Hi! I'm new top volvos and this forum. I have a 2005 S60 that surges under acceleration from like 3500-4500 rpm (warm or cold) and doesn't idle when it's cold. I just bought it and it had 213,000 miles on it. I've replaced the front o2 sensor (threw a code for that) and the alternator is new.
Now it's not throwing any codes and I'm not sure what to do next....Any ideas?

 

my list of possible suspects - ECT (temp sensor), plugs/coils, fuel pressure sensor, ETM (throttle body). I'd also look for intake and exhaust air leaks which can mess with the O2 sensing. I also recall a post where the OP suggested doing r&r on the PCV system cured his surging at speed. You can measure resistance of the ECT sensor from its connector, pull the plugs to check color/oiling, measure fuel pressure at the rail (assuming your year still has the shraeder valve on the end of the rail), read up on ETM symptoms for worn contacts, spray around for air leaks - vaccuum lines, tubing, feel/listen for exhaust leaks (particularly by the flex pipe and the EGR tube).