Vacuum Gauge Tests

Why the vacuum gauge is my favorite diagnostic tool for engine diagnosis.

This tool is not only inexpensive, but also provides the quickest route for diagnosis of restricted exhaust systems and leaking EGR valves.

A leaking exhaust gas recirculation valve will cause a low but steady vacuum reading at closed throttle.

This is because EGR flow increases manifold pressure and lowers vacuum.

The engine is designed to tolerate metered amounts of exhaust gases as the throttle is opened. But the efficiency of the cylinders is too low at idle for the engine to accept exhaust gas into the intake charge at closed throttle.

Hence the egr valve is always closed at idle. Or at least it should be unless it is hanging up or not sealing due to carbon deposits on the pintal.

In cases where the valve is stuck wide open, the engine will not start at all. When you have a stuck open EGR valve and the vehicle will not start you will see a vacuum cranking reading of zero.

Where as if the valve is closed a cranking vacuum reading should be around three to 5 inches of mercury. Read more