Performing an Engine Diagnostic: Vacuum Tests

Testing an engine’s mechanical condition is required when the cause of a problem is not located during the preliminary inspection. The common testing procedures are those for engine vacuum, cylinder power balance, cylinder compression, cylinder leakage, oil pressure, and cooling system operation. (Refer to Test 2 for the latter two tests) These tests are designed to locate internal engine problems before disassembling the engine. 

CAUTION: Be sure to use approved exhaust ventilation equipment when operating a vehicle in an enclosed area.  

 

Procedures for performing engine vacuum tests:

NOTE: Accurately diagnosing problems with a vacuum gauge can be difficult. Study and compare readings to diagnostic charts. 

1. Perform a vacuum gauge test to determine engine condition and performance.  

Connect the vacuum gauge to an intake manifold vacuum source. Connect a vacuum hose to an accessible intake manifold vacuum connector and extend it up to the vacuum gauge.  

  

Connect the exhaust ventilation equipment  

  

Start the engine and allow it to reach normal operating temperature. 

Observe and record the reading with the engine at idle speed. Normal idle vacuum is approximately between 18 in and 21 in. A low reading or an erratic vacuum gauge needle indicates a problem.  

  

Shut off the engine. Disconnect the test equipment. Disconnect the exhaust ventilation equipment. 

  

2. Perform an exhaust restriction test to determine if the exhaust system is restricted or has excessive back pressure.  

  

Connect the vacuum gauge to an intake manifold vacuum source. Connect a vacuum hose to an accessible intake manifold vacuum connector and extend it up to the vacuum gauge.  

  

Connect a tachometer to the engine.  

  

Connect the exhaust ventilation equipment.  

Start the engine. Gradually accelerate the engine rpm from idle to 2,000 rpm. Observe and record the reading. 

Maintain the engine speed at 2,000 rpm for about 10 seconds or longer. Observe and record the reading. The vacuum gauge should hold steady or increase slightly. The vacuum reading gradually decreases if the exhaust system is restricted.  

  

Shut off the engine. Disconnect the test equipment. Disconnect the exhaust ventilation equipment.  

  

3. Perform a cranking vacuum test to determine if the engine is mechanically sound. Connect the exhaust ventilation equipment.  

Start the engine and allow it to reach normal operating temperature. Shut off the engine. 

Connect a vacuum gauge to a non-ported vacuum source on the intake manifold.  

  

Disable the ignition system.  

  

Completely block the throttle valve so that air cannot enter.  

  

Crank the engine. Observe and record the cranking vacuum.  

  

Disconnect the vacuum gauge.  

  

Unblock the throttle valve. Enable the ignition. Disconnect the exhaust ventilation equipment. 

The following are possible results for this test:    

• Satisfactory cranking vacuum indicates a mechanically sound engine.   

  

• Uneven or pulsating cranking vacuum indicates defective valves, defective piston rings, a defective head gasket, or an uneven cranking speed.   

  

• Below normal cranking vacuum indicates excessive resistance in the battery cables, a defective cranking motor, or excessive mechanical drag in the engine.   

  

• Uneven cranking vacuum indicates uneven compression or a defective starter.  

   

NOTE: If the cranking vacuum is uneven, perform a cranking engine cylinder compression test or a cylinder leakage test. Satisfactory engine performance cannot be obtained until existing compression or vacuum leaks are eliminated. 

  

Based on the tests, determine the necessary action to correct any problems. Include further diagnosis and/or repairs.