System Checking
With engine at proper idle speed, remove oil filler cap. If valve is working normally idle speed will noticeably change. If no change in engine speed is noted, valve is faulty. If engine idles unevenly or will not start, the valve pin is not closing fully and excess air is being drawn past lower plate valve and into the intake manifold. Remove clip and check for correct assembly of valve and diaphragm. Check diaphragm for any punctures.