Absolutely, if the battery was disconnected, idle speed is going to be too low. You probably read one of my replies as I've posted this numerous times. Everything in the Engine Computer's memory gets lost when the battery is disconnected. Fuel trim numbers and individual sensor characteristics are relearned automatically as soon as the engine is restarted. "Minimum throttle" however needs a specific set of conditions for the relearn to take place. Specifically, it needs to see a steady, low voltage from the throttle position sensor, near 0.5 volts, for at least seven seconds, and it must see very high intake manifold vacuum during that time.
If the computer sees the signal voltage from the throttle position sensor bouncing even a few hundredths of a volt, it knows you have to have your foot on the gas pedal. As such, it leaves engine speed up to you and won't do the relearn. You can achieve high manifold vacuum in the shop by snapping and releasing the gas pedal, but that won't last seven seconds, indicating the conditions for the relearn haven't been met.
The only possible way to have a steady TPS reading and high manifold vacuum for seven seconds is by coasting with your foot off the accelerator pedal. When it sees that, it takes the reading from the TPS and puts that in memory. From then on, every time it sees that same signal voltage, it knows it has to be in control of idle speed. After that, if you do something that produces a lower signal voltage at idle, the computer will start to use that as the minimum throttle voltage. An example would be to replace the throttle position sensor. No two sensors are ever exactly alike, so the computer has strategies to know when to relearn them.
If you have a scanner, you can read the throttle position sensor voltage. It will be close to 0.5 volts at idle and around 4.2 to 4.5 volts at wide-open-throttle. You can also read the "desired idle speed", the "actual engine speed", and the "idle steps", from "0" to "256". Those steps refer to the idle speed motor you replaced. That is not a regular motor like we think of with brushes and a rapidly spinning armature. Instead. it has a magnet on the end of a threaded shaft, surrounded by four electromagnetic coils. The Engine Computer pulses those coils with varying voltages and polarities to create a slowly revolving magnetic field. The magnet follows that, and as it rotates, it extends or retracts the threaded shaft. A pintle valve on the end of the shaft closes or opens an air passage around the throttle blade to adjust the amount of air entering the engine. At the same time, it adjusts the amount of time the injectors are pulsed open to control the amount of fuel.
If you find the automatic idle speed, (AIS) motor is on step "0", minimum throttle hasn't been relearned yet. For a properly running engine, step "32" is typical at idle and when in "park". It will be considerably lower, but not "0" if there is a vacuum leak. That leak causes high idle, and the computer is trying to lower it, without success. To put this in perspective, with a single cylinder misfire on a V-8 engine, you'll find the AIS is closer to step "50". More fuel and air is needed to make up for the dead cylinder.
If you listen next to the idle speed motor, you can usually hear them run for about one second, most often when the engine is stopped. I've also heard a few run when the ignition switch is turned on. In either case, it is being positioned to put engine speed near 1500 rpm for engine startup. That's what gives it the nice "idle flare-up" for a few seconds when you start the engine.
In a Chrysler class I attended, the instructor wanted to demonstrate how much control of engine speed the computer had. On a V-8 Jeep engine, he unplugged the fuel injectors, one by one until he had seven unplugged. With each additional one unplugged, engine speed dropped, the idle steps increased, then idle speed came back up to the "target idle speed". Obviously, the engine ran very poorly, but it did stay at normal idle speed. As I recall, it didn't reach step 200, so there was still some control range left.
There's nothing to synchronize the AIS motor to the computer. If you remove one but leave it plugged in, at some point just before the computer positions it for engine startup, you'll see the valve extend fully, equating to fully closing the air passage, and step "0". In case it moved on its own, that maneuver puts it back in sync. When it's installed, the pintle valve closes fully well before the shaft is fully extended. That's the position where the computer starts counting the number of steps it wants it to be at. The point of this comment is the actual step number is not important. What's important is the step number is going up or down as necessary to maintain desired idle speed.
Failure of the AIS motor is extremely uncommon. You're more likely to run into cut wires or corroded connector terminals if the system isn't working. Back in the early 1990s, it wasn't uncommon to find the air passage plugged with carbon. That resulted in low idle speed even after minimum throttle was relearned. The passage was very easy to clean with Carburetor Cleaner once the AIS motor was removed. I only saw that happen on the 3.0L Mitsubishi-built engines, but by the mid '90s, gas had better additives to prevent that carbon accumulation. I had 440,000 miles on my '88 Grand Caravan and never had to clean that passage. In some other posts you'll see a reference to cleaning the throttle blades, but that caused more of a stumble or hesitation on GM vehicles when trying to accelerate from a stop. It's more important to inspect that air passage.
Mar 23, 2026 at 3:09 PM
