Bubbling Gas Tank?

I have a 2004 Mazda 3 2.3L hatchback. I just had the engine replaced because the Crank Shaft in my old one broke at about 84,000 miles. Weird, mechanic never saw that before.

Anyway, after 40 miles my check engine light came on. And I know sometimes it can be because the gas cap isn’t tight enough. So after driving home from work I unscrewed the gas cap and I heard a bubbling sound coming from the tank. Almost like the gas was boiling. I listened for 4-5 minutes, then heard a click and it stopped suddenly. Anyone know what this was? Don’t want to start my car until gas vapors dissipate, so not sure if my check engine light has turned off.



I think the fuel pump relay may be sticking. (It won’t shut off)

Fourty miles of driving on your new engine and your check engine light comes on and you think it is because your fuel is boiling? How about it being caused by a engine installation error or part breakage when the parts from your old engine were transfered to your new engine?

The check engine light can indicate that a problem has been anywhere on the car. You won’t know where until you get the code scanned for. Bring the codes here for advice.

That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here.

Regarding warning lights:

  1. if the coolant temp light comes on, shut off the engine ASAP

  2. if the oil warning light comes on, shut off the engine ASAP

  3. if a FLASHING MIL/CEL comes on, shut off the engine ASAP

    ASAP means driving to the berm of the highway right now and not waiting for the next exit.

But if the MIL/CEL is not flashing, then it’s not an urgent indicator.

I’d take it back to the guy who replaced your crankshaft. Considering the size of the bill for that, he should be willing to read the codes and give you a diagnosis for free. He may well have forgotten to hook up a vacuum hose or something. Some codes don’t turn on the Check Engine light until they have been seen on several successive starts or “drive cycles”.

Boiling sound from the gas tank? I doubt it is boiling but I don’t know what it is doing. It’d make me nervous too. The facetious solution would be earplugs or turning the radio up, but I think you are right, you need to find out what it is. You can experiment with turning the ignition on without starting the car and seeing if the noise is present, where it seems to be coming from, etc. The more you can tell the mechanic you take it to, the more likely you are to get a valid diagnosis. Do you know anyone who is interested in cars? Maybe you can get them to investigate. In financial circles, that is known as “offloading the risk”.

The only source I could imagine for bubbling would be if there were air in your fuel line and it was being drawn back into the gas tank through gravity (the gas draining back down the line) when you shut down the engine. For that you’d need a bad check valve and a source for air to get back into the line to allow the draining…perhaps the same leak that allowed air to be drawn in when the car was running is now allowing it to be drawn in and let the gas run back to the tank. I’m thinking a real poor gas line connection here, probably introduced when the engine was replaced.

Anyway, I too think you should go straight back to the shop that put in the new motor. Post back and let us know what they found.

Allow me to amend. With the pressure in the line a leaky fitting would spray gas when running, then it would draw air back in when the gas ran back into the tank. You mentioned gas vapors?