Not true. I do not doubt you got your engine running better, but your conclusion is news to the engineers who designed the system. The idle speed motor is a motor-controlled vacuum leak. The engine computer opens that valve in "steps", to increase air flow at idle, and increases the on-time of the injector pulses at the same time, to increase fuel. Those two together are how the computer adjusts idle speed.
One of the items of confusion on Chrysler products is the low idle speed after the battery was disconnected or run completely drained. Most of the data lost from memory in the engine computer is rebuilt as soon as the engine is restarted and the vehicle is driven, but that does not apply to "minimum throttle". You have to perform a very specific step to initiate the relearn to take place for minimum throttle. Until that is done, the engine may not start unless you hold the accelerator pedal down 1/4", you will not get the nice "idle flare-up" to 1500 rpm at start-up, and the engine will tend to stall at stop signs.
When minimum throttle hasn't been relearned, the IAC valve will remain fully-closed at step "0". The engine will run just fine at any speed when you hold the accelerator pedal down just a little. When minimum throttle is learned, the engine computer takes a reading from the throttle position sensor when it knows your foot is off the accelerator pedal. Any time after that when it sees that same reading, it knows it has to be in control of idle speed. At any reading higher than the minimum throttle TPS voltage, even as little as 0.01 volt higher, it knows you have your foot on the pedal, and you are in control of engine speed. That means the air flow into the engine is being controlled by the throttle blade in the throttle body. The IAC valve is not part of the story. If you read the step number the computer has set that valve to, you will see it is either on step "0", meaning fully-closed, or it is around step 20 - 25, in preparation for the idling vehicle coming to a stop. If the valve were to become gummed up, (which is very uncommon), and stuck in the closed position, the only symptom would be too little air at idle. That would cause stalling. "Loading up", "excessively rich mixture", or "black smoke from the exhaust", are symptoms that have never been associated with the IAC valve. If the valve were to stick in the open position, you would have a lean condition, and possibly a high idle speed.
To verify the IAC valve is working, (Chrysler's name for it is the "automatic idle speed motor", or "AIS motor"), the engine can be run up to 2000 rpm, in 200 rpm increments, with a scanner. That will prove the motor is working, the wiring to it is good, the computer has control over that valve, and the air passage is not plugged with carbon.
As further proof the AIS motor will not cause too much fuel, when minimum throttle has not yet been relearned, you will find the computer keeps it at step "0" all the time, meaning it is fully-closed. It forces it there so it knows exactly its physical position, and since it knows later exactly how many steps it has pulsed it to, it will be in calibration. There is no means of monitoring the AIS motor's position, so to insure it remains in calibration, the computer regularly sets it back to "0" steps when you are driving at highway speed. It pulses it closed more than necessary to insure it made it to fully-closed, then it counts the number of pulses it opens it up, and assumes the motor responded appropriately. The live data screen on the scanner will show the actual engine speed, and the "target idle speed". When the computer desires a higher idle speed, it will pulse the AIS motor, then observe whether engine speed responded as expected. If it did not, it simply continues to pulse the motor one way or the other as necessary.
If the AIS motor were to be open while driving, the air flow through it would be so tiny compared to the air flow through the throttle blade, it would be insignificant. Any lean condition would be detected by the front oxygen sensor, then the engine computer would adjust the number of milliseconds it holds the injectors open during each pulse. A fully-closed AIS motor is the normal condition at off-idle, so it cannot cause an excessively rich condition.
As a point of interest, while the terminology is different, the physical motor is identical to that used on GM's engines, and they work the same way. The motor is a "stepper" motor. Instead of brushes and commutator bars, they have four sets of coils that are pulsed with varying voltages and polarities. The armature follows those varying magnetic fields, and as it rotates slowly, it turns a threaded shaft that extends or retracts a pintle valve that controls air flow past it. As usual, Ford does it quite differently.
