Positive Fuel Tank Pressure, gas coming out of lid (E320/M104)

[John5788]

Hello,

I have a strange problem that I haven't seen before. When I open my gas cap, gas spills out of the hole like it is overfilled.

I recently filled up the tank at a gas station then parked the car after driving maybe 5 miles, nothing out of the ordinary. The next day, I noticed heavy smell of gasoline in the passenger cabin, so I opened the gas cap and a decent amount of gasoline came rushing out of the hole.



I wasn't planning to drive that car any time soon so I just put the gas cap back on and continued working on some other stuff. The next day, I checked the gas cap again, and gasoline spilled out. This happened yet again for a third time today.

I have not driven the car or even started the engine since I parked the car on Friday. Today is now Wednesday, so 5 days has elapsed since I last drove this car.

What is causing this overpressure and gasoline to come spilling out of the gas cap when I remove it? Sure the evap vent valve is probably broken and not letting excess pressure out, but unless a chemical reaction is happening inside my gas tank, why is there so much pressure being built up while the car is sitting idle on the street?

A video for reference: Watch 26702 | Streamable

This video is from the 2nd time I opened the gas cap. I got the video recording on my phone started as soon as I felt pressure on the gas lid while removing it to check.

 

[gsxr]

Slight pressure in the tank is normal, but usually only air escapes, not liquid.

Does gas spill out ONLY when the tank is near completely full, i.e. driving 5 miles after a fillup? And, was the car level, or angled slightly towards the passenger side?

I'd drive it more than 5 miles and park on a level surface, and see if the issue persists. It also wouldn't hurt to replace the vent valve below the tank on the driver side, along with the rubber hoses.

 

[John5788]

Slight pressure in the tank is normal, but usually only air escapes, not liquid.

Does gas spill out ONLY when the tank is near completely full, i.e. driving 5 miles after a fillup? And, was the car level, or angled slightly towards the passenger side?

I'd drive it more than 5 miles and park on a level surface, and see if the issue persists. It also wouldn't hurt to replace the vent valve below the tank on the driver side, along with the rubber hoses.

This is the first time I've witnessed this happening in any of my W124s. I have 2 of them full tank parked on the street one in front of the other. Actually one is a S124 so the tank is configured a little differently. The wagon is not spewing out gas from the cap, but the E320 is.

The car is parked so the passenger side is a little bit lower.

So are you thinking, the gas pump must have slightly overfilled my tank and whatever offgassing is going on inside my tank is just causing it to spew out? Nothing out of the ordinary except my evap vent valve or charcoal canisters are not allowing the excess pressure to escape? I did refuel at a station I normally do not fill up at.

 

[Bogeyman]

Thermal expansion?

 

[gsxr]

The car is parked so the passenger side is a little bit lower.

With a full tank and parked with the passenger side lower... yep, you can get liquid coming out if you remove the cap.

So are you thinking, the gas pump must have slightly overfilled my tank and whatever offgassing is going on inside my tank is just causing it to spew out?

Correct.

Nothing out of the ordinary except my evap vent valve or charcoal canisters are not allowing the excess pressure to escape?

The pressure is normal, although there's not really any way to tell if you have "excessive" pressure. The usual problem is when the tanks have vacuum instead of pressure, which causes the tank to collapse slightly (sides cave in)... that's the opposite of what you are experiencing now.

I did refuel at a station I normally do not fill up at.

Probably unrelated, unless the new station nozzle kicked off "late" and filled higher than you're used to.

 

[nocfn]

When your light for the fuel goes on, put in 19 gallons. I always go with known quantity from the light on - and try for 19 gallons on 126 and 124

I would also change that vent GSXR spoke of as it is most likely exceeded OE life. Lastly, Charcoal canister less likely but again, not a bad idea to replace.