Have you checked the "idle motor"? I had a similar porblem on both of my 240's, and one of my 740's. ( all non-turbo). when at idle, smack the idle motor with the butt end of a long screw driver. After a few smacks, if the idle drops or fluctuates, it is the cause of the high idle and usually the high emissions readings. It sells for about 165-185 at the dealer and even parts stores get 125 or so. I've learnd that you can take 'em off clean them with PB Blaster or such and reinstall. When first removed rapidly rotate the motor and listen for the rotation of the internal parts. If its dirty it will be slow to respond. When sprayed with cleaner repeatedly, it should loosed up and it will be easy to hear the clicking as you rotate/shake it in your hand. this may not always work, but it is worth a try and a few minutes of yor time. In each case my idles returned to factory specs of 750rpm. By the way, its usually located just forward and below the air intake bellows (driver side) and one large rubber hose connects to the flexible air intake from the air cleaner and the other goes to the intake manifold. Just disconnect the electrical connection and a couple of nuts and disconnect the hoses at the the intake and manifold. Just reinstall the same way so as not to reverse the air flow! Good luck. Jim