I don't know about your specific failure mode however I doubt it has anything to do with the fuel level in your tank. FYI, many S60s and S80s were manufactured with fuel pumps that are prone to leakage. The plastic housing develops (stress ?) cracks and are problematic. They lose pressure causing hard starting issues. A hint is that sometimes they will start to emit a gas smell inside and/or outside of the car. This is how we caught ours - and yes the top of the housing was cracked and leaking when the shop showed it to me. Our independent Volvo mechanic says it happens all the time and keeps spares as he sees so many. NHTSA has a case open on this : 07V152000 and Volvo will replace them under recall only if you live in Arizona or New Mexico under their campaign number # R176. I spoke to Volvo USA and they claim that the heat causes the problems only in these two states and have refused to compensate me. Additionally, now they are saying it's the additive in the gas in these states. I find both excuses their tech people and management have given me to be less than truthful. Seems like Volvo has gotten great training from FORD Customer Service.