fluctuates and worsens with higher rpm Weak valve springs


Part 2: Step-by-step instruction

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. Record your observations
    Make sure you keep a record of the readings and needle movements you experience when working with different engine components.