I have a 92 GMC with a LT1 in it. Whoever did the engine swap used the stock A/C so there is no HVAC. The vacum line on the side of the intake had a cap on it. Should I take the cap off and just let it suck air or should I leave it capped off. Or can I run it to the open hose on the bottom of the EVAP purge SWITCH.Also there is no line going from the purge canister to the air cleaner. Will that effect anything. The reason why Im asking these questions is because it will run good for about 10 minutes then start to run rough.

 

You want to leave the HVAC port plugged off. It would be a vacuum leak if open.

This pic might help a little for your question about the EVAP:
http://shbox.com/1/evap_sol.jpg

....unless you are really referring to the round EVAP switch that was used on 96-97s. The hose on the bottom of it is a vent and has no connection.
http://shbox.com/1/1996_evap.jpg

 

OK Ill plug it off . It had a rubber cap on it that had dry roughted. So Ill take that off and put a hex bolt in to seal it good and tight. Thanks for mentioning that they only used that EVAP switch on 96 and 97 models. Since i bought the truck Ive been trying to figure out what year the engine is. I knew it was a 95 or newer but thats it. So you just narrowed it down for me. If that rubber cap on the HVAC line was leaking would that cause the problem I mentioned in the origanal post. Tanks for the help I appreciate it.

 

A small vacuum leak will usually cause a high idle. It may not be the cause of your problem, but get it taken care of and re-evaluate.

 

OK thanks again for the advice.

 

Do it right - use a vacuum cap, since that's its intended purpose. Putting bolts and screws in is sloppy. A pack of vacuum caps will cost you a buck.