The next thing is to observe if the idle speed jumps up to 1500 rpm for two or three seconds right after starting the engine. My experience has been that when minimum idle has to be relearned, you don't get that "idle flare-up" that every engine does, and usually the engine won't even start on its own unless you hold the gas pedal down just a fuzz.
If you do get the idle flare-up, that proves the automatic idle speed motor is working and the computer has control over it.
A better approach is to find a mechanic with a scanner to see what "step" the automatic idle speed (AIS) motor is on. This isn't a motor like you normally think of with brushes and a spinning armature. It's four coils of wire that are pulsed with varying voltage and polarity to set the armature to various positions. As it slowly turns, a threaded rod extends or retracts a pintle valve to expose more or less of an air passage around the throttle blade. At the same time the computer does that it also commands the fuel injectors to remain open slightly longer to spray more fuel. That's how it adjusts idle speed.
The computer will position the AIS motor to one of 256 positions. For a properly running engine, step 32 is about typical. On a V-8 engine, that motor has enough control to keep the engine running when six cylinders are disabled. Obviously it won't run well, but that's how much control it has.
If you find the AIS motor on step 0 or very low, minimum throttle hasn't been relearned. If you find it very high, as in step 50 or more, it is trying to make up for something that is causing the engine to not produce sufficient power. A misfire is the most common cause but you should feel that at idle and notice the power loss on the highway.
I use the Chrysler DRB3 scanner. That also has a function that allows you to run the idle speed up to 2000 rpm in 200 rpm increments by just pressing a few buttons. If the engine doesn't speed up, either the AIS motor isn't turning or the air passage is plugged with carbon. The motor circuit is monitored by the computer for continuity on all four wires but it is not monitored to see if it's actually turning, so in the absence of any related fault code, you might just have to remove it to check the passage.
Some clues include observing how the problem came about. If it started immediately after some other service, it's unlikely the blocked passage is the problem. That was a fairly common problem on the 3.0L Mitsubishi engine in the minivans and cars in the '90s but you don't hear much about that anymore with the better additives in today's fuels. My experience has been that the motors are not defective very often but I do read a lot that people replaced them to solve a problem. The same part was used in GM cars for many years. Haven't heard of them having much trouble either.
One last thing to consider is the brake light switch. There can be up to three separate switches built in. Only one section is for the brake lights. One person solved their idle speed problem by holding the brake pedal up with their toes while doing the coasting. My thought was their cruise control wouldn't set if that part of the switch failed to stay on when the pedal was released, but they said their cruise control always worked fine.
The DRB3 scanner will also show the state of the brake light switch under the "Cruise Control" menu.
If you do remove the AIS motor to inspect the air passage, you can move the pintle valve by hand. It moves really hard but it can be done. Squeeze hard on it to retract the valve about 1/8". (That's a lot). If the idle speed is too high when you restart the engine, and it stays there, replace the motor. You still might have to hold the gas pedal down to get the engine started. Once it's running, if the idle speed remains too high from the valve not extending properly, it isn't going to be the most fun to drive because the computer won't be commanding the extra fuel to go with that extra air. Idle speed will be high but power under load, as in accelerating, will be down.
Normally when you're driving, the AIS motor closes completely and the computer commands the correct amount of fuel based on the amount of air going through just the throttle blade. The valve needs to open quickly when it looks like engine speed is going to drop too low but the computer doesn't do anything until your foot is off the gas pedal. When minimum throttle is learned, the computer puts the throttle position sensor's voltage in memory. Anytime after that when it sees the same voltage, meaning your foot is off the gas pedal, that's when it knows it has to be in control of idle speed. Otherwise it leaves engine speed up to you.
If this problem started occurring randomly a while ago, carbon in the air passage is a more likely possibility. I've also read where people recommend using a toothbrush and carburetor cleaner to scrub a film of carbon buildup off the back side of the throttle blade. I'm not sure why that affects performance but it seems to solve some idle speed issues and some stumbling and hesitation problems on acceleration.
Also look for leaking vacuum hoses. The extra air will cause an increase in engine speed, and the computer will respond by trying to lower it down to normal. It does that by closing the AIS motor AND by reducing the amount of fuel spraying in. Now you may have the correct idle speed with insufficient fuel, and anything that causes a momentary drop in speed results in stalling. If you watch a normally operating AIS motor on the scanner, you'll see it constantly bouncing up and down a few steps while the engine speed sounds to you like it's holding perfectly steady. The computer is responding to something, and that "something" might be what is leading to the intermittent stalling. Two of the more common causes are having the transmission in gear which lowers idle speed, and the AC compressor kicks in putting a load on the engine. Even turning the steering wheel makes the power steering pump work harder which loads down the engine. A sudden need for more current from the alternator can take a couple of horsepower. The computer is responding to all of those things. That's why it's normal for the AIS steps to be constantly adjusting.
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Thursday, March 29th, 2012 AT 3:29 AM
