Purpose
To check for restriction in the air intake system likely to cause engine performance problems.
NOTE: High intake or exhaust restriction can cause a large amount of black smoke.
Tools
• Gauge Bar (magnehelic)
• Test line
Procedure
1. Inspect the following parts for restriction, damage or incorrect installation:
• Air filter inlet and ducting (includes hood)
• Air inlet hoses and clamps
• Air filter housing, filter element, and gaskets
• Chassis mounted CAC and piping
• Air filter restriction indicator or gauge
NOTE: Intake restriction should be below 25 in H2O. When the filter element reaches maximum allowable
restriction, the yellow indicator will reach the top of window and automatically lock in this position.
2. Remove air intake restriction indicator or remove line to instrument panel restriction gauge from air filter housing.
3. Attach test line to tap for air filter housing.
4. Connect line to magnehelic gauge or manometer.
5. Run engine at high idle, no load.
6. Record reading on Diagnostic Form.
• If restriction is more than 3.13 kPa (12.5 in H2O), replace air filter element.
• If restriction is more than 3.13 kPa (12.5 in H2O), and a new filter is in place, check for obstructions in air inlet.
• If restriction is less than 3.13 kPa (12.5 in H2O), continue with Performance Diagnostics.
NOTE: An equivalent test, using the instrument mounted restriction indicator, can only be done while operating the engine at full load and rated horsepower. The true maximum air filter restriction for this test is 6.22 kPa (25 in H2O).
Possible Causes
• Clogged air filter element (dust, dirt, or debris)
• Snow in air filter inlet
• Ice in air filter inlet
• Plastic bags or other foreign material in air filter inlet
• Collapsed air filter
• On engines recently repaired, rags or cap plugs may have been left in the intake system.
