Air Ducts Important to Engine Efficiency
Question: I noticed recently that cracks have developed in the ducts that go to my Mazda’s air cleaner. Although the car seems to be running fine, I recently tried to cover the cracks with duct tape; the tape came loose after a few days. How important are the ducts?--B.B.
Answer: These are known as carburetor-preheater ducts, and they are fairly important to fuel economy, emission controls and performance. The ducts tend to deteriorate, often developing cracks or holes after three or four years.
The ducts are designed to provide the carburetor with air warmed to a specific temperature. If the preheater duct is leaking, the air going to the carburetor will be too cold, resulting in a loss of power and in higher emissions.
A leak in the fresh-air duct will also affect air temperature, because the air inside the hood that will be drawn into the carburetor is usually much warmer than the air drawn from the grille, where the air inlet is usually situated.
These ducts are easy to replace. Auto-parts stores now carry replacements for most cars. Replacement is certain to work better than your duct tape, which isn’t designed for a hot, greasy environment.
Q: I own a 1987 Ford Aerostar Mini Van. So far it’s been a workhorse, even though pieces of trim keep falling off. I tried to change the spark plugs on the V-6 engine. I was able to change the plugs on the driver’s side only, because there just is no way to get to the ones on the other side. Is there some kind of special tool, or does the engine have to be dropped to get the plugs out?--L.S.A.
A: Although engines are getting harder to work on, fortunately you can still change a spark plug without disassembling the vehicle. The Aerostar engine compartment, like those in most modern vehicles, has little extra room. As a result, you need to remove the air-intake duct that runs from the air cleaner to the fuel-injection manifold.
In addition, you need to use a socket wrench with a universal joint-type socket of the correct size for the spark plug. You can probably get by with a universal-joint attachment that would plug into the socket.
Q: I have a 1976 Pontiac Grand Prix. When the motor is cold, it starts right up. When the motor is hot, I have to hold the pedal to the floor for it to start. I figure I am getting five to six miles to the gallon. What do you think the problem is?--G.P.
A: You have a stuck choke or a badly malfunctioning choke. A choke operates when the engine is cold by restricting the air flow to the carburetor, causing the air to move faster and pick up more gasoline.
A choke that is stuck shut works just fine when the engine is cold. The problem comes once the engine reaches operating temperature. If the choke is shut all the time, the engine is getting much too much gasoline, accounting for your bad gas mileage.
The chokes on older cars are fairly simple devices that operate with a spring that is sensitive to heat. When the engine reaches a certain temperature, the choke begins to open up. The choke is visible inside the carburetor when the air cleaner is removed, and it should be open when the engine is hot. Your mechanic should be able to determine whether the choke is operating properly.
Ralph Vartabedian cannot answer mail personally but will respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, You section, The Times, Times Mirror Square, Los Angeles 90053.
