Dear Car Talk:
I have a strange question about my old 2006 Saab SportCombi Wagon. Every morning I drive over a very steep and windy mountain road.
The start of the road is about three miles from my house, and it’s typically cold in the morning, so the heat is on.
As I drive up the mountain, I’ll make hairpin turns (rather quickly, which is why I bought the Saab). That causes the vents to stop blowing heat and blow cold outside air. I’ll straighten out of the turn, and the vents return to heat then go into another chilly turn!
I’m curious what causes this! -- Connor
You have a vacuum problem, Connor.
Have you tried changing the bag? It might be full of cat hair. Actually, the blend doors inside the ventilation system -- the flaps that direct either heat or cool air into the cabin -- are controlled by vacuum motors.
Vacuum is produced naturally by the engine when the piston draws down. You create the greatest amount of vacuum when your engine is idling and the throttle is closed, and the least amount of vacuum when the throttle is wide open and you’re accelerating hard.
And you say you’re accelerating through those uphill turns. That means you’re creating a low-vacuum situation.
Normally, that shouldn’t be a problem. Most cars use check valves and vacuum reservoirs to maintain enough vacuum during hard acceleration to keep things working. But there’s obviously something wrong in your car.
You might have a bad check valve or a cracked or leaky vacuum reservoir. Or you might have a small vacuum leak somewhere else in the engine that’s draining enough vacuum so the reservoir can’t do its job.
So take it to your Saab dealer, Connor. Sorry, that was unnecessarily cruel. I’m sure you have a mechanic who has kept the thing running for the past decade and a half. Ask him to check it out and see if he can figure out why the vacuum is dropping too low. He can check for vacuum leaks using something called a “smoke test.” That test does not involve any fire, Connor. So don’t get your hopes up about getting a new car with the insurance money.