What engine are we talking about here? From your descriptions of the cap, it sounds more like you are talking about the 10 mm, CM6 plug cap. The CM6 cap does have a spring towards the bottom where I have never seen such a spring on the 1/4 - 32 cap. Also the CM6 cap does not lock on with the 1/8 turn, it just pushes on but quite hard like the smaller cap. Just wanted to be sure which cap you have. The cap illustrated in the PDF a few posts back was the CM6 cap. Either cap being a bit loose can cause problems.

On the CM6 cap some people remove the "C" spring from around the base of the cap and substitute a screw type hose clamp to retain the cap. This actually works quite well and is a very positive means of cap retention though this method would not work on the 1/4" cap.