Hamtaro : Check air con compressor , when it's going to kong or needs servicing it causes excessive engine load as well. And also ask mech to check ur alternator output when engine is on, see if it's supplying enough wattage.
If gbox welding then can gao dim then ok lor, but where is ur mech welding ?
When flooring the gas pedal, usually will have some petrol smell , depends on how strong it is. Are you running a fuel pressure regulator ? If yes check the hoses running in and out of it. Also check the fuel pressure regulator itself, it can leak as well. Also Check near your fuel tank , there should be 2 hoses running out of it, check if they are leaking or not.
Hope that helps.