Checking Engine Vacuum
Warm engine to normal operating temperature. Disconnect brake vacuum hose at manifold and install vacuum gauge to hose fitting on manifold. With engine at idle, read vacuum gauge. Normal engine vacuum reading should be less than 17.72 in. Hg with engine at idle and A/C off. While monitoring engine vacuum reading, quickly open and close the throttle several times and then allow engine to idle. If engine vacuum is within specification, test is complete. If engine vacuum is not within specification, check for vacuum leaks and repair as necessary. If no vacuum leaks are found, check for engine mechanical problem. See MANIFOLD VACUUM READING DIAGNOSIS table.
MANIFOLD VACUUM READING DIAGNOSIS
| Vacuum Gauge Indication | Possible Cause |
|---|---|
| Needle Steady, But Lower Than Normal Reading | Intake Manifold Gasket Leak Or Damaged/Disconnected Vacuum Hose |
| Needle Temporarily Stops Or Is Steady Above Normal Reading When Engine Speed Is Slowly Reduced From High Speed | Exhaust Backpressure Too High Or Clogged Exhaust System |
| Needle Intermittently Drops To Lower Than Normal Reading | Leak Around Cylinder |
| Needle Vibrates Rapidly At A Certain Speed As Engine Speed Is Gradually Increased & Then Vibration Increases With Engine Speed | Weak Or Broken Valve Springs |
| Needle Vibrates In Narrow Range Above & Below Normal Reading | Defective Ignition System Or Throttle Idle Adjustment |