Team-BHP - Pressure build up in fuel tank

Today, while driving my car (2019 petrol Swift), I noticed random dings coming from the instrument cluster - the same sound that usually plays when the low fuel warning light comes on. Interestingly, the dings seemed to occur whenever the car went over bumps, though no lights lit up on the cluster. It was a particularly hot day (Chennai summer).

After stopping the car and getting out, I noticed a strong smell of petrol. Out of curiosity, I opened the fuel cap and was surprised to find a significant amount of pressure being released from the fuel tank.

What could be the cause of this?

The pressure being released from the fuel tank is a normal thing. I've noticed this in all cars starting with my 2003 Maruti Versa (MPFI BSIII). However even I have experienced what you have mentioned (accompanied by a strong smell) over the last year or so with my 2007 Suzuki Grand Vitara when the car runs for about 30 minutes. Surprisingly, the odour disappeared when I displaced the volume in the tank with fresh fuel (eg. when I did 1400 plus kms in 5 days in Dec) - that was like almost 130 liters of fuel or two fill drain cycles. Another few months of low usage and the odour is back. Release the pressure via the filler cap and the odour diminishes and then disappears. I can associate this only with the Ethanol content in fuel (can't think of anything else). I was initially suspecting the vapour canister to have gotten clogged. However, ruled this out as the odour had disappeared when the car was used a lot. Ethanol's initial boiling point is lower than that of motor spirit from what I've heard.

Quote:

Originally Posted by kpkeerthi (Post 5966271)

After stopping the car and getting out, I noticed a strong smell of petrol. Out of curiosity, I opened the fuel cap and was surprised to find a significant amount of pressure being released from the fuel tank.

What could be the cause of this?

Pressure build up in fuel tanks of cars and motorcycles are absolutely normal. Especially in a hot city such as Chennai. There's nothing to be worried about - go on driving your car as usual and do not keep opening the tank lid to "let the pressure out!"

The pressure build-up is normal, and our cars are designed to handle it through the EVAP system. I will let you explore what the EVAP system is yourself.

The smell, well, that needs some diagnosis because the fuel delivery system is basically a one-way (it lets air in but doesn't let it out) sealed system and no fuel smell must emanate from it. There must be a leak/crack somewhere, and that somewhere must "most likely" be in the activated carbon canister or the hose that connects it (since you mentioned that this happens during a hot day).

Next, the pressure that you are letting out of the lid is actually combustible gas. You are wasting combustible petrol vapour that, I am sure, could have given you a few kilometers at least.