By "throtle control sensor" I assume you mean the throttle position sensor. That actually has very little to do with fuel metering and nothing to do with idle speed. It simply tells the Engine Computer where the throttle is and its direction and rate of movement. The computer constantly compares sensor readings and operating conditions to each other, in part, to determine when there's a problem. For example, it knows that when the throttle position sensor has a low voltage, as in it's near idle, the mass air flow sensor had better not be reporting large intake air volume and the map sensor had better not be reporting high load on the engine. 99 percent of the time a problem with the TPS will be detected electrically and a fault code will be set. Once in a great while though it can report an incorrect value that is within the acceptable range. That can result in surging or "hunting" at lower engine speeds.
Idle speed that's just too high is caused by a mechanical issue that is almost always related to a vacuum leak. There's a couple of ways to verify that. One is to pinch off each vacuum hose one at a time. If pinching one drops the idle speed, follow it to its branches and pinch off each one to narrow it down. The problem is that doesn't work if there's an intake manifold gasket leaking. For that you may be able to spray water over the engine while it's running and still cold to see if it gets sucked in somewhere. You can also use a smoke machine to inject a white non-toxic smoke at 2 psi into a vacuum port, then you watch for where it sneaks out. Worn bushings in the throttle body assembly can cause a vacuum leak too.
Tuesday, February 11th, 2014 AT 3:35 PM