Fast Idle in the Computer Warms Engine in a Hurry
Question: I own a 1984 Cadillac Cimarron, which is mechanically the same car as a Chevrolet Cavalier. When I start the engine cold, it idles at 1,200 revolutions per minute and runs fast for three or four minutes. I know this is way too fast, but I have been told by Cadillac service reps that the electronically controlled fuel system is set up to run that way. They claim it would take a different computer chip to have the car run at a lower speed. Is this true, and can you offer any suggestions on how to lower the idle speed? --J.M.G.
Answer: The GM mechanics were telling the truth. The car has a central computer that is set up to run the engine at about 1,200 r.p.m. when it is cold, which is a lot faster than you are probably used to. Some GM cars are set up to idle at 1,600 r.p.m. when cold. The idea is to get the engine warmed up in a hurry to minimize emissions.
The computer should be examining the engine temperature and idle constantly and will slow down the idle within 10 seconds if the engine is at the proper temperature. If the engine runs fast for more than three or four minutes, though, that may be a bit long. Sometimes, you can tap the accelerator pedal lightly and trick the engine into idling down, just like you used to do with a mechanical choke.
The fast idle and the base idle are both controlled by the central computer. There is no “set screw” that determines the idle, so there is essentially nothing to adjust. The only way to change the idle speed or warm-up speed is to reprogram the computer, which would indeed require a computer chip. Even if such chips were available on the market, it would be against the law for a mechanic to tamper with the engine’s emission system.
With computerized control, the car will theoretically always idle at the proper speed. The down side is that the computer will compensate for potential problems by increasing or reducing fuel to keep the idle speed at the proper setting, thereby potentially masking other problems.
Because the car seems to be performing properly, the fast idle speed is an irritation that you may just have to live with.
Q: I bought an Oldsmobile Cutlass Ciera nine months ago. About a month ago, I noticed the smell of burning plastic and took the car to the dealer, who found a plastic shopping bag had attached itself to the tail pipe. I have had the car in twice to have it removed and was told that it would eventually burn itself off. Are there any dangers in these fumes? When I turn on the air conditioner, the odor is really strong. --M.E.
A: Depending on the chemical composition of the plastic, it could be a toxic fume. You ought to be able to have it removed by the dealer. Otherwise, you ought to have the car vents or windows closed when you are idling. When the car is in motion, it should not be a problem, because the plastic is presumably in the rear of the car and the fumes should be swept away behind the car.
Mailbag
A recent reader letter complaining about a run-on or dieseling problem in a Mercedes Benz Turbo Diesel drew some potentially valuable reader suggestions. One Mercedes expert suggested that the problem stems from a faulty vacuum line or vacuum pump; the Mercedes uses a fuel pump that relies on engine vacuum, rather than an electric fuel pump. This means that when the ignition switch is turned off, the supply of fuel is not automatically shut off.
