When I crank my Chevy year 2000 2500 series pickup the rpm idles at about 600 rpm. When driving and I take my foot off the accelerator the rpm drops to about 1300 rpm and doesn't go below that until I apply brakes. So I coast along at about 35 - 40 mph at 1300 rpm.....Also, if I am driving and take my foot off the accelerator the 1300 rpm drops to 600 rpm as soon as I shift into neutral. Anyone have an idea what is causing the problem? Sure hurts my gas mileage: