Erratic idle speed suggests a vacuum leak. The most common cause would be a disconnected or cracked hose. Many of them are hard black plastic, about half the diameter of a pencil. Certain gaskets can also cause a vacuum leak. If the leak is bad enough, you might also notice that you have to push harder than normal on the brake pedal. Power brakes use engine vacuum for the power assist, so low vacuum means less power assist to the brakes.
Listen under the hood for a hissing sound while the engine is running. You can also use a spray bottle to spray water over the hoses and gaskets while the running engine is still cold. If the idle speed slows down or changes, or you see the water getting sucked in, you've found the leak.
A defective MAP sensor can also cause the symptom you describe, but once the readings go outside of preset limits, it will be detected by the engine computer, and it will turn on the "Check Engine" light. At that point there will be a diagnostic fault code memorized in the computer. Do not disconnect the battery at that point until the code(s) have been read or that valuable information will be lost.
You can also make the computer display the diagnostic codes yourself to see if there are any stored. Only codes that affect tail pipe emissions will cause the Check Engine light to turn on. There are a lot of other codes that will direct your mechanic to the circuit with the problem, but won't cause the light to be on.
Sunday, December 6th, 2009 AT 4:14 AM