About two months ago my 1996 GMC C1500 5.7l would not immediately start. I had to give it a little gas to get it started. Once started, the truck would stall as soon as I shifted into gear. I replaced the Idle Air Control Valve, this seemed to fix the problem. Sunday before last I noticed an extremely high idle in park, approximately 2100 RPM. The idle in gear was about 1100 RPM. When I ran a scan, the P0102, Mass Air Flow Sensor code was returned. I replaced the sensor and all seemed fine. I did not drive the truck again until Wednesday. When I started driving Wednesday there was a bit of a rough idle and the Service Engine Soon Light illuminated. I ran another scan and this time P0175, system too rich bank 2, was returned. Feeling that it was too coincidental that I would keep having these kinds of issues, I reinstalled the MAF sensor, reset the computer this past Saturday. The vehicle idled fine, no Service Engine Soon, and no codes. I did not drive the truck too far Saturday. Sunday I drove the truck between 75 to 100 miles with no problems. I then drove the truck another 15 miles to and from dinner Sunday evening, no problems. Monday when I started the truck the high idle issue has returned. A scan returned no codes. Does anyone have any ideas?