why can't it be back pressure? The air in the tank has to go somewhere.

My '12 also has this problem, when I filled it for the first time (it's new to me), I had to dribble the fuel in. I have to examine/test the evap system to check that it does vent through the canister when pressure builds up in the tank.

If yours suddenly stopped filling right, I would perhaps start with looking at the evap venting solenoid (probably not the right term), that vents air through the canister, then out to air.

 

Did you end up replacing the cannister or solenoid or both?

 

Most likely the solenoid valve off the charcoal canister has failed shut. Other option is a kinked vent hose or the charcoal canister is plugged up.

 

Well, it sounds like I have some troubleshooting to do. I appreciate the leads fellows.

YouTube to the rescue🤞 1st video showed cannister failure where the carbon pellets broke free and fouled the lines.

I might add that I have NO check engine light iluminated on the dash at present...could that be forthcoming as the venting problem continues to get worse?

I don't own a reader to decifer OBD2-Codes.

Need to investigate whether or not GM has a TSB covering this model year.

Hate to be at the mercy of the dealer, but I just don't want to be changing parts for the sake of changing parts.

 

Same thing happened to my truck. I always thought it was the gas station nozzle because it didn't happen all the time. No CEL until the day I took it for a smog inspection. It took a while to diagnose but ended up being a plugged charcoal canister. And don't try to flush it with water like the YT video.

 

Did you replace it or dealer?

 

No, I've got a '12 Silverado 3500 that I'm working on. I tested the vent solenoid, and it opens/closes on command and passes/blocks airflow today, and tested that the gas tank pressure sensor is registering, so tomorrow the truck should be mobile finally, and I'll see how filling it up goes.

 

Very well...so if I may...by what means did you give a command to cycle the solenoid? And the sensor? Thanks for your time.

 

You mentioned you hacksawed off the top of the canister...did I read that correctly.

There is a vent valve out there that eliminates the long hose that trails to the rear of the truck. It's just a stub section of hose (clamps) and housing w/foam filter...easy access for routine cleaning👍

 

My 2013 has this issue when it's really cold outside. I usually rotate the gas nozzle as far sideways as I can get it and still be in the tank. Start pumping and after about 30 seconds slowly rotate it down and in. It says running after that. No lights or codes ever come up been this was for years.

 

That's an interesting take on what seems to prevalent with these trucks. Damn over engineered emissions...that all started years ago with a smog pump and an EGR valve. Both wreaked havoc ever since.

 

Agreed

 

Well, I'm thinking I may just head over to Auto-Zone local in my little podunk town and have them hook their scanner to my OBD2 port and see what pops up. I've got to start somewhere.

 

There has been many others before me with the same problem and some have had good results with their version of the 'FIX'. Everything from changing out every component with zero results to breaking up the the carbon in the canister and reinstalling...etc. So, am I to assume there is no exact science to solving this weak link. Tell me it isn't so.

 

It isn't so.

If you know how the system works, have the right tools, actually diagnose where the problem is, you can fix it.

Random people who dont know how the system works, and just throw parts the problem and hope, is a "maybe" situation.

For example, I just filled up my truck, and I was able to get it to fill at full speed, vs the last time, where I had to dribble fuel in.

 

In all fairness you have a leg up on most of us especially with the likes of an ASE grade diagnostic tester.

 

Yes, that is the thing. If you put the time, money and effort into it, you can fix the problem.

I spent the money for the tools, as well as access to the full service manual, so I can fix most everything on my trucks. The tools aren't cheap, but they are cheaper than going to a mechanic to fix pretty much anything other than wiper blades.

 

It is the EVAP system. You have a valve stuck in the closed position. Nine times out of ten, it is the control valve and filter unit on the left side by the rear axle. Not that hard to replace. GM has several TSBs on the problem.

 

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Is this valve you're referring to...?

 

When my 04 started doing that, my check eng light also came. The codes said a problem with evap system. I found out that my fuel cap was no longer sealing properly. I bought a new cap at my local parts house. My problem was solved!

