Quite often, working on a vintage or late-model Mustang is fun. A large part of that is due to an intriguing upgrade, such as installing a four-barrel carburetor and intake in place of a two-barrel.

The sweet smell of antifreeze when the heater was turned on, along with tiny puddles of coolant on the passenger-side floor mat, were obvious signs there was an issue with the heater core in our ’91 ...