So recently I have been getting a gas cap loose warning. Its not I have checked. i am now getting a P0455 Evaporative Emission (EVAP) System Large Leak Detected -PCM. Anybody know of any common issues that cause this? What to look or listen for?

 

evap purge selenoid is first guess

not sure how common on lsj. very common on LNF

 

Check your evap lines, your lucky you have a large leak the small ones suck to find. Or it could be your gas cap is faulty, if you can get your hands on a gas cap checker to make sure its working properly that would be great so you can rule the cap off the list

 

K thx will look into it tomorrow.

 

Vent valves are the #1 cause for evap leaks in EVERY GM i've ever worked on. They use the same valve in 90% of their vehicles. The solenoid stops working and the valve gets stuck open causing the large evap leak.

In my experience large evap leaks are either vent valves or gas caps every time. Unless there is a evap hose that is disconnected or cut in half.

Purge solenoid failures mostly throw small evap leak codes

 

Where is the vent valve located. And how do I tell if the gas cap is screwed/

 

Looking at the prices for the tester i would just buy a new cap lol

 

from what we have seen on the LNF its almost always the purge selenoid on the manifold. i am on my 4th one

 

Op does not have an lnf. And im basing this off the vehicles that come through my shop. Haven't had a single CSS come through except for mine. But they all have vent valves and they use the same one in pretty much all of them.

Vent valve is normal back by the gas tank above your fuel filter iirc from my wife's old SS/NA