EVAP System Mystery

[shima]

Hi guys,

I think something may be going on with my fuel system. In particular, I think it may be an issue with my EVAP system.

Here is what happened this morning...

• Fuel gauge was at 1/4 tank full.
• I went to the station and pumped gas until the hose shut off. The pump showed that I pumped 10.X gallons (can't remember the exact number).
• I thought that was a bit low, so I started pumping gas again. The hose shut off again at 11.4 gallons.
• I got back into my truck and started it up. The fuel gauge went up to 3/4 tank full.
• I drove off and went to work. Shortly after pulling out of the station the fuel gauge settled just above 1/2 tank full. It stayed there for the whole drive.

Before pulling into the station this morning, I was thinking that maybe my fuel gauge was broken or malfunctioning. After doing the math, assuming I have a 31 gallon gas tank in my K5, I think the fuel gauge is actually showing the right amount. It's a little odd that it jumped up to 3/4 tank before settling down to just over 1/2, but that doesn't seem like a tremendous issue (correct me if I'm wrong!).

Instead, it seems that the gas pump is shutting off prematurely, which has to mean something is wrong with my EVAP system? Do these trucks have a common point of failure that you all have seen a million times already?

 

[frankin5]

Would have nothing to do with Evap. There is a vent hose off sending unit to filler neck that allows air to escape the tank while filling. Some fuel hoses are very sensitive to gas pressure caused by filling. Could have been the angle you were added fuel that caused the problem.

 

[shima]

Thanks for the reply frankin5, I'll play around with it and see if I get some different results.

 

[Fordum]

Next time you got to fuel up. look at the nozzle. On the bottom side, close to the end, you will see a small hole. As you dispense fuel, air gets sucked in that hole.
As soon as its blocked by fuel splashing back, the nozzle trips. If your vent tube from the tank has gotten crimped or stopped up, the air in the tank will bubble back up the main filler tube, and when even a small amount hits that hole, the nozzle trips.
Also, if you have been having lots of really hot days, the fuel in the ground will tend to foam, causing the same thing.
Sometimes the hole will suck up a small amount of fuel and it will form a little blockage. If a nozzle keeps tripping, I will pull it out and, without putting my mouth on it, blow into that hole. Sometimes that fixes it.
I used to drive a '71 F100 with the tank behind the seat. The only way to fill that truck other than just a trickle, was to put the nozzle in upside down.

You would not think the simple act of fueling up a truck would be so involved would you?

 

[dyeager535]

Would have nothing to do with Evap. There is a vent hose off sending unit to filler neck that allows air to escape the tank while filling. Some fuel hoses are very sensitive to gas pressure caused by filling. Could have been the angle you were added fuel that caused the problem.

On all trucks with EVAP that I know of, the vent hose goes to the EVAP canister. Unlikely, but if the hose was pinched or somehow blocked (or the in-tank valve has failed) then the tank can only vent through the cap, which can cause problems with filling, especially with any of the vapor recovery nozzles that require a tight fit. The EVAP canister itself shouldn't normally be able to stop the tank from venting properly, although I'm sure somehow it's possible.

 

[shima]

On all trucks with EVAP that I know of, the vent hose goes to the EVAP canister. Unlikely, but if the hose was pinched or somehow blocked (or the in-tank valve has failed) then the tank can only vent through the cap, which can cause problems with filling, especially with any of the vapor recovery nozzles that require a tight fit. The EVAP canister itself shouldn't normally be able to stop the tank from venting properly, although I'm sure somehow it's possible.

Yeah, I jumped to thinking it was the EVAP system because of a similar problem on my girlfriend's Focus. She couldn't put gas into her tank without the pump shutting off. I saw a video that implicated a part called the "vapor canister vent valve / solenoid", and they said it is part of the EVAP.

I went to hang with a buddy last night. When driving home I noticed that my fuel gauge was showing 3/4 tank full. Makes no sense, to me!

