Unless you want wet feet, clean your cowl drains

I was helping a friend of mine sell a 2007 Ford Mustang that had been sitting in her driveway for a few months. In addition to its dead battery, the Mustang had an odd problem: There was standing water on the carpet.

I assumed that the windows had been left open while the car sat, but my friend swore that wasn’t the case. I next assumed that the problem was what sometimes happens with vintage cars: the windshield gasket hardens and shrinks, a gap forms, and water gets in. That did not appear to be the case either.

I looked up the problem online and was surprised to learn that it’s actually pretty common. The fifth-generation Mustang and several other cars with cabin air filters in the firewall have what only can be described as a glaring design flaw.

Most cars have a cowl (a well) at the base of the windshield that holds the wiper motor and hides the wiper linkage mechanism. Since it’s recessed, the cowl would naturally fill up with water if there was nowhere for it to drain, so there are typically two or three drain holes at the bottom for water to exit into the engine compartment. These are usually finished with rubber flaps or tubes that guide the water forward and past the firewall insulation. The problem is that when dirt, leaves, and twigs fall into the cowl, as they inevitably will on a car that’s left outside, they can clog up the drain tubes. When it rains, this makes water back up in the cowl.

While it’s not great for the wiper motor to be sitting in standing water, the more visceral problem is that, if the car has a cabin air filter in the firewall, the water has nowhere else to go, so the undocumented “autoflood” feature is invoked and water seeps through the filter, into the passenger compartment, and onto the floor.

You can usually see the rubber drain tubes from inside the engine compartment. They hang down from the underside of the cowl. It’s pretty easy to unclog them by reaching up from the bottom and cleaning them out with a rubber-gloved finger or a popsicle stick, but it’s really best if you clean out any leaves and dirt from the cowl, or else they’re likely to clog up the drain tubes again soon.

Accessing the interior of the cowl usually requires removing the windshield wiper arms, then popping off the clips that hold on the plastic cowl cover. If the windshield arms have been on there for a long time, penetrating oil and a small puller may be needed to pull them off their splined shafts on the wiper linkage. The cowl cover may be one or two pieces.

When I removed the two-piece cowl cover in the Mustang, I found a lot of standing water on both sides of the cowl.

The cabin air filter on the Mustang is on the right side, as it is on most cars that have one in the firewall. On some cars, you replace it from inside the glove box, but on the Mustang it’s actually inside the cowl. For some reason, in the photo below you can’t see the water line, but it’s about halfway up the air filter.

I pumped out the standing water with a small electric pump, then unclogged the rubber drain tubes, washing the dirt out of them. When I inspected the bottom of the cowl, I was surprised that it was actually fairly free of leftover leaves and other debris. I could’ve probably unclogged the drain tubes without pulling the cowl cover off, but I didn’t know that in advance. Plus, the cabin filter was pretty dirty from sitting in standing water, and I wanted to change it anyway.

I replaced the filter, buttoned things up, dried out the rugs with towels, a fan, and sunlight, and knocked back most of the damp smell with an ozone generator. It’s rained several times since, and the problem has not recurred.

As you can see, it’s a good idea to prophylactically clean those cowl drains. Unless, that is, you want to use the “autodampen” feature.

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Rob Siegel’s new book, The Best of the Hack Mechanic^TM: 35 years of hacks, kluges, and assorted automotive mayhem, is available on Amazon. His other seven books are available here, or you can order personally-inscribed copies through his website, www.robsiegel.com.