Gas leak only after fill up - bad gas cap?

[penz08]

Hey guys,

I recently noticed that a few times, there was a small puddle of gas right at the rear wheel driver side (same as the gas cap). I usually filled it up to the very top, but since then I am careful and stop after two clicks. Maybe I forced too much gas in there before. It seems its leaking from the gas cap, I can see some dry gasoline on the paint from the gas door. As soon as the tank goes down a quarter, no leak, nothing. I suspect de gas cap, but could it be something else? I drive a 2018 Mazda 3 GS.

Thanks!

 

[eljefino]

Probably your filler neck, they are metal and tend to fail. Got any check engine light codes?

 

[FowVay]

It could be the fuel pump gasket, the filler pipe or even the evap canister if you are "topping off" with each fill-up.

You should get underneath and find where the issue is post-haste and park outside away from anything of value until you do.

I had a similar issue with my BMW after fill-up. I'd come out to the garage and could smell gasoline. It was such a small leak that it dried before I could identify it. It turned out to be the fuel pump. There's a big gasket on it that seals it to the tank and when I'd fill the tank it would seep ever so slightly.

 

[Trav]

Hey guys,

I recently noticed that a few times, there was a small puddle of gas right at the rear wheel driver side (same as the gas cap). I usually filled it up to the very top, but since then I am careful and stop after two clicks. Maybe I forced too much gas in there before. It seems its leaking from the gas cap, I can see some dry gasoline on the paint from the gas door. As soon as the tank goes down a quarter, no leak, nothing. I suspect de gas cap, but could it be something else? I drive a 2018 Mazda 3 GS.

Thanks!

Any CEL or codes for EVAP?

 

[Astro14]

I suspect the filler pipe, but a lifetime of overfilling like this has probably damaged the charcoal canister as well. It’s designed for vapor, not liquid fuel, which is what you get in there when the tank is filled past pump shut off and the filler neck has fuel in it - the expanding liquid fuel has nowhere to go but into the canister, destroying it.

Put a new cap on and see what you get. You might get lucky with just a cap.

 

[penz08]

Any CEL or codes for EVAP?

nop, no code, nothing, I do suspect filler neck, when its half tank or 3/4 tank full, no leak.

 

[penz08]

I suspect the filler pipe, but a lifetime of overfilling like this has probably damaged the charcoal canister as well. It’s designed for vapor, not liquid fuel, which is what you get in there when the tank is filled past pump shut off and the filler neck has fuel in it - the expanding liquid fuel has nowhere to go but into the canister, destroying it.

Put a new cap on and see what you get. You might get lucky with just a cap.

I suspect filler neck too, no code, nothing at all, only leaks when full.

 

[Trav]

nop, no code, nothing, I do suspect filler neck, when its half tank or 3/4 tank full, no leak.

It could be a bad cap but I doubt the filler neck or a bad fitting on the tank. You would get a CEL for a very small, small or large leak code.
If I had to guess (which we do, the car is not here to look at) I would suspect a bad possibly plugged tank vent valve, it is located in a small line which runs up the filler neck near the rear wheel. Fuel heats from returning fuel when the engine runs expanding fuel in the tank, the vent valve equalizes the pressure if plugged it will force fuel out somewhere probably the cap which does have +2/-0.5 psi pressure release. The valve is #17 you may have to remove the wheel well liner to get to it, not a big job. If the cap does not do the job look at this.

https://parts.walser-mazda.com/a/Mazda__3/52401772__6407875/FUEL-SYSTEM-COMPONENTS/5419585.html

 

[penz08]

It could be a bad cap but I doubt the filler neck or a bad fitting on the tank. You would get a CEL for a very small, small or large leak code.
If I had to guess (which we do, the car is not here to look at) I would suspect a bad possibly plugged tank vent valve, it is located in a small line which runs up the filler neck near the rear wheel. Fuel heats from returning fuel when the engine runs expanding fuel in the tank, the vent valve equalizes the pressure if plugged it will force fuel out somewhere probably the cap which does have +2/-0.5 psi pressure release. The valve is #17 you may have to remove the wheel well liner to get to it, not a big job. If the cap does not do the job look at this.

https://parts.walser-mazda.com/a/Mazda__3/52401772__6407875/FUEL-SYSTEM-COMPONENTS/5419585.html

View attachment 252838

I looked under the car last night, I could see it dripping, seems to come just before the rear wheel, driver side, looks like close to the fuel pump, but I really doubt the fuel pump seal is leaking, I bought the car new and its just over 5 years old. Removing the fender liner is a good idea, inspect for leaks there too.

 

[penz08]

I also forgot to mention, I got the car undercoated with Krown, is it a possbility that it clog the vent?

 

[Trav]

I would guess anything like that could clog small openings.

 

[penz08]

UPDATE *******

We took off the back seat and unscrew the fuel pump cover, the ring holding the pump was so rusted out, the car is only five years old. Anyway, only the dealership has the tool to remove the ring and have access to the pump? any truth in this?

 

[someonesomewhere]

UPDATE *******

We took off the back seat and unscrew the fuel pump cover, the ring holding the pump was so rusted out, the car is only five years old. Anyway, only the dealership has the tool to remove the ring and have access to the pump? any truth in this?

Flathead and a hammer is what I've always used.

 

[penz08]

I actually went today, the parts were in, rubber ring and metal cover, plus fuel pump sending unit. it was 130$ total for parts, 180$ for labour lol, Oh well! its fine now, no leaks or gas smell.

 

[SC Maintenance]

Anyway, only the dealership has the tool to remove the ring and have access to the pump?

What kind of car? I have this one. Most use a hammer. Hammering on a gas tank always made me a little uneasy.

 

[eljefino]

You can use a brass punch if you're worried about sparks. If a tank isn't dripping wet up on top I'm not worried. You can also mist it down with water before working.

 

[Nukeman7]

You can use a brass punch if you're worried about sparks. If a tank isn't dripping wet up on top I'm not worried. You can also mist it down with water before working.

I always wonder how many people actually own a brass punch when it is mentioned in the service manual? I own several hundred tools, but not a single brass punch. I just improvise and use a wooden paint stirring stick when striking the locking ring on an in-tank fuel pump.

 

[penz08]

What kind of car? I have this one. Most use a hammer. Hammering on a gas tank always made me a little uneasy.

View attachment 257713

its a 2018 mazda 3 GS auto with the 2.0L

 

[penz08]

You can use a brass punch if you're worried about sparks. If a tank isn't dripping wet up on top I'm not worried. You can also mist it down with water before working.

its was very wet yes, puddle under the car after a fill up.

 

[taintedtattoo]

Hey guys,

I recently noticed that a few times, there was a small puddle of gas right at the rear wheel driver side (same as the gas cap). I usually filled it up to the very top, but since then I am careful and stop after two clicks. Maybe I forced too much gas in there before. It seems its leaking from the gas cap, I can see some dry gasoline on the paint from the gas door. As soon as the tank goes down a quarter, no leak, nothing. I suspect de gas cap, but could it be something else? I drive a 2018 Mazda 3 GS.

Thanks!

NEVER EVER fill it to the top, there are expensive emission goodies that will fail, stop at the first auto off click.