1987 Jeep Wrangler XJ - The Invisible Gremlins
Idle Air Controller (IAC)
More commonly known as the idle air controller, this electrical solenoid controls the amount of air passing into the engine at idle. It is found on the throttle body and through the movement of an electrically controlled cylinder. Even if the butterfly in the throttle body is fully closed, air gets into the engine and it still runs. When the Jeep is first fired up, the engine's rpm should go up to around 2,500. This is a sign that the IAC is working. It allows for a fast idle when the Jeep is cold, and also allows for more air (and, thus, fuel) to get to the engine when there is no throttle input. If you've ever crawled your Jeep, it seems to have a mind of its own when encountering an obstacle - it just doesn't want to die. This is due to the IAC. When the IAC fails, often the Jeep will die at stoplights or at trail obstacles. It could also idle high or erratically. We hate to be a sellout, but consult the FSM pertaining to your particular Jeep for proper testing procedures.
Throttle Position Sensor (TPS)
This could possibly be one of the most maligned sensors in the modern world. The throttle position sensor's purpose is relatively simple: provide the computer with a quick input as to demands made on the engine. If you mash the throttle down, this sensor is the first to tell the computer to advance the timing and dump the fuel; kind of like the secondaries on a carburetor, but way more sophisticated. Look at it as vacuum secondaries in a tuxedo. It is always located on the opposite side of the throttle body from the throttle cable. At heart, the TPS is simply a variable resistor and just alters the 5-volt input signal. In practice, it's a nasty, temperamental sensor that will keel over if it gets wet or too hot. Most common symptoms of a dead TPS are hesitation and backfiring. View Related Article
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