between each injector you will find a fabric coverd rubber pipe, (the spill return)this takes unused fuel back to the tank,although this is the low pressure side of the fuel system it will draw in air,leading to extended cranking,you may not think there is any thing wrong with them,but they are cheap and easy to change.(push fit) if that fails to do it, the pump pick up can spilt and draw in air but this only happens(generaly) at one fuel level and is much morework to check(tank out ) and lots more money