Cowl vent leaks are the Mustang owner’s worst nightmare. Just about any vintage Mustang in need of restoration is going to need some sort of cowl panel restoration. Some cowls have minor leaks and can be patched, others need complete replacement panels (and we can thank the reproduction metal companies for making this step much easier), and the worst of them need complete replacement cowls.
There is very little in the way of diagnosis—simply run some water into the cowl vent and see where it leaks from. On the worst cars, you can also see rust damage from under the dash—with the heater case and the driver-side fresh air vent removed. It’s pretty much a given that the cowl will leak when you test it in this manner. The only things left to do is remove the top of the cowl area and survey the damage.
But why is the Mustang cowl design so prone to rust perforation? Well, it’s a simple fact that the cowl of the early Mustang was poorly designed. The cowl, as it was welded together, had no access for proper paint application or rust inhibitors. Back then, metal wasn’t dipped in EDP primer as it is commonly done now on modern cars; the protection came strictly from paint application after the body was assembled.
The drainage of the cowl was poor, and any large items, such as leaves, twigs, and paper, could easily slip past the large cowl vent slots and dam up the drain areas—causing standing water to sit around the unprotected metal “top hats” of the cowl vent areas. Though the design changed for the ’69 model, it’s interesting to note that Ford used a similar design for the Fox-body Mustang from 1979-’93—a large cowl vent opening with a single air inlet on the passenger side. The drainage was improved, but the vent slots were plastic and smaller (allowing for paint application inside the cowl before the vent was attached), and the metal was EDP-coated as well.
Project ’66 needed cowls, of course, as do most ‘65-’68 northern cars. We didn’t know what we would find—and except for looking under the dash (the interior was gutted for paint and body work), we didn’t see any major rust perforation. We hoped cowl patch panels would complete our job efficiently. To be prepared, we ordered a left and a right cowl patch panel from The Paddock and sent them over to Classic Creations of Central Florida, so that when we arrived to shoot this article, they would be ready for assembly. Take a look.