I am not an expert but from past experience this has happened to me. (1.) Once while priming a friends car with gas down the carb (aloong time ago) I got some gas on the disti and it leaked down the shaft. When cranked blew the oil fill cap off like a mortar. (2.) Last Feb. (more recent) I replaced an engine in a 98 Z71, 350 vortec, FI. All went well but noticed every morning the lower hose was collapsed (no spring either). After getting on this site and talking it thru discovered the hose to the overflow tank was clogged. The system was not open to the atmosphere. Hope something helps.