LazyPup:

I had the exact same problem on my 1998 V70 GLT (LP Turbo) last year.

I checked fuel cap, filler neck, all the lines, etc. I reset codes several times and each time after a few days the Check Engine light would reappear. I tried different brands of gas. I tried everything I knew of and finally gave up and proceeded to my dealer since a special test is required to diagnose the EVAP system on these cars.

Dealer replaced the EVAP valve (apparently sticking), charcoal canister (full of fuel) and the little filter between the two. No problems since.

We could not pinpoint the exact cause, but there are two theories. First theory was that the EVAP valve failed and caused everything. Second theory is that we may have been overfilling the tank. Normally I stop when the pump clicks off, but our driveway is on a bit of an up grade so if I had filled up and then immediately parked the car at home in the heat of summer fuel may have expanded (as it became warmer) and forced its way up into the evap system lines. Sounds a little far fetched, but possible.

Either way, after $400 in repairs my paranoia level was raised considerably and I now NEVER fill the tank to full. I max it out at 16 gallons which is two gallons shy of full. That Volvo fuel gauge is VERY accurate, BTW, so this is easier to do than you might think.

I have also heard tales of the rollover check valve in the gas tank sticking and causing similar problems. That particular problem requires the tank to be dropped to repair.

The EVAP test at your dealer is probably your best bet to diagnose the problem. Good luck with yours!