I do have a go-no-go guage, from Sir Tools, that I am using to determine if the washer is the appropriate thickness. It is showing that all of the existing washers are too thick. If I put in a new HLC, the correct thickness washer is 4.05 mm, according to the guage.

I'm still wondering what a leak down test and a compression test would show if the washers were too thick and the valves did not fully close?

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1989 420 SEL