I'm sure someone has asked this already (everything has been asked/answered by now), but I can't find anything using the search feature. Probably a matter of me not thinking of the correct search terms.

I am having a lot of trouble filling my gas tanks. The auto shut off on the fuel dispensing nozzle just keeps on tripping. This happens at multiple pumps at multiple gas stations where my other cars work fine, so I'm pretty confident this is a problem with my car. Even pumping at the slowest rate causes the shut-off to trigger, and it does it continuously from both tanks near empty all the way until the tanks are actually full. Fuel does not appear to back up the filler hose/pipe.

The FSM has a list of possible causes for 'Poor Fuel Fill Quality' (as they call it), but I can't find an actual troubleshooting procedure. If I just knew exactly how everything in the evap system was routed I could probably come up with my own troubleshooting procedure, but I can only find a 'generic' evap schematic in the FSM (something that would apply to all cars, not the C5 specifically, page 6-503). If anyone knows where I can find a detailed, C5-specific fuel line and evap line/component schematic, that would help me a ton.

Possible causes as listed in the FSM include:
1. The check valve is stuck closed... I assume they mean the check valve in the filling pipe. I don't think this is my problem because fuel does not seem to back up and spit out as I fill. And I can dump fuel in from a gas can without any problem.
2. The fill limiter vent valve is stuck closed... could be my problem... does anyone know how to test for this?
3. The Evap canister is restricted... again, could be... does anyone know how to test for this?
4. The Evap canister vent solenoid is stuck closed... that was easy to check with the car on my lift, mine seems fine unless it only sticks when hot or something
5. Restricted EVAP pipes... again... anyone know how to test for this?

There are a couple of other things listed that seem unlikely to be the cause.

Any help/advice would be greatly appreciated... it would be nice to be able to fill my car faster than an electric car can recharge.

 

Do you use the nozzle's auto fill setting or do you hold the nozzle while squeezing the fill trigger? I had similar problems with some gas stations. I interpreted the issue with the Vette's opening angle for the nozzle to fit in as the problem. So now I just hold the nozzle while squeezing the fill trigger. No further problems.

 

I've tried inserting the nozzle to different depths, I've tried holding the nozzle at different angles (can only adjust a little once inserted), I've tried every possible flow rate (including the slowest the nozzle will allow, which is less than the first 'latch' for automatic filling)... the nozzles click off continuously no matter what I've tried so far.

However, I did recently read that inserting the nozzle completely upside down might override the auto-shut off feature... I have not tried that yet.

What I end up doing is squeezing the nozzle as gently as possible with the nozzle inserted as little as possible into the filler opening on the car, and this will usually dispense a few cups of fuel before clicking off, then I repeat that many, many times. It takes forever.

 

The check valve is at the top of the right tank, two lines come off it, one goes to the canister, the other goes to the fill tube. There is also a pressure switch at the valve that triggers a code. The canister has 3 lines, one going to the vent valve, one to the check valve, and one to the purge valve. You can access the lines from a rubber access panel in the passengers rear wheel well and test things with a vacuum pump.

The check valve is a float inside the tank, some have had them fail open, some have had them come apart, but I don't think I have heard of one stuck in the closed position. The only other option would be the vent valve failing closed.

 

Hi !!...Yes, there is no diagnostic tree for this I believe but you can check the vent valve (normally open) by removing the 2 wire connector....pink is power and the white wire is the control wire from the PCM...if you disconnect the harness you can give the valve 12 volts on the pink pin with a jumper wire and ground on the white...you should hear the valve click as you connect and then disconnect the 12 volt power to it...when 12 volts is applied you should not be able to blow through it...checking the harness side circuit integrity can get you in trouble if you don't know what you're doing and you can possibly fry the PCM...I'd just check for voltage on the pink wire at the harness and leave it at that...to check the control side you would have to turn on the PCM driver with a bi directional scan tool anyway to check the ground side. To check the cannister probably would be best to remove it (tough) and see if you can pass some air through it...I'd put my money on the vent valve...GM's have a pretty high failure rate on these !!

 

Thanks, but that's number 4 in my list which I mentioned was the only easy thing to check... mine seems fine. I removed it, blew through it, applied 12v to it a few times to cycle it closed/open. I don't think that's my problem, like I said, unless it only sticks closed when hot or something. It is above the exhaust pipe, so it probably gets pretty warm. But I'm thinking my problem is elsewhere.

 

Thanks, this helps a lot. But what should I expect when applying vacuum (or perhaps a little pressure) to each of the different hoses I can access through that access panel? Since I don't know exactly where the various check valves are located, I'm not sure what I should expect.

And most of what you said makes sense to me just from my understanding of how an evap system and fill valve works, but what is the purpose of the line going from the check valve to the filler tube? Is that line attached to the tank side or the canister side of the check valve?

 

What you expect will depend on the line...I would expect the line from the tank to the evap canister to have no vacuum at all if the tank check valve is open...which it should be. If you get vacuum on that line, the valve is bad and tank needs to be changed (valve is not serviceable). On the line going to the purge valve, its a long line so it will take a lot of vacuum, but you should be able to build vacuum on it. On the line going to the vent valve, if the valve is working, you should get no vacuum, if vacuum builds, the valve is bad or line is plugged.

The line going to the fill valve connects just past the restricter the pump goes into. Here are some pics to help:


Fuel System Diagram

Fill Tube w/vent

Good luck!!

 

Just FYI...the EVAP system uses VERY LOW vacuum and pressure signals when it performs its onboard tests !!...1 psi max testing pressure as you can see here and around 1/4 psi or so vacuum !!...you don't want any more issues than you all ready have...like a damaged fuel tank pressure sensor !!

 

I did more research on what makes these fuel nozzles shut off. There is a small hole near the end of the nozzle that sucks in a little air as you are fueling. When that gets covered by fuel it can't suck in air anymore and that is the long-standing and main trigger for nozzle shut-off.

But I know on my car I can pour fuel in from a gas-can just fine, and fast... fuel is flowing into the tank, so I don't see how fuel can be backing up and covering that sense port on the nozzle.

So I researched some more, and many fuel stations have gone to a nozzle that has another shut off mechanism. If the nozzle is determined to be too horizontal they will shut off. The idea being that if the nozzle falls out of the car during refueling it would lay flat on the ground and this would shut it off. I'm thinking that's my problem, because the cure for this is to insert the nozzle upside down (so it is no-where near horizontal), and when I did that today, BINGO... worked fine.

First I inserted the nozzle the normal way and after it shut off about 6 times while only getting in about 3/4 of a gallon of fuel, I flipped the nozzle upside down and proceeded to fill the rest of the tank with only one shut off.. the one that should happen once the tank was full.

I think the nozzles near my house just don't like the angle of the refueling spout on a C5.

 

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