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In my Mercedes C180 T sometimes I smell fuel. I can reproduce it with the following steps:

  1. when the engine is stopped after a route and become cold
  2. first time when I open trunk door or only I press the trunk opener button when it is locked
  3. after that if I dont start the engine in 5-10 minute I can smell a fuel, but I cant see any leakage

If I repeat the step 2 and 3 after 3rd step, I cannot reproduce the problem. So I need to warm up the engine and wait to become cold again to reproduce the problem.

What can be the problem?

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    Where do you smell the fuel? When you are standing at the rear of the car (behind the open trunk hatch), or when standing near the fuel fill cap or while seated in the driver’s seat?
    – zipzit
    Dec 30, 2021 at 1:20
  • I cannot exactly locate the source. I smell it also in front and rear outside the car. After I smell it outside If I start the air conditioner I can also smell inside. Dec 30, 2021 at 8:14

1 Answer 1

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I suspect there is a leak somewhere in the fuel vapor recovery system.

Fuel vapor recovery system illustration

There is a whole lot going on here.

  • The system is totally designed to reduce fuel vapor emissions.
  • Start with a totally sealed fuel filler cap. Now think about that a moment. As you use fuel, it leaves the fuel tank and gets burned in the engine. If the fuel filler cap is totally sealed, what happens? Sure, the inside of the tank becomes a vacuum. Ever seen a vacuum applied to an empty beverage can?
  • So yeah, that fuel vapor system allows air to enter the fuel tank.
  • What about vapor inside of the tank?
  • The system provides a slight vacuum to capture those fumes (and turn them from a gas to a liquid).
  • Periodically in the drive cycle, the system sends that captured vapor (now liquid fuel) to the engine where it is burned.
  • I can go on, but it's a engine computer controlled system with multiple inputs and outputs... Lots of places for a leak to occur.

another vehicle fuel vapor image

I'm guessing there is a leak somewhere in the engine compartment end of things, that only occurs occasionally in the vapor recovery cycle. Fuel leak there would definitely be smelled both inside and outside the vehicle.

I suspect it's possible to have a leak that isn't captured by code within the OnBoard Diagnostic (OBDII) system but I'd certainly like to see what stored codes are in your system. You may or may not have a check engine light.

If my guess is correct, you can probably spot the leak by a close inspection of the hoses and valves in the engine compartment. Safety first, engine off please! Use a flashlight, really inspect each and every hose, valve and fitting looking for a slightly brown "varnish" which would indicate a previous leak site. I doubt you'd see wet fuel. Its an intermittent issue.

And while you are there, inspect all hose type fittings at the engine for possible vacuum or fuel system leaks. Look closely at every thing plugged into the intake manifold.

And an inexpensive purchase of a wireless OBDII diagnostic tool might be appropriate, so you can read the engine/fuel system computer codes yourself. There are some decent free code reader apps. Handy for friend's and family's vehicles, too.

Let us know what you find!

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  • On board computer does not show any error, check engine leamp is off. Later I will read stored error codes. Dec 30, 2021 at 10:22
  • Presuming zero error codes and not choosing a repair shop or dealer to find the source of fuel smell/leak leaves you to do the extensive diagnoses of the entire fuel system. Are you familiar with your MB fuel system routing? Can you measure fuel pressure to observe for pressure bleeding off when ignition is first turned on without starting if a leak exists?
    – F Dryer
    Dec 31, 2021 at 20:14
  • @FDryer My posting here is based on the assumption that the leak is intermittent. A fuel systems leak (from tank to engine) would be constant. A fuel vapor recovery leak would be VERY intermittent, based on how the system works. I suspect the leak only occurs during the cycle where captured liquid in the carbon cannister system is sent to the engine to burn.
    – zipzit
    Dec 31, 2021 at 20:44
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    I suffered an intermittent fuel leak that came and went on its own until it became constant. I checked under the hood several times because fuel vapors came into the interior. After some time passed with no visible evidence of fuel leaking, I popped the hood and saw a small fuel puddle under the fuel rail test valve, directly over the engine of my V6. Dipping a finger verified the source of the leak from a damaged Schrader valve core. Replaced it and no more intermittent fuel leaks.
    – F Dryer
    Jan 1 at 19:45
  • Finally I took it to dealership, they replaced one fuel pipe in front side of the car, it solved the problem. Mar 16 at 8:25

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