1986 Nissan 720 4X4 pickup Z24 I mileage: 250,000. The problem started with the idle speed fluctuating from about 1000 to 2000 rpm when the vehicle was sitting at an idle. This fluctuation was fairly rapid (about every 5-10 seconds). Then after a couple of hundred miles, the idle speed stayed at 2000-2400 rpm. Now when accelerating, the engine surges terribly and is getting bad fuel mileage. When the surge occurs a small amount of black smoke puffs out of the exhaust pipe. There is also some kind of relay under the hood, over the right fender beside the battery making a rapid irregular clicking noise. Could this be related? I think the problem is in the emission control system but I'm definitely in over my head. Can you help? Thanks a lot!