P0455 check engine light code help

[AdamE36M]

I have a 96 328is with 198k miles that im trying to get smogged and i keep getting a P0455 code. I cleared it and for 2 days and about 70 miles the engine light didnt come back on. At 80 miles it turned on again, i cleared it and went to try and smog it and what do you know, it came back on during the test.

So far i have tested the vent and purge valve with a power probe and both are opening and closing as they should. I have also smoked the system and there appeared to be no leaks in the system. I also did the gas cap test on the smog machines connector and it passed as well.

Im at a loss for what else it could be. Even tho both the vent and purge valve tested fine and opened and closed, could there still be an issue with them? Im not sure where else i should be looking in the system for the problem.

[strad]

Got this code on my 97 328 when I owned it. I suggest you pull the expansion tank in the left rear wheel well down far enough that you can inspect the fittings on top of it. One was cracked on mine. I had to replace the expansion tank to correct. Tank was 100 or so as I recall. And the code went away.

edit: to give credit where credit is due, I looked at that tank first, because research here and other sites directed me to look at those fittings. There was an actual hole in the plastic on mine -- it was quite visible.

[AdamE36M]

Ok, I pulled out the expansion tank and it doesn't look like there was any cracks on the fittings although there was a decent amount of dirt covering them. I will double check tomorrow to make sure but any other suggestions? What about the sensor on the side of the expansion tank?

[AdamE36M]

Could it possibly be the pressure sensor on that expansion tank that is not working correctly?

[i328]

I have the same code and a hell of a time trying to figure it out as well. Supposedly if you gently jiggle the fuel expansion tank is should sound like some marbles are knocking around inside. That's how it's supposed to sound, which means that the pressure sensor is not stuck and working fine.

[AdamE36M]

Update: I got the car to pass smog. I took the gas cap, vent valve, purge valve, and the fuel tank pressure sensor off of an m3 my coworker has and knew that they were all in good working order. I believe it was the fuel tank pressure sensor that fixed the issue because my other valves were functioning fine when I tested them with a power probe as well as my gas cap.

[Eric93se]

Do you have the newer style gas cap or the older one with the spring loaded tabs? I ask because I had an evap leak over a year ago and a new gas cap solved the problem, or so I thought. It lasted a while and came back again (both are the same brand 'blas' or whatever), turned out the new design sucks balls. The rubber seal on the new one was good but after tightening the cap there would actually be a small amount of play in the cap (I could wiggle it). That alone will make a huge vacuum leak, I could actually smell gas when opening the gas door and taking a whiff (very very subtle though). So what I did was use the new rubber gasket on the old gas cap and that pretty much solved it. The old rubber gasket was very hard.