Any ideas on what could be wrong ?

In a perfect world fuel tanks have nothing but fuel in them. In reality, the older they are, the more crap they accumulate in the bottom of them and when you run low on fuel it can get sucked into the system. Possibly the problem is only a blocked fuel filter. But if water/algae gets through your filter then your injectors may be blocked. Tell your mechanic that you ran out of fuel. If you did previously, find a new mechanic!

Kalon
May 2008