2003 chevy 1500 has a code that says large evap leak. Where do I find this leak?

"Where" to find the leak is in your evaporative emmissions system. Most of the time, a good visual inspection will turn up the culprit. First, check the fuel cap. Look at the rubber O-Ring that seals it against the fuel fill tube. If it is cracked or looks like it is crushed down, replace your fuel cap. If that all looks OK, then you can check the rest of the system for obvious cracks, or leaks. Start with the Evap Purge line. You can identify this in the engine compartment as a plastic vacuum line about 5/16" in diameter and it has a service port on it with a green cap. Check the plastic line for cracks. Then check the rubber components that connect to this line on both ends. One end will go into the Purge Solenoid and the other will go into a steel line that runs along the frame rail under the vehicle. Follow this steel line to where it goes and inspect all connecting hoses, tanks, etc. there will also be rubber hoses at the fuel tank area and some that connect to the fuel fill tube. Check all of these for any visual signs of damage or leaks. Also check for badly corroded metal parts - especially near the fuel fill tube. Sometimes the metal tubes get rusted through and cause leaks.

If a thorough visual inpection does not lead you to a suspect, then the system will have to be "smoke-tested". This is where the system is filled with smoke under a slight amount of pressure so the leak can be seen where the smoke comes out.

Hard to do at home ,but if you take the green cap off the evap test port ,and blow cigarette smoke into it ,an assistant can sometimes see the leak. Could be fuel cap ,or a line along the way,takes alot of smoke ,find a smoker !