Fuel Odor From Air Vent.
 

Recently I started to use heater, I noticed very strong fuel odor from the air vent when I turned on heater, it went away if I turned off heater.

Does this indicate I have fuel leakage under the hood? any ideas?

Thanks

Forgot to mention:

The car is a 92 400E with 200K km, I have just replaced fuel pressure regulator, distributor caps and rotors.

Yea I would say that you have a fuel leak somewhere under the hood. Have a good look under the hood at all the fuel related components while the engine is running.

I tried to look under the hood, couldn't find any leak, I tried to rev the engine, but still no leakage and even no smell under the hood.

When I walked around the car, I smelt some strong gas odor from the passenger side rear wheel fender, so I guess I have leakage in the fuel pumps or fuel filter.

But does it make sense? I mean when I was driving, how could fuel odor from rear get into the air vent?

If you say you smell it when driving, it means you would have to be more thorough in checking the entire fuel distribution system. Start with the front where the smell will be pushed into the cabin. I wouldn't be surprised that the fuel mixture is too rich - just taking a wild turkey shot.

Check you gas cap. My 300 ['86] has an mb cap that has two parts inside and if they get out of alignment the smell fills the car. I may be wrong but it has happened to me several times when the gas station fellow was careless and just twisted in the cap.

Good luck.