 

[soonertodd]

If the canister or vent hose gets plugged it will do that,next time before you fuel up remove the vent hose from the tank and see if it will fill up properly

 

[dyeager535]

As your problem is all over the place, so too will be my story.

I've had some issues where I just couldn't fill my truck up. I even posted about it. Nozzle kept shutting off all the time, and it was obvious it was trying to vent through the fill neck vent line. Fought with that for a long time...ran a true vent (for some reason, the idiot that put the truck together put a bolt in the vent hose, not sure who that guy was...) but that didn't solve it. Fuel gauge kept reading oddly, like maybe the sending unit was flawed... pump gas, would be over full, then shortly be at 3/4, then 1/4, seemingly whatever it wanted to do. Disconnected sending unit wire with key on, gauge responded properly. Grounded the sender wire, gauge responded properly. Huh.

Decided to take a spare gauge with me on my trip the other day, and swapped it out halfway through the trip. It read fairly full. I got to a station that had the "old" style nozzle with no vapor recovery at all. Set it on trickle, and low and behold, 3 gallons later, fuel level was up to the gas cap in the fill neck. Drove it about 165 miles, fuel level was about 2/3 on the gauge, and it took 10 gallons to fill it back up to the fill neck. The next time I drove 200 miles, and put in 12 gallons. So the ~16.7MPG was repeated, and the gauge now reflects fuel consumption as I'd expect. The wonky gauge wasn't working right, but I also didn't want to trust what I was seeing, that the tank was actually full.

Yours obviously wasn't, if you could put 10 more gallons on, but sometimes those nozzles will click off seemingly randomly. When full, it will very quickly shut the nozzle off.

 

[shima]

As your problem is all over the place, so too will be my story.

I've had some issues where I just couldn't fill my truck up. I even posted about it. Nozzle kept shutting off all the time, and it was obvious it was trying to vent through the fill neck vent line. Fought with that for a long time...ran a true vent (for some reason, the idiot that put the truck together put a bolt in the vent hose, not sure who that guy was...) but that didn't solve it. Fuel gauge kept reading oddly, like maybe the sending unit was flawed... pump gas, would be over full, then shortly be at 3/4, then 1/4, seemingly whatever it wanted to do. Disconnected sending unit wire with key on, gauge responded properly. Grounded the sender wire, gauge responded properly. Huh.

Decided to take a spare gauge with me on my trip the other day, and swapped it out halfway through the trip. It read fairly full. I got to a station that had the "old" style nozzle with no vapor recovery at all. Set it on trickle, and low and behold, 3 gallons later, fuel level was up to the gas cap in the fill neck. Drove it about 165 miles, fuel level was about 2/3 on the gauge, and it took 10 gallons to fill it back up to the fill neck. The next time I drove 200 miles, and put in 12 gallons. So the ~16.7MPG was repeated, and the gauge now reflects fuel consumption as I'd expect. The wonky gauge wasn't working right, but I also didn't want to trust what I was seeing, that the tank was actually full.

Yours obviously wasn't, if you could put 10 more gallons on, but sometimes those nozzles will click off seemingly randomly. When full, it will very quickly shut the nozzle off.

Man, it surely is bizarre. We have an air compressor in my garage, so I disconnected the vent hose and blew some air through it last night. My girlfriend helped me out by putting her hand over the fill tube, she confirmed that she felt air coming out. So hopefully that helps me out. If it doesn't, I guess I'll take a closer look at the canister.

The fuel gauge is another story. I wouldn't be so upset about it if I could get my trip odometer to roll back to 0, but as you already know it is stuck. So I've got to watch my general odometer.

 

[dyeager535]

Don't forget that the vent line is a two way valve. Pressure in, pressure out. To test it properly you need to put air into the tank and see if the vent line is venting.

I hadn't considered this until I saw a Spectre sending unit ad that mentioned resistance to ethanol, but the "rollover valve" that is on the tank side of the sending unit on the evap line, is partially plastic, probably rubber inside. It would not be unreasonable to expect an older sending unit to have problems there, since everything else rubber in the fuel system the ethanol in the modern gas destroys.