This has been an ongoing thing with the Cool Fuel Modules since they starting using them. Mercury has a service bulletin out on them. Yu need to check and see if your motors have have that service performed. Mercury will supply the parts to fix this, BUT - they must be installed by a Certified Mercury Service person. If you decide to do this, clean all your injectors on both motors, Fuel lines, replace pump housings, fuel filters and filter strainers.
