My truck is having problems idling and at low RPM but once I get it up and go it is fine. I know this could point to a vac line being cracked or disconnected. How do you test to find out where the leak is?

 

Couple of methods, with engine running, pull and replace vacuum lines one at a time, whichever line makes NO change while in plugged is the bad one. Other method involves a can of carburetor cleaner, make sure it's NON-CLORINATED. I fondly know if you can even get the chlorinated stuff or not but make sure it's non. With engine running, spray connections of vacuum lines and anywhere you think is suspect. Idle should come up when you find the bad spot. Make sure to check the Pcv hose. It's a black line running from the back of the drivers side valve cover, around the back of the intake manifold, up the passenger side of the intake manifold and goes in. The 90* rubber elbows on the ends crack.

 

I meant while un plugged