 

Glad it was a simple fix for you. Although my '17 doesn't use the traditional cap... it incorporates a spring loaded double flap if you will. Somehow GM thought this was a better idea. Apparently, its makes it almost impossible to syphon.

 

UPDATE - Crawled under the truck to get a better look at the Evap System components and their layout as it applies to the '17 model year.

Vapor Canister Vent Solenoid - GM (84148819)

Attempted to locate the inlet/breather checking for any obvious obstruction...well didn't get very far when it was discovered the vent line follows up along the gas fill hose and terminates on the upstream side of the filler neck housing. Between the inner bed wall and outer fender. In fact, even with an inspection mirror I couldn't put eyes on the breather housing. Why in the hell did GM locate what should have been accessible for routine maintenance.
I do understand its somewhat protected from road debris, etc. But, its literally out of reach. So, not knowing whether or not the foam filter is dirty/heavily restricted or what have you at the moment I can't answer that question.
How many of you '17 owners are aware of this and what was your solution short of dismantling the filler neck, etc.

 

For my '12 3500, it clips to the top of the top of the plastic piece that the filler neck mounts to. You can possibly reach up and press the sides of the vent housing together to get the vent off, or you may be able to push the clips holding it on, towards each other, then out of the housing, through the open fuel filler door. Be careful to not bust the clips, as there were only 2 of them for me, and with only 1, it won't stay on.

 

...and it was only 2 screws and a push pin, to detach the filler housing from the truck bed, which then makes it no problem to remove the vent.

 

Very good...I will attempt to get to it...via the fuel door. Btw, do you recall what kind of shape your foam filter was in?

 

It varied. I have 2 '12 3500's, one a crew cab, 8' dually, the other a crew cab short box srw. I got both which the engines and underside covered with thick, caked on dirt that had been built up over a long time, not just a single event.

The dually, which was overall in worse condiition than the other truck, as the bed sides were damaged and flopping around, the filter at the end pretty clean.
The srw, the filter was caked with hard packed dirt and was mostly plugged.

The p.o. had done something with both systems, as the vent hoses weren't clipped down like they were originally (even though the clips were still there) but rather were just ziptied to whatever was nearby.

 

Fair enough, really apprecaite your time. I'll see what transpires in this case.

 

My truck bed is off, so it was easy to take some pics to show you my setup:

But, IDK if your setup is the same as mine.

 

That's the culprit! Great pics!

So, am I to assume I'll be looking for a foam (serviceable) breather element inside of that housing?

 

IDK how serviceable it is, other than banging dirt out of it. It didn't readily come out the housing of my parts truck, but I didn't really go at it.

 

Well...I will get at it soon...its not my daily driver and is garage kept. I need to check that breather off the list of suspect faults and move forward.

 

If that's what you want to do, then I would suggest you do what I did, with a scanner that can show you the evap vent solenoid position (if it's open or closed). If the scanner shows it's open, unplug the vent hose from the canister, clean the end off, and try blowing throw it. If you can, it's not the problem when filling (other than if there's some reason the evap system orders the vent to be closed while you are filling, which it shouldn't).

Then, if you can blow through that tube, take out the evap canister (3 quick release fittings for hte evap lines, take your time and don't bust them, examine them, and then find youtube videos that show how each type releases), then clean off and try blowing through each of the ports, while plugging one of the other two, then switching which is plugged. You should be able to blow through each port, so air can exist either of the other ports.

If you can't blow through all the ports, then the charcoal inside is preventing that from happening. You can either replace the canister, try banging it around (but not hard enough to damage the housing or ports) to loose the charcoal, or there's a variety of youtube video's on "cleaning them" (which I haven't tried, and don't recommend), and try blowing through the ports again. Repeat until you can blow through all the ports, or give up and buy another one (new or used).

If you can blow through the canister ports, then remove the gas cap, clean off and try blowing through the evap line (that plugs into the canister) that goes to the gas tank. You should also be able to easily blow through that (do NOT inhale from that line, unless you want to get very sick).