Low Idle and Catch Tank Making Noise

[mtnbiker26]

Coming home from work last night I let off the gas for a corner and felt the car buck a little. I thought my EGR valve might be stuck open again. Made it home and it stalled in the driveway. Started it back up and idle was pretty low and I could hear a very faint banging sound from the firewall area that sounded very similar to jungle drums or those street performers who play the plastic 5 gallon pails on the sidewalk. I was listening to a EDM type station and actually thought it was a song fading in. I tracked it down to the cylindrical canister on the firewall towards the passenger side where the AC lines penetrate. The screwdriver stethoscope trick makes it sound like a flapping diaphragm or cavitation type noise. I can feel it pulse with my hand too. Section 1-16-2 of the service manual lists the part as "Catch Tank". I suck at vacuum related stuff so I'm hoping someone can diagnose this for me. Do I replace the catch tank or do I have another problem somewhere else in the vacuum system? Thanks in advance.

 

[mtnbiker26]

I pulled off the catch tank and found that it had quite bit of liquid gas in it. I filled up two days ago and gave the pump handle an extra squeeze after the first click (which I never do) so maybe I pushed some gas past the charcoal canister and all the way up to the catch tank. The vacuum system trying to pull air through an overfilled catch tank might be the cause of the weird noise. Did a short test drive and it seemed okay.

 

[pcb]

You may have gas in your air filter too.. Part #6.

Some guys that overfill their gas tank get it filled with gas then the handle at the pump keeps shutting off when they try to refuel.

 

[mtnbiker26]

Good timing. I'm pretty sure the catch tank is okay. I poured a ton of fuel out of it, visually inspected it, then plugged one port with my finger and blew into the other port and it seemed sealed. I was just googling around and discovered the function of the purge valve solenoid. The noise in the catch can would come and go about every 30 seconds so I think the purge valve is opening and drawing in air which was going through the gas in the catch can like air goes through the water in a bong. I'll see how it is tomorrow. Thanks.

 

[pcb]

You might have trouble next time you fill up...

If it takes forever to fill your tank, you know what I look for... Your air filter may be a gurgling bong too.

 

[mtnbiker26]

Thanks, I'll keep that in mind too.

 

[mtnbiker26]

I've filled it to the "FULL" line a few times and the problem hasn't re-occurred. I waited until it was on on "E" and then put in 10 gallons rather than let the pump decide when it was full. I read somewhere else online that the charcoal canister near the tank (part #4 in the above diagram) has internal partitions which can fail over time. If this is accurate then maybe an overfilled tank could push liquid fuel through the charcoal canister allowing it to be drawn up to the evap tank. It doesn't matter much now because I just signed the papers for a 2016 3i Grand Touring.