Was the battery disconnected or died recently? If it was, minimum throttle must be relearned by the engine computer. Until this is done, the computer doesn't know when you have your foot on the gas pedal and when it must be in control of idle speed.
It knows your foot is off the pedal when you're coasting. The strategy it uses is higher than normal manifold vacuum for seven seconds. High vacuum will occur if you snap the throttle open and closed real fast, but not for seven seconds. You have to coast down from highway speed for the high vacuum to occur for seven seconds or longer.
When these conditions are met, the engine computer looks at the voltage from the throttle position sensor and puts that in memory. The next time it sees that same value, it knows it has to control idle speed.
Most technicians will perform a test drive specifically to prevent this customer complaint after doing any work that requires disconnecting the battery.
Drive on the highway, then coast for seven seconds without touching the brake or gas pedals.
There should also be an idle "flare-up" when you start the engine. If it doesn't run up to about 1500 rpm when you start it, there could be a problem with the idle speed motor or its air passage could be plugged with carbon.
Friday, June 5th, 2009 AT 1:55 AM