if the cooling system begins to boil surprisingly it has been found that the cold impact resistance of a multi layered fuel line can be very high if a brittle barrier layer forms the middle layer of the tubing in this case its the impact resistance achieved by the impact resistance of the internal and external layer it has also been found that unmodified polyamide 66 and polyamide elastomers based on polyamide 12 are eminently suitable as barrier layers this barrier effect in fuel lines is required in particular with respect to the aromatic constituents of the fuels which are usually highly toxic polyamide 66 is superior to polyamide 6 and in particular the polyamides from long chained monomers such as pa 11 pa 12 and pa 12 12 are advantageous polyamide 66 is so compatible with polyamide 6 on the one hand and polyamides 11 and 12 on the other hand that no delamination could be observed in the tubing made from such layers according to the invention this same advantage has also been observed for polyamide elastomers based on polyamide 12 blended with copolyamides based on polyamide 6 and polyamide 12 monomers therefore the multi layered fuel lines of the invention have.