Why Does the Defroster Make Shirley's Kia Run Rough?

Dear Car Talk

Dear Car Talk | Mar 05, 2015

Dear Car Talk:

I have a 2012 Kia Forte. It's a great car, except that in the winter when I turn on the defroster, the engine runs really rough. As soon as I turn off the defroster and turn the switch to "heat," the engine runs smoothly again. I want to know if you have heard anything about this, or is this normal?

-- Shirley

It could be normal, Shirley. If the engine runs just slightly rougher when the defroster is turned on, that could simply be because the defroster uses the air conditioner.

Because the air conditioner removes moisture from the air (i.e., it conditions the air), when you turn on the defroster in most cars, the air conditioner is automatically switched on, so the air blowing on the inside of the windshield is dry air. And because the air conditioner imposes an additional load on the engine (it takes a lot of engine power to run), some cars with small engines will seem to run a little rougher when the AC is on.

But it should be barely noticeable. So test this theory yourself by turning on the air conditioner without using the defroster. If the car runs smoothly, then it's not normal, and I'm guessing you have a vacuum leak.

There are little "doors" in the ventilation ducts that open and close to direct air from one place to another. They're called blend doors. They're usually operated by vacuum motors. And if the vacuum motor that directs air to the windshield has a leak of some kind, that will make the engine run rough when you engage that setting.

The reason it makes the engine run rough is because the vacuum is generated by the pistons when they go up and down. If there's a leak in a vacuum hose, it allows extra air to get sucked into the combustion chambers, and throws off the carefully calibrated fuel-to-air ratio.

The bad news is that in order to get at this blend door, your dealer may have to remove the dashboard. The good news is that Kias have a five-year, bumper-to-bumper warranty. And last time I checked, the dashboard was still between the bumpers.

So take the car to your dealer and show him what's happening, and ask him to fix it. Good luck, Shirley.

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