1984 Chevrolet Blazer:
Runs Rough Cold, Possible Carburetor Icing

Question:

My '84 Chevy Blazer runs rough under certain conditions; always below 45 degrees and usually after a rain. (There's no problem in warm weather.) It seems like it's not running on all cylinders, the engine bogs down and sometimes it stalls. Usually, this happens about 15 minutes into a highway trip.

My mechanic has ruled out the carburetor. He's replaced the distributor cap, rotor, wires, plugs, coil and the EGR valve, none of which has helped. My cousin, a mechanic in Michigan, suggests it could be carburetor icing.

Answer:

Carburetor icing is a definite possibility. Every modern carbureted engine has a method of warming intake air during colder ambient temperatures. In some cases, ice can build up in the throat of your carburetor even though the outside air is significantly above 32 degrees F. It's like the wind chill factor you hear on the weather forecast every day. The faster the air moves, the colder it gets. Icing can limit the amount of air entering the engine. This changes the air/fuel ratio, giving you symptoms like those you describe. Even without icing, cold air will hinder the combustion process and cause drivability problems.

The system that's designed to deal with this condition has various names, depending on the manufacturer, but we'll just use "thermostatic air cleaner." This term is pretty universal. When your engine is cold, the system uses vacuum to open a little door, diverting warm air from the exhaust manifold to heat your carburetor. As the engine warms up, the thermal switch releases the door to allow fresh, cooler air from the main intake to enter the air cleaner, carburetor and engine. Here are some things your mechanic should look for:
1.) Make sure that the tube from the exhaust manifold to the air cleaner is in place and properly attached.
2.) Check all vacuum hoses. Make sure that they are attached and not kinked, cracked or cut.
3.) Check for vacuum in and out of the thermal switch when cold. Then make sure it gets to the vacuum diaphragm that opens the little door.
4.) If the door remains open. Check the diaphragm for leaks.

Now, if you and your mechanic have gone over the entire system and everything works properly, start looking elsewhere. You've eliminated this system as a possible cause of your drivability problem, and you haven't wasted any money.

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