in my recently purchased 2002 C70. 1700 RPM is the sweet spot, let off the gas and it lurches and bucks. Dealer squared away the battery cables and one vacuum line, but discovered that the engine appears to have been changed, and they surmise it's an interfacing issue with the ECU and engine (?).ETM appears fine. Possibly a factor, the tires are suspect and are adding a degree of "out of round" or something; I shift to neutral to separate the two effects and am convinced it's not totally the tires. Ideas?

 

Not sure on this. Maybe something with the transmission?
Wait and see what (Moderator) Tech says.

 

Throttle body clean?
Good tune up?

 

Yep, I was thinking it was ETM and had the Volvo dealer upgrade software, then clean the throttle body. It was then they discovered the liklihood of an engine changeout. Not sure about the tuneup and plugs, 98K miles on car, who knows on engine (it looks newer though). Sorry on double post, didn't look like anyone checked the C70 section, for obvious reasons