Glenn R

I've run into this a couple times. But last Friday there was a new development.

On some hot days, I'll pull in after a session and when I shut down, I can hear the gas in the tank boiling. Yes, boiling. It happened a couple times during the 2019 driving season. I didn't drive in 2020. But this year, I've had it happen again. Only on hot days and even with the heat shielding I added on the engine compartment fuel lines.

In addition to the insulated fuel lines, I tried to run a fuller tank on really hot days and that seemed to help. However, last Friday, I was at Pitt Race for a track day. The afternoon was one big session where we could enter or leave the track at anytime. It was hot but I wanted to run a long stint. I went out with better than 3/4 tank of gas. At the 45 minute point, the tires were obviously too hot and I came in to check tire pressures and take a look at the "tread" (Toyo RR).

When I shut down, I heard the boiling sound and it was quite vigorous. As I walked around the car, checking the tires, I ran into the new development. Fuel was leaking onto the ground behind the left front wheel. I assumed it was one of the connections between the stock lines that run to the tank and the SS lines I ran from about that point on the car to the engine. I called it a day and headed home.

I have now had a chance to inspect the situation and discovered the fuel did not come from the fuel line connections. It was coming from the carbon canister!

I'm running premium pump gas. And I know the boiling point of gas, especially with ethanol, is pretty low. I don't notice any performance issues. And it happened so rarely that I wasn't to worried. But now that it resulted in a fuel leak and the fact that I'd like to run some endurance races, I need to fix it.

Has anyone run into this problem and how can I resolve it?

aventari

What muffler are you running?

944Cup

Remove the carbon canister.

Glenn R

It is a stock muffler

I can do that. But how would that cause the gas to boil? I don't understand the relationship.

CL944135I

Do you have the heat shield above the muffler in place? Early or late car? If you have the late fuel tank/plastic, does it have all the heat shields on it?

Glenn R

It is an early car (85.1) and has all the heat shields in place.

GPA951s

So on a related issue that im working on. I will say that you need to install a fuel pressure gauge. I think what is happening is that your residual fuel pressure is dropping to quick when you shut off the car. More pressure= higher boiling point… i was working on my audi S4 and was having a weird issue were the coolant was boiling in the small hose at the back of the engine after i shut it off … low and behold i found a small leak.. not big… but just enough to hold “ some” pressure but not enough to raise the boiling point where the hose doesnt go.. snap snap snap for about 10 second after i shut it off… not sure about the carbon canister, but in any case, they are meant for vapor and not liquid..

Jeff951NJ

Are these problems happening while you are driving at Pitt Race buy chance? I only ask as 80+% of our race field has had a issue with there tank pressurizing only at this track. Our ATL fuel cell actually expanded so much hit our driveshaft causing the cell to rupture.