Fuel Barfing Gas at the Track

[Tango_Yankee]

So, I took the car to the track for the first time a couple of weekends ago. I quickly learned that, in hard left-handers, the gas cap wasn't up to snuff for holding back the gas slosh. It even managed to do it on a mostly empty tank. I checked afterwards, and my filler neck grommets seem to be in good condition. The gas only seemed to be leaking from inside the fuel door not from the bottom of the filler neck, as you can clearly see in the picture. When screwing the cap on, it feels tight and there isn't much play to it. After pitting multiple times and quadruple checking it, I know the cap was on as tight as it could be.

The most peculiar part, though, is that pressure would manage to build up inside the tank and I had to vent it through the gas cap. I'm aware that the gas tank vent needs to be addressed, but this means that the gas cap is sealing tight enough to keep the fumes in. I'm genuinely stumped as to where so much gas is coming from.

I'm posting this to ask what I can do about it. I looked around online and didn't get any clear answers. I feel like just getting another gas cap online would just result in the same problem unless my cap is missing some sort of seal that I'm unaware of. Any info would be greatly appreciated.

 

[General karthief]

The vent (or lack of) is likely what is causing the issue.
Does it still have the vapor canister?
and what is that hanging down on the right side of the pic under the car?
if that is a open header pipe then expect to be a fire ball upon exiting a turn if this issue is not corrected

 

[Tango_Yankee]

The vent (or lack of) is likely what is causing the issue.
Does it still have the vapor canister?
and what is that hanging down on the right side of the pic under the car?
if that is a open header pipe then expect to be a fire ball upon exiting a turn if this issue is not corrected

I'm very well aware of the danger, which is why I'm trying to correct this before the next meet. Though, I would imagine the open header would have been more benificial because then my tailpipe and exhaust near the spill itself was not hot.

It does still have the cannister, but I fail to see how a lack of pressure release would lead to such a massive spillage of gas. I'm going to try and look over everything properly today. Why do you think that could be? I can't say I have had a lot of experience in messing with the fuel system on these cars.

 

[Noobz347]

...because you're sloshing gas and creating a pressure excess. When the gas hits the top of the tube, it's forced out instead of falling back down the tube.

Any other time, as you circulate fuel from the tank, to the engine heated fuel rails then back again, only the vapor is pressed past the seal on the gas cap.

Def worth looking into.

 

[Hoytster]

As stated above, you need to check the vent valve on the tank and eliminate the pressure build up. This is a problem a lot of fox/sn95 guys deal with on the track. Most resolve it with a new valve and/or gas cap. Some run the vent line out the back of the car away from the exhaust so if it spills out the vent it doesnt catch fire. For the extreme cars, guys will add flapper valves or baffle their tank to reduce fuel slosh.

The general consensus is that it's the heat absorbed by the fuel from pumping as well as from the exhaust that causes the fuel to vaporize and build pressure in the system if the vent is inadequate. I always had spilling issues in my 95 when I would run the fuel low and allow it to get hot. The couple people I knew that went to side exhaust never had an issue with pressure build up after the change.

 

[General karthief]

What does a side exhaust have to do with venting fuel?

 

[JD1964]

....except for the gas coming out, nice picture! Nice car

 

[Tango_Yankee]

...because you're sloshing gas and creating a pressure excess. When the gas hits the top of the tube, it's forced out instead of falling back down the tube.

Any other time, as you circulate fuel from the tank, to the engine heated fuel rails then back again, only the vapor is pressed past the seal on the gas cap.

Def worth looking into.

Ah, that definitely makes more sense. It didn't even occur to me that the extra vapor would cause enough of a high-pressure build-up behind the gas to push it out like that. It also makes sense because it would take a couple laps before it started doing that. Thanks!

As stated above, you need to check the vent valve on the tank and eliminate the pressure build up. This is a problem a lot of fox/sn95 guys deal with on the track. Most resolve it with a new valve and/or gas cap. Some run the vent line out the back of the car away from the exhaust so if it spills out the vent it doesnt catch fire. For the extreme cars, guys will add flapper valves or baffle their tank to reduce fuel slosh.

The general consensus is that it's the heat absorbed by the fuel from pumping as well as from the exhaust that causes the fuel to vaporize and build pressure in the system if the vent is inadequate. I always had spilling issues in my 95 when I would run the fuel low and allow it to get hot. The couple people I knew that went to side exhaust never had an issue with pressure build up after the change.

I cannot say that I've ever checked the valve to see if it's in working condition, but that's the first thing that I'm going to do. Do I have to drop the tank or can I cut an access panel in the hatch floor? And running the vent line out the back is certainly not a bad idea.
As you can see in the picture, I was running a dump pipe out the underside of the car, which I think bought me some time for heating up the tank, but as I mentioned before, after a few laps, it was a problem again.

What does a side exhaust have to do with venting fuel?

The side exhaust prevents the gas tank from getting as hot as it normally would with the rear exhaust running right next to it.

 

[Tango_Yankee]

....except for the gas coming out, nice picture! Nice car

Thanks! She's getting there. I have a lot of other nice pictures without the gas sloshing out

 

[2Blue2]

....except for the gas coming out, nice picture! Nice car

It is a cool picture, fun to see.

 

[Hoytster]

I cannot say that I've ever checked the valve to see if it's in working condition, but that's the first thing that I'm going to do. Do I have to drop the tank or can I cut an access panel in the hatch floor? And running the vent line out the back is certainly not a bad idea.
As you can see in the picture, I was running a dump pipe out the underside of the car, which I think bought me some time for heating up the tank, but as I mentioned before, after a few laps, it was a problem again.

I did see you were dumping the exhaust out the side. Sorry, I was just trying to reinforce that once you get your venting issue fixed you hopefully won't have much trouble since you are dumping out the side already.

You can cut an access hole, but I would personally just drop the tank since it's not that bad of a job IMO. LMR has a nice step by step on what all is involved in case you've never done it.

How To Remove Mustang Gas Tank (83-97) - LMR.com

Watch this installation video on how to remove and install a new fuel/gas tank in your 83-97 Ford Mustang!

lmr.com

 

[2000xp8]

Yeah, I would not cut a hole in the floor to save 5 minutes worth of work.

2 strap bolts, 1 bolt on filler neck tube bracket, 3 bolts under the door (not sure if these need to come out, but I always take them out so things move easier).

If you run the car down to a couple gallons, it's really simple.

 

[Tango_Yankee]

Will do, thanks guys. I'll drop the tank when I get it in the air to do the tail seal on the transmission and work on that vent.