93' 850 226k miles. Car battery cable became disconnected the one day and then all of a sudden my car was idling at 1500 and surging between 1500 and 2000 without the foot on the acceleration. In drive it will surge when you are gearing down or sitting at a light. Had everything cleaned with 2+2 and took the car into my brothers volvo. Put it on the OBD and got codes for a RPM sensor. Pulled the RPM sensor off the other car and put it in and still the same problems. So we are pretty much perplexed?

 

Try disconnecting the battery for about a half hour, then take the ECU module out (one of the two silver modules under the square black box cover on the front left side of the engine bay). Clean the contacts with alcohol and a small brush, and reinstall. The loose connection could have scrambled the ECU (hopefully didn't fry it)