Mark Houlahan
Tech Editor, Mustang Monthly
February 1, 2013
Photos By: Courtesy of National Parts Depot
Shown here is the typical weld-in cowl patch, as seen from under the dash and installed on the passenger side (the heater box inlet butts up against the opening).

The well-travelled vintage Mustang owner knows that the hobby's biggest four-letter curse word is r-u-s-t. Rust has ravaged its fair share of vintage Mustang body panels and one of the more problematic areas is another four-letter word that brings chills to Mustang owner's spines—the cowl.

The '65-'68 Mustang's cowl design is certainly not one of Ford's better ideas. The cowl panel is made up of an upper and lower sheetmetal stamping that is welded together and welded to the firewall as well as the windshield framework. Unlike today's coated metals and robotic painting processes, the cowls in '65-'68 Mustangs were simple sheetmetal parts spot welded together and given a coat of paint, certainly not the most rust preventative measures. Combined with the sealed design, it was a rust issue just waiting to happen.

Starting in 1969, Ford made the cowl area a bit more accessible via a removable grille panel, mainly for servicing the wiper motor and transmission, which also made it easy to keep the lower cowl clean of debris. This debris is what causes the rust issues in the '65-'68 design.

While Ford did design a drainage trough on each side of the cowl that exited the cowl sides into the fender area, the drains easily get clogged with pine needles, tree leaves, dirt, and other debris that enters the cowl vent opening. This debris traps moisture in and against the steel of the lower cowl panel. The combination of air and moisture over time initiates the metal's transformation into rust. As the rust worsens, causing penetration of the cowl panel around the cowl vent's "hat," the ensuing perforation allows future water ingress (natural rains, car wash water, etc.) into the passenger compartment, soaking the carpet and causing further rust issues with the floor pans, the Mustang's next biggest rust issue.

If you're one of the lucky few to have a garage-kept vintage Mustang or perhaps lived in a southern and dry climate, then you most likely have nothing to worry about. However, for the rest of us, cowl vent rust and leaks are a real problem. Some may say that the only proper repair is to cut out the rusty cowl portions and weld in patches or a new cowl. While we agree with this wholeheartedly, we also understand that not everyone has the budget for a complete cowl replacement. Not to mention some cowls may have minimal rust while others look like Swiss cheese. So for a cowl with just a couple of small holes, a complete replacement is a bit of overkill.

Thankfully there are several solutions that vary in price, practicality, and the amount of labor involved. Read on as we go through each solution and the pros and cons of each.

If you suspect you have a cowl leak, a bucket of water or a stream of water from a hose will help diagnose the situation and pinpoint where the leak is coming from. Just don't mistake a leaking windshield gasket for a cowl leak. It is often best to gut the underside of the dash (gauges, wiring, wiper assembly, heater box, etc.) so you can get a clear view of the complete cowl to determine where any issues lay. If your car has a nice paint finish and you don't wish to repaint the cowl and/or pull the windshield, some shops will install the cowl patches from inside the car, but the welding/sealing is more difficult in this manner and, as such, be prepared to pay a little more. Speaking of cost, we've outlined the parts cost and typical labor costs for each option below.

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Cowl Vent Cover

While few would consider this device a proper repair, the cowl vent cover does make an effective stop gap for those on a tight budget. The cowl vent cover is just that, a cover for the vent opening. It uses a foam seal and spring loaded retainers to seal the vent opening from water intrusion. It does not fix the rust issue, but it will prevent further damage and keep your interior and your feet dry for those daily drivers out there with cowl rust-through.

The cowl vent cover does block the cowl's airflow to the heater and driver's side fresh air vent, so many owners keep their vent cover at the ready in the back seat or the trunk and only pop it on when the weather turns wet. The cowl vent cover offered by National Parts Depot (NPD) uses high-quality billet knobs and virtually unbreakable Lexan material for the cover. Pricing is $25 for the '65-'66 and '69-'70 model, while the '67-'68 Mustang cowl vent cover runs $31.50. Naturally, there's no installation labor for this DIY fix.

Cowl Vent Collar Repair Kit

On the market just about as long as the cowl vent cover, the cowl vent collar repair kit is a simple DIY fix for basic cowl vent leaks around the spot welded flange of the "hat," or collar, area. If you have major rust perforation on the floor of the cowl panel, this repair kit will not help. However, if you have just basic perforation around the ring/base of the collar, you might be able to make an effective repair using this repair kit. Included in the kit are two plastic molded collars and a tube of silicone adhesive. The plastic collars fit up into the stock cowl vent collar area and when properly installed/sealed will create a new collar area that will prevent water from entering the passenger cabin.

