If the solution was that simple, he would have done it before trying to sell the vehicle; right?
The most common cause of high idle without a corresponding increase in power is a vacuum leak. That can be due to a loose or cracked vacuum hose, a leaking intake manifold gasket, or a sensor problem. Start by pinching off various soft rubber vacuum hoses or spraying water over the engine while it's still cold to see if it gets sucked in somewhere.
To determine if this is due to a sensor providing incorrect readings, you need a scanner to view the idle control "steps". Typical for a properly-running engine is around step 32 out of a range of 256 steps. If you find it's at step "0", the Engine Computer is trying to lower idle speed without success. If you find it's at a higher step, as in step 50 or more, the computer is requesting the higher speed in response to something. You need to compare sensor readings to what seems normal. For example, if it's 50 degrees outside but the coolant temperature sensor says it's minus 20 degrees, the computer is going to raise idle speed. Minus 20 degrees is a valid value so no diagnostic fault code would be set for that sensor circuit.
Friday, October 9th, 2015 AT 3:09 PM