on my 2011 3/4 silverado hd, I have a yellow engine light on. The codes are 174 and 496. MAP and o2 sensor. The only sysmptoms I'm getting are when the fuel cap is removed ther is a vacuum whoosh. The the vehicle is hard to start. Sounds like the evap purge valve. Not sure but never had one cause a vacum. Ive seen pressure, so i guess it could go the other way as well. help
2011 Chevy Silverado HD 3/4 ton
If your EVAP purge valve is bad, it should have also thrown a code. If you would like to test it yourself follow the link below. A hard start is a symptom of a bad purge valve, especially after you fill up at the pumps. http://www.samarins.com/glossary/purge-valve.html
You may want to first repair your error codes before pulling your purge valve to test it. A bad MAP sensor will cause fouled plugs and a rich fuel mixture which could be the cause of your hard start issues. Fix the codes first, then check your purge valve.