For your loading issue, you have to look at how the engine computer calculates the amount of fuel needed. Every other manufacturer in the world uses a mass air flow sensor, with a few exceptions for one engine here and there, to measure the weight of the incoming air. For those engines, it is critical there cannot be any air leaks in the fresh air tube, or any vacuum leaks. If any air sneaks in that does not go through the mass air flow sensor, it will not be included in the fuel metering calculations. Chrysler is the only manufacturer that has never needed a mass air flow sensor to make their engines run right. Their main fuel sensor is the "manifold absolute pressure", (MAP) sensor. This sensor is also identical to what GM uses. In fact, GM had a huge failure rate right after they sold them to Chrysler. They caused a lot of crank/no-start conditions on Chrysler products. On GM engines, they just caused problems from the incorrect barometric pressure readings they took.
With the new, redesigned MAP sensors, failures have been very uncommon. Most of the trouble is related to wiring or to mechanical problems associated with that sensor. We used to see cracked or deteriorated vacuum hoses going to the MAP sensors but by the mid 1990's, those were eliminated. Now the MAP sensor is plugged into the side of the throttle body. That eliminates vacuum leaks in its hose, but it contributes to other problems. Often it has to be unplugged while performing some other repair. We still have to look for corrosion between mating terminals in the connector. Unplugging and reconnecting the connector can cause the terminals to scratch clean spots through the film of corrosion that creates a good connection, but that is temporary. Reconnecting it can also allow a speck of dirt to get between two mating terminals.
When a MAP sensor does fail, it can do that in a variety of ways with different results. They are fed with 5.0 volts, then their acceptable range of signal voltage is approximately 0.5 to 4.5 volts. Anything outside that range will set a diagnostic fault code. More importantly, an incorrect signal voltage that is still within that acceptable range likely will not trigger a fault code unless some other defective condition is detected. The engine computer will accept the voltage, and use it to calculate fuel needs. While we do not do it, we could measure engine speed with MAP sensors because they are sensitive enough to see each tiny pulse of increased vacuum when a piston takes a gulp of air. A signal voltage that is incorrect by as little a 0.1 volt will have a noticeable effect on engine performance.
Given all these things to consider, I know how easy it is to work in this area, and inadvertently cause or cure a problem. Naturally I blamed it on what I thought I had done. Since you are not under a time restriction from working on a customer's truck, if this were mine, I would stick the old AIS motor back in, then see if the problem returns. If it does, you can have confidence in your solution. If the engine still runs okay with the old motor, you will know there was some other cause, and it may show up again in the future.
For all the work you did, you probably read the diagnostic fault codes by now. Did you find any related to the MAP sensor? The engine computer can also set fault codes for electrical problems related to the AIS motor. The wire inside them is pretty tough, so an electrical failure will usually be caused by the connector terminals first, then, based on the age more than the mileage, rubbed-through wires second.
Regardless if there is a fault code or not for the AIS motor, you can also observe if it is working by listening to it. Have a helper stop the engine while you are listening under the hood. As soon as the engine stops, you will hear the buzzing sound it makes when the computer runs it in fully, then back out a calculated amount. That is in preparation for the next engine start and the idle flare-up to 1500 rpm. You can also plug in the old motor, then watch the valve move when you turn the ignition switch on and off. If you see the valve move at any time, the system is working. On some engines, the valve resets to "0", then opens for starting when you turn the ignition switch on, but most are when you turn the switch off.
Do not try to run the engine with the AIS motor removed. This system was covered to great detail by the Chrysler instructors. To show how much control the computer has over idle speed, they used to unplug injectors, then watch how many steps the computer needed to set the AIS motor to, to get idle speed back up. There is enough control to keep a V-8 engine running at the desired idle speed with six injectors unplugged! Obviously it did not run well, but it did maintain idle speed without reaching its maximum step of 256. If you totally remove the AIS motor, the air passage will be open much more than step 256, and with eight working cylinders, you do not want to risk pistons going through the hood!
As a final point of interest, we used to perform cylinder balance tests by disabling one cylinder at a time, and watching how much idle speed dropped for each one. We cannot do that today because the computer keeps idle speed correct, but we can watch how many additional AIS steps it takes to maintain idle speed. A properly-running engine will typically be around step 32. That will go to around step 50 with one cylinder disabled.
Sunday, November 12th, 2017 AT 7:05 PM
