Dealership might be blowing smoke on air-conditioning

Dear Car Talk:

I have a 2007 Mercury Marquis. The A/C never can make up its mind as to where to direct the air flow. It comes out the dashboard vents, then changes its mind and sends the air out the defrost vent. Then to the floor.

There’s no real pattern. I’ve spent $1,800 at the local Ford dealer, and the problem never changes. They claim to have replaced stuff under the dash, but I’ve seen no evidence that they ever touched it. They have touched my money though. — Herman

Geez, I think these guys owe you a lot of money back, Herman.

The first thing I’d check would be the vacuum reservoir. The “blend doors” that direct the airflow under the dash are controlled by something called vacuum motors. And the vacuum needed to operate them is produced by the downward motion of the pistons inside the cylinders.

Every engine produces plenty of vacuum at idle and at low speed. But when the engine runs faster — when you begin to open the throttle — vacuum drops.

To make sure the blend doors don’t go crazy when the vacuum drops, lots of cars have something called a vacuum reservoir, which is a simple plastic container, about the size of a Nerf football, that stores vacuum. Its job is to provide vacuum to keep the blend doors from closing and opening haphazardly when you accelerate.

So, I hope they replaced that.

If that’s not it, it could be something as simple as a bad connector, hose or check valve. The easiest way to find the vacuum leak (which is what I think you have), is to use a smoke machine. As a bonus, you can also throw a helluva party in your 2007 Marquis while getting your diagnosis. Be sure to bring your Pink Floyd albums with you when you go the shop. We’ll introduce smoke into the system instead of vacuum. If there’s a leak, we’ll see it.

That’s what your Ford dealer should do next. And if they find a faulty vacuum reservoir or a bad connector or check valve, I think they owe you $1,800 back, minus the cost of the $150 vacuum reservoir, the $5 tee connector or the $3 check valve.

Or you might want to just go to another mechanic and start over with a smoke test. If they figure it out, then go back and ask Ford for a full refund, Herman. Or go to small claims court if you need to. I think they’re going to owe you.