Our recommendations for installing include cleaning and prepping the cowl panel's underside to prevent as much future rusting as possible, then gluing the plastic cowl hats in place. These steps will require the removal of the driver's side fresh air vent and the heater assembly. We've been told by paint and body experts that silicone adhesives can trap moisture, so you might want to consider alternative adhesion solutions like panel bonding adhesive. You might also consider removing the front fenders and cleaning the cowl vent drains of debris. We've also seen people drill access holes in the cowl side to spray in rust converter and/or undercoating to help patch the area as well for a two-pronged approach using the cowl vent collar kit. The collar repair kit sells for $15 at NPD and is generally considered a DIY solution, however you'll be looking at an all-day job to get to the bottom of the cowls.

Cowl Vent Sheetmetal Patch

By far the most popular repair option are these cowl vent repair patch panels. The left- and right-hand panels are sold separately, but most shops we've talked to over the years tell us if a Mustang needs one side, the other can't be too far behind. Let's face it, with the labor involved to remove the car's front sheetmetal, pull the windshield, drill out 200-plus spot welds, and then cut out the rusty cowl vent area, do you really want to go through all that labor/cost again a couple of years later for the other side? We didn't think so, which is why these patches are usually done in pairs (just remember you have to order each side separately).

The cowl patches are stamped from 19 gauge steel and feature a full patch area from firewall to windshield frame with the vent collar already spot welded into place. The worst rust-through is usually in the valley area surrounding the collar, so these patch panels are usually an effective repair when properly installed. We've seen these installed a couple of different ways; butt-welding is going to be your best method. You can save yourself some time if the upper cowl area at the wiper pivot is solid by cutting around it. This way, you don't have to worry about the wiper pivot mounting being spot on and in the correct place with the patch install.

The patches themselves aren't too hard a hit on the budget at $55 each in the NPD catalog, but labor is going to hurt on this one. Expect the typical Mustang restoration shop to get about 15 or more hours of labor for the repair. You can save some money by removing your front sheetmetal, windshield, heater, etc. in advance, but still expect a good eight to 10 hours of labor with the car delivered in such a state. Depending on your locale's hourly labor rate, you're looking at anywhere from $1,500-$2,000 or more for the installation of these patches.

Complete Cowl Vent Panel

If your cowl has serious rust-through issues outside of the standard patch panel repair areas, the fix used to be a series of metal patches/strips welded in as needed to repair the issue. Now, thanks to new stampings, the complete lower cowl panel with collars is available for repairing cowl rust issues on '65-'66 and '67-'68 Mustangs. A cowl panel is available for the '69-'70 Mustang as well, but it is complete with the upper panel included. The complete lower cowl panel requires the same amount of tear-down labor for access to the cowl area and just a little more repair labor (figure a couple of extra hours) to remove the complete lower panel and weld in the new panel versus the two separate patches placed at the ends of an original cowl lower panel.

The cowl vent lower panel for '65-'68 Mustangs is designed around the '67-'68 Mustang cowl stamping and will fit the '65-'66 Mustang with minor tweaking. Due to the larger repair panel and stamping of the assembly, the full lower cowl panel is roughly twice the price of both left and right patches, ringing the register for $209. However, any good shop will tell you that you can easily recoup the cost of the full lower panel when you start adding up the cost of labor and materials to hand-make a bunch of small patches and weld them in to repair a perforated cowl panel outside of the two patch panel repair areas. For'69-'70 owners, the complete two-piece cowl is $465.

Last but not least, after you've made your cowl repairs, be it a patch panel or a full lower cowl panel, you might want to consider adding a new upper cowl panel to your parts list. If your '65-'70 Mustang's upper cowl panel is just as rough and you hate the thought of reinstalling a rusty upper panel over that freshly repaired lower cowl panel, then these new reproductions are your answer. For the '65-'66 Mustang, you'll have to buy your upper panel as part of a complete cowl “tank” with the upper and lower together ($434), just like the '69-'70 version mentioned above. For those with '67-'68 Mustangs, you can buy the upper as a separate part in NPD's catalog for $229 if you need it for your cowl repair or restoration project.