Had this happen to my Sierra, every once in a while it wouldn't start. After making an appointment to have it towed I went out the day before to make sure it wouldn't start and it popped right off. Did it again a few months later, waited a few days and it started. Figured out it was happening after it rained for a few days with high humidity. Changed out cap and rotor and has never happened again.
You must have an air leak letting in moisture some where if the cap doesn't have a crack in it.
You must have an air leak letting in moisture some where if the cap doesn't have a crack in it.