Carburator icing: A new product from Betty Crocker.

Dear Car Talk

Dear Car Talk | May 01, 1991

Dear Tom and Ray:

I have a 1980 Mazda GLC hatchback that has a problem no one has been able to fix. Sometimes when the temperature drops into the 20's, and the humidity is high, the car will just die while I'm driving. If I pull over and let the car sit for a short time, it's fine again. What's going on?

TOM: Judy, it sounds to me like you have carburetor icing. Even though that may sound like something Betty Crocker sells, it's actually the build up of ice inside your carburetor.

RAY: We know that water (or humidity) plus sub-freezing temperatures equals ice, right? Well as you're driving along, your carburetor is sucking in that cold, humid air. And to make matters worse, there's this thing called the Venturi effect. This guy Venturi discovered that when air is forced through something shaped like the throat of your carburetor, it's pressure goes down and the mixture gets even colder! Before long, you start making ice.

TOM: That would be OK if you were driving an Alfa Romeo, because you'd be making Italian ice. But in most cars, when enough ice forms, the carburetor throat gets plugged up and the car stops running. Once it stalls and the Venturi effect stops, the latent heat of the engine melts the ice, and the car is fine again--for a while.

RAY: But this shouldn't be happening. The air coming into the carburetor is supposed to be pre-heated. There's a paper hose that brings hot air from the engine compartment to the carburetor. My guess is that the hose has fallen off.

TOM: Back up for about 25,000 miles and see if you can find a short, paper hose on the ground somewhere. If you can't, ask your mechanic to give you a new one. If it turns out that your hose ISN'T missing, ask him to take a look at the "blend door," which lets the hot air into the carburetor, and the blend door's thermal switch. Good luck, Judy.

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