Have a 2000 Elantra (Best car I ever owned). A year ago check engine light came on and tested PO455 evap leak. Service dept tested and found no trouble but after light came on again decided to replace Purge valve and canister. After this light would come on once in a while and then go off. Last week light came on and stayed on and took it to dealers service and they did pressure test of evap system and fuel cap and everything is perfect. Am now driving around with check engine light on and service dept seems stumped. Any suggestions??

 

By "service department" I'm guessing you're taking it to a dealer. First, unless the car is under warranty, there isn't much reason to take it to dealer. They will generally be more expensive than an independent repair shop.

If they did do a pressure test correctly, it must be something they are missing. That code indicates a large leak somewhere in the evap system. Check for old, worn out or cracked emission control hoses, check to make sure the gas cap seats correctly, check the condition of the filler neck and make sure any emission hoses are in place. I've never seen an evap pressure check done, but I suspect that they have to remove one or more hoses to accomplish this, unless its done via the gas tank filler neck. Its possible they forgot to reconnect a hose.

 

At the dealer, we can force a self test to determine if the attempt we made actually reapired the fault.. Independent Joe does not have the software to force the evap test..

I can have scanner in hand, and manually close off air line when I hear the CCV click, and work along withthe self test to find the fault..

BE AWARE,, the yellow blow hose-gauge thing is not all that they claim it to be.. I got ran around with it with no leak detect, then run self test, fail... wasted a day & 1/2 looking for leak... problem -- Hyundai tool is pressure to system... self test is via vacuum... on a wild guess, I replaced fuel cap, PASSED the scan tool forced self test..

 

yes evap issues can be very tricky if intermittent and i have spent hours chasing my tail on them, but you live and learn... in my experience with hyundai, i would remove the charcoal canister assembly and remove the canister close valve and see if the charcoal bag has ruptured and forced coal into the CCV, causing it to stick and throw a P0455.
good luck

 

there are things dealers can do that other shops cant and having the ability to command the vent valve shut is kind of essential for evap. Also the purge valve wouldnt be the problem or it will throw a purge code i think P0465 or something like that because the computer can sense the difference in unwanted purge and evap pressure loss. Vent valves are pretty common since theyre electric controlled and they constantly open and close. Nissan even has a big issue with them. I'd try a vent valve or gas cap first. Gas caps are pretty common too. they'll leak internally or the seal will dry out and stick. sometimes greasing it can help.