is used on a car which has the oil filler cap made of metal. Newer ones are plastic.

The idea is to check for vacuum at the oil filler hole while the engine idles. A metal cap, loosened but not lifted off, will not jiggle if the vacuum is OK. A plastic cap will jiggle no matter what. Dead test.

Try the palm test: Engine at idle, and not fully heated up, close the oil filler hole with your palm as see if you detect vacuum. Or hold the cap and lift it gently.

If no vacuum, that small diameter hose is not delivering. Usually the brass fitting on the intake manifold. Gets hard carbon crud in there, so take the fitting off the manifiold to clean it. Use a 3/32-inch drill bit, and be careful not to drill out the brass itself.

Good Luck,

Bob

:>)
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240s: 1986 244GL, 1988 244GL, and 5 others.