Pumping fuel out of fuel cap!!

[pixielee]

The AA were called out when my Merc A Class 140 began pumping fuel out of the cap on a downwards slope at work. Although it was obviously a pressure problem the guy could not work out what was causing it. A day at the garage ( Mercedes specialist) revealed no more light on the matter! I am now driving it with the cap on and promptly unscrewing it everytime I do not drive it.
It ran fine for several years ( 04 Reg very low mileage 12.5K) It was NOT a hot day, the fuel tank was only half full and it was parked on the hill on a regular basis for work until I began cycling in - conveniently after this event. When I take the cap off it blows air OUT every time.
My concern is that the car is mainly used to transport my 2 yr old daughter around and I don't like to use it without the cap on (illegal and can spill petrol) but am really concerned about building up pressure. There seems to be no answer0- has anyone else had this trouble or know what is causing it?

 

[ernieh]

Carbon filter / Vent

Welcome to the Forum.

Sorry to hear about your problem. Others will know if it a common problem with your model or not and might have a better answer, but modern fuel systems are normally vented through a carbon filter, often located behind one of the front wings. This, I believe is to allow for expansion / contraction of fuel in the tank and also to aid venting of the tank while refuelling, without allowing excess fuel vapour to pass into the atmosphere. It might be worthwhile checking out the pipework between the tank and the filter to see if it is blocked. A first step would be to disconnect the pipe at the filter end and then work back to the tank. However, do be careful of breathing fuel vapours and of using naked lights around the tank and its pipework.

Let us know how you get on.


Ernie

 

[jibcl500]

Sounds to me like you need to replace the charcoal filter.

jib

 

[mattkh]

Hi
Do you also smell any fuel in the car sometimes ?