Just a thought. My truck is a 1999 K2500 small block. Not long after I got it I experienced severe vapor lock symptoms during the first summer. It didn't repeat but reoccurred the next summer as soon as it got hot out. It seemed similar to an issue going back many years on a family vacation. My Dad's '76 Ford Granada crapped out on the drive through the mountains of BC on a hot day. It turned out to be a gas cap issue. Changed the cap and drove on to Ontario issue free. I know the fuel cap is a much more important part of the fuel system on today's cars. I thought, what the heck, I'll try it on the truck. Cheap fix. Never had an issue again. A couple of years ago I was working with a millwright and he was having again, a similar issue with his '2000 Tahoe. He laughed at me when I suggested that he replace his gas cap. I joked that in my opinion GM made lousy gas caps. I convinced him that it was a cheap repair that could do no harm. Worst that could happen is you get a new cap. My friend didn't like taking advice from the service and parts guy but he asked me to order him a cap on the next parts order that day. Yup, no more issues. He felt good again sending his wife and kids out in the Tahoe once more. His wife felt good that she didn't have to drive his rusty Jetta anymore as well.
Exactly. Pre-emissions we used simple bi-directional vented caps so that a vacuum wouldn't be
developed in the gas tank while the fuel pump was busy emptying it -or- have excess pressure
develop as a full tank on a parked vehicle warmed up over the course of a hot day.
But in order to help control the evaporative emissions, our gas caps were upgraded by installing
small one-way air valves. I think of them as an 'air diode', in that air is allowed in as needed, but never
allowed back out. (Relieving excess pressure is where the charcoal cannister gets involved.)
The point is, just like diodes in the land of electricity, these one way check valves in our gas
caps can & do fail, and when they do, they can definitely cause 'vapor lock' (ie: no fuel delivery)
symptoms.
Good on you for sharing your experiences. Staying on top of these intermittent failures is
how we can keep our aging rides in the DD or better level of reliability.
Cheers --