evap system

i have a 1999 suburban 40,000 mi.and 3 times or so ive had it come up with trouble code PO440 evap emission vaccum leak somewhere replaced gas cap with new from dealer. my question i know this is a very complex system but outide if inspecting the lines for breakage where should i start i also notice that on the charcoal canister there are two lines connected they clip on but one does not look secure and it conned=cts to the purge valve. im very sorry for this long post sometimes im long winded i appreciate any imput... thanks

A P0440 code is considered a large leak in the EVAP system. Most times it is caused by a loose gas cap.
There have to be certain perameters that are met for the PCM to run the EVAP tests.
Check all you lines and fittings and make sure they are secure.
Don't beat me up for my next comment.........
Your GM dealer has a tool that will pressurize the EVAP system and fill it with smoke to make the leak easier to find. You can always pay for the diagnosis of your problem and then fix it yourself.
Some of your EVAP problems are difficult to diagnos, but a large leak should be fairly easy to find.

thanks sounds like good advice

just wondering because ive been doing some extensive reading.Is it at all possiblethat i have a bad Fuel Tank Pressure Sensor.thanks in advance for any help

It is a possibility.....Have seen a FTP cause a P0440 once or twice ( 1 was leaking)
Before you go and drop the tank to replace it, eliminate the possibility of a leak first.

ok the only reason i thought that was because the check engine light has come on for po440 and gone off while driving later that week if i had a leak i would think the light could not go off on its own.

If the light went back off, that means the Evap test ran and passed....could it be possible that you left the gas cap loose and filled your truck up again sometime between when the light came on the first time then went off?

On some trouble codes, the light will turn off if the PCM runs the test and the system passes. There will still be a code stored in history along with some freezeframe and failure records to help with diagnosis.

no cap was tight and since then the light is back on for a week now.

i just was looking at vent valve for canister in a diagram and noticed it has a plug in to it. i have a plug hanging underneath my hood in engine compartment guess where it plugs into? canister vent valve... somebody unplugged it maybe when i had alarm put in thank you for help ill see how i do now

This would explain your leak....the PCM couldn't close the vent valve.

Thanks for all your time GMMerlin.Its REALLY appreciated.Oh and the check engine light went out already.Thanks again.

Glad I could help:bigthumb: