Throttle Icing
by Jim Kerr

Winter can be snowy and cold but usually the humidity levels are low. Recently, however, the humidity has been high, painting the landscape with heavy white frost. Although beautiful, this frost can cause trouble with your vehicle in the form of throttle plate icing.

Several people have recently asked me about problems with their vehicles and all of them could be symptoms of throttle plate icing. If it happens, it can be very frightening to the driver. Here are a couple symptoms of throttle plate icing and how you should react if it happens to you.

One symptom is the throttle plate sticks in an open position. You are driving at a constant speed for a few minutes (usually on the highway) and when you release the gas pedal, the vehicle will not slow down. Stepping on the brakes may slow the vehicle, but the engine is still producing a lot of power so it can be extremely difficult to stop the vehicle.

Tapping on the gas pedal in an attempt to free the stuck throttle plate is a no-no. This can cause the throttle plate to stick even further open! Here is another no-no: never place the transmission in neutral or step on the clutch pedal. The engine now has no load on it and the stuck throttle will cause the engine to over rev immediately. Engine parts can bend and break with expensive consequences. I have overhauled several engines that were damaged by drivers in this manner.

The correct response from the driver is to turn the ignition key to the OFF position, NOT TO LOCK. Then hold firmly onto the steering wheel because there is no longer power steering, and coast or brake to a safe stop. Now the throttle plates can be freed without causing any damage to the engine or the car. Letting the car sit for several minutes will usually cause the problem to disappear. The heat from the engine rises under the hood and melts the ice on the throttle plates. The vehicle will then start and run fine.

A second symptom of throttle icing is a lack of power. As the ice builds up around the throttle plates, the airflow into the engine is blocked off. In a fuel injected vehicle you will only notice the power loss, but if you drive a vehicle with a carburetor you may notice black smoke from the exhaust; a very rich air/fuel mixture. The ice has the same effect as having the carburetor's choke fully on: the air is blocked off. Again, letting the vehicle sit for several minutes will let the heat of the stopped engine melt the ice and the vehicle works fine.

Throttle plate icing occurs because the shape of the passages past the throttle plates causes a low air pressure area to form. This low pressure area cools the air and any moisture in it can collect and freeze in the passage. The air temperature and humidity conditions have to be exactly right for this to happen, just as they have been on occasion this Winter.

Manufacturers have used several methods to reduce the possibility of throttle plate icing. One method is to pass hot coolant from the engine through passages near the throttle plates to heat the area. Another method uses a temperature sensitive vacuum valve and door inside the air cleaner to draw air from around the engine's exhaust manifold instead of outside. This hot air prevents ice build up.

Some vehicles have no method to prevent throttle plate icing, relying instead only on the heat from the engine. Removing the air cleaner ductwork that pulls in outside air can help. Now the engine receives underhood air heated by the engine. Be sure to re-install the ductwork in the spring or the engine will be using very hot air that decreases both fuel economy and power.