I bought a used 88 gmc sierra-appx 152,000 miles - 5.7 liter about 2 months ago. It has been well maintained. The previous owner just replace the fuel pump recently before he sold it.
Anyway, the truck has been running fine, but lately after about 15 minutes of driving it will stumble and run really rough (feels alot like a vacuum leak) and for a few seconds up to 1 minute and either dies (will start right back up. So far) or it will clear up on its own. Obviously its not a vacuum leak since it " clears up by itself" sometimes. I am not sure where to begin. Somebody mentioned a purge valve. Where is that located on my truck? I know where the canister is, but kind of confused as to where this purge valve is. BTW, the smog pump has been bypassed as the previous owner said that the bolt broke off in the block and he just took the pulley off and got a shorter belt to bypass it.
Also, when I first start it after it sits for a while/over-night, the engine turns for about 5 seconds, and finally starts and it shoots out white smoke and it clears up. The truck ALWAYS shoots out white smoke when I first start it, but it does not smoke at all once it is started and running.
Anyway, if anybody can offer some guidance as to where to begin with troubleshooting, I would appreciate it!