Need some help! My 1990 GMC Sierra 7.4L used to run like a raped ape up until a few months ago. Now it struggles to get above 65 mph on the highway. It has 207,000 miles on it, so have replaced the following; timing gears and chain (18 months ago), distributor (a year ago), coil (a year ago), EGR valve (a year ago), fuel pump (a year ago), catalytic converter (18 months ago), throttle position sensor (9 months ago), Maniforld Absolute Pressure sensor (9 months ago), coolant temperature sensor (6 months ago), sparkplugs+plug wires (6 weeks ago - cap and rotor replaced 12 months ago), oxygen sensor (1 month ago). Starts and idles fine. Go wide open throttle and I hear a "whistling sound", shifts hard at 20 mph and again at 40 mph. Most days abut 65 mph is tops - occasionally will run over 70. No real power anywhere from 0 - 65. Any ideas of where to go from here?
