1st Gen R53 Cooper S Hard starting when below 1/4 tank of fuel

It is totally normal overnight for the fuel pressure to drop enough that you have to prime the system again. I always do it, every time I start, just because. If it needs to be primed after just a few minutes at a gas station, then there's an unusual leak somewhere, most likely back into the tank (you'd smell gas if it was anywhere else). I am guessing the fuel pump itself has some sort of check valve to the line it feeds that may at some point in time begin to fail.

The tank itself can be fully sealed with perfect gaskets to the outside world and the pressure will still leak out of the fuel line back into the tank, as there is always air in there that will compress, plus the gas cap will never seal perfectly.

Prime and drive - I had my car parked for 5 months before starting it Saturday - two prime cycles for good measure, started right up, just a few cough moments after that and purred right out of the driveway.

 

Probably something in the system that isn't holding the pressure like it used to. Check valves are usually in the pump itself.

watch this - seems like exactly what you are experiencing

Testing for a failed fuel pump check valve - YouTube


Here's a different approach - seen on a DeLorean site, but same idea - just need to put one of those between the line that leaves the tank and fuel rail

Inline Fuel Check Valve — Joe's Projects

 

Thanks for posting the fix.

 

The good thing is, your fuel pump didn't totally fail and leave you stranded like mine did a few years ago on a club drive.

Motor On !