Blocked tank vent; evap canister causing overflow at filler?

12bridn · Post by **12bridn** » Thu Jun 22, 2006 4:45 pm

Hey, new guy here.

I have a 2004 ZR5 which, as of two days ago, started taking gas from the pump in sips. The auto shutoff on the pump would activate after 30-50 cents worth of gas was pumped. I figured it was a venting problem, but I decided to search this forum to see if it was a common problem anyway. I found a few threads on overfloewing filler necks and vents, and thought that a blockage at the filler vent was the root of my problem. After removing the fender insert, I was also surprised to find no vent hose at the filler neck. A little more searching and I found the evap canister. It has a hose in and a hose out. I followed the "in" hose to a tee, and figured one of the hoses should be the vent, so I unplugged the hose at the canister. Problem solved. If you lie on your back and slide under the rear of your truck, look to the right and forward of the spare. That is the evap canister.

KTM 950 on/offroad motorcycles have a similar evap system, with a charcoal canister integrated into the vent and vacuum system. If the rider would drop the bike, the canister can flood with fuel, causing the ventilation to be blocked and causing rough running or no running at all. When I removed the hose from the canister on my truck, a small amount of gas dripped out. I suppose the same thing that happens on the KTM happened to the canister on my truck, causing a block in the ventilation, therefore allowing only the filler neck to fill before shutting off the pump.

Maybe the filler overflow problems some guys are having are caused by a similar situation, but there is only a partial blockage? I am going to remove the canister and inspect it, and maybe keep it off and just add a waterproof filter of some sort, or route the vent higher...

Post by **HenryJ** » Thu Jun 22, 2006 4:53 pm

Wow, that took a while for a first post

A good one though.

What have you done to solve the problem? You can just leave that pipe open since it will throw a code when it tests the system.

12bridn · Post by **12bridn** » Thu Jun 22, 2006 5:00 pm

Haven't investigated too far yet. My main goal was to be able to fill up the tank in less than a half hour.

BADs Crew · Post by **BADs Crew** » Fri Jun 23, 2006 3:01 pm

Welcome to the site. Great first post. I have never heard of this problem before. Is this an issue with GM products across the board or is it just the 04 Crew Cab?

Post by **HenryJ** » Fri Jun 23, 2006 3:24 pm

BADs Crew wrote:...Is this an issue with GM products across the board or is it just the 04 Crew Cab?

I am not sure exactly what years are involved. Some of the crewcabs have the smaller filler hose with no attached vent and plastic fuel tank. These seem to be the ones having "issues".

My 2001 has the larger filler hose with a parallel vent hose and steel tank.

Here is some more information:
Fuel Filler Neck Pipe Leak - Best Fix?

GM data wrote:Fuel Spits Back/Out When Filling Tank (Replace Fuel Filler Pipe Assembly) #03-06-04-058 - (Oct 2, 2003)
Fuel Spits Back/Out When Filling Tank (Replace Fuel Filler Pipe Assembly)

2003 Chevrolet S-10 Crew Cab Pickups

2003 GMC Sonoma Crew Cab Pickups

Built Prior to July, 2003
Condition

Some customers may comment that when filling the fuel tank, a small amount of fuel may spit back/out.
Correction

Replace the fuel tank filler pipe assembly with P/N 15124302. This new fuel tank filler pipe has an updated check valve as part of assembly which has been released to correct this condition. Refer to the Filler Tube Replacement procedure in the Engine Controls sub-section of the Service Manual (SI Document ID #692074).

12bridn · Post by **12bridn** » Fri Jun 30, 2006 2:58 pm

Well, it seems it didn't really fix the overflow issue. I have narroed it down to the narrow filler neck. There is a short period of time between when the auto shutoff is tripped and when the pump stops completely. During this time, a small volume of fuel is expelled from the hose. Due to the small filler neck, the same volume of fuel fills a longer length of the neck, causing the overflow. I think the only solution is to somehow fit a larger filler neck.