Diagnosing an Engine Vacuum Leak with Propane

[Christopher R.]

I'm trying to determine whether my carburetor or intake manifold has a vacuum leak. To diagnose, the manual says to squirt propane along the base of the carb and the intake manifold. Rpms are supposed to increase when the propane gets sucked into the leak.

Squirted propane all along the base of the carb and the base of the manifold, especially the ends. Nothing happened. So I thought I'd try squirting it directly down the throat of the carb. Still nothing happened.

What am I doing wrong?

[Duke W.]

Re: Diagnosing an Engine Vacuum Leak with Propane

I don't think you're doing anything wrong. The test results indicates no vacuum leak in the area you checked, so it's somewhere else or some other problem.

Vacuum leaks cause rough idle due to a lean mixture, but also increase idle speed. Pull off the vacuum hose from the VAC of an engine with full time vacuum advance and you see what happens - increased speed, but rough running. Adding propane richens the mixture and smooths out the engine.

You might want to give the year/engine option of the car, symptoms that lead you to believe there is a vacuum leak, and any other information that you even suspect might be useful to a diagnosis.

Duke