The oil compounds in fuel is what is causing those odors. Other then lots of time, you will find little to help. I would suggest that the carpet padding under the carpet be removed. Even if its just a small square of the padding, you can purchase a few feet of padding at most stores where you can patch it back in so not to have a dent on the carpet surface.
Trying to wash out the odor only spreads the oily byproducts around more. Steam cleaning may help if you can apply a blotting cloth under the carpet to soak up any of the fuel remnants that bleed down during the steam cleaning process.
There has been suggestions to use coffee grinds, and vinegar and alcohol to remove the smells. However grinds will do nothing to remove the chemical itself that is causing the odor, and will only mask it, the vinegar method will only dilute the chemicals and spread them around and the last method will only remove the color of the carpet creating a stain due to bleaching action.
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