What is the ideal vacuum gauge reading at idle for the engine?

The ideal vacuum gauge reading at idle for an engine is typically between eighteen to twenty inches of vacuum, which indicates that the engine is operating efficiently under the current load conditions. This range reflects proper air-fuel mixture, healthy intake valves, and adequate manifold pressure, contributing to smooth engine operation. When the vacuum reading falls within this range, it suggests that the engine has good compression and is not experiencing any significant leaks or restrictions in the intake system, which helps ensure optimal performance. Understanding this vacuum reading is crucial for diagnosing engine performance issues, as it can signal problems if the vacuum levels are lower or higher than expected.