2000 sierra high idle HELP!

2000 GMC sierra, 186,000 miles. Idles at 3000 rpm in park. Drops to 1500 when in gear. Replaced idle valve, pc valve and entire throttle body. Engine still idles at 2800, rpms dropped about 200 when the throttle body was changed out. Also all hoses were changed and there is no check engine light on. Need help. My mechanic has given up.

Have you tried changing the throttle position sensor,I have had the same problem and that was the fix.I have had same symptoms from idle air control valve also,but sounds like you already changed that. you can look on autozone web site on procedure to check throttle position sensor with a volt meter.hope this helps.