2003 GMC Envoy - Replaced gas cap without results. Where do I start looking for a vacuum leak? Check engine light is on and the meter says that it's a vacuum leak. I replaced the gas cap but that didn't correct the problem. I looked online trying to find a schematic for the vacuum system and haven't yet found it. What would be your suggestion?
Finding a leak in this system is almost impossible without a smoke machine. You might be able to find one at an auto parts store that borrows or rents tools. It injects a white non-toxic smoke at 2 psi, then you search for where it's seeping out. The alternative is to hope to find something real obvious like a rusted-off hose clamp or dry-rotted hose. Other possibilities include a leaking purge valve or charcoal canister. There's nothing visual you can see with those types of leaks.