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Yeah, but it won't prove anything Posted by RS [Email]  (#15)  [Profile/Gallery]
(more from RS) on Fri, 3 Jan 2014 16:40:42
In Reply to: Re: Could I simply just unplug the AIC to isolate?, Saabath89  [Profile/Gallery]
, Fri, 3 Jan 2014 16:06:14
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When the AIC on an LH 2.4 car, the AIC valve will go to the unpowered position. (If you disconnect and remove your AIC valve, look in it and you'll see that there's an open slit that air can pass through. They did that so, if one of the wires to the AIC valve breaks or the transistor that controls the electrical pulses to the AIC valve dies, the car won't die.) When the valve is unpowered, the car will idle around 1500 RPM.
Here's a bit of setpoint control 101 from an instrumentation and control engineer: the idle fluctuates when there's a vacuum leak because the AIC control loop is tuned for an induction system with no vacuum leaks.
When the throttle position sensor indicates that the throttle is closed the ECU controls the position of the AIC valve to maintain its setpoint idle speed.
If there's a vacuum leak, idle will tend to creep up because additional unmetered air is bypassing the throttle plate. The ECU will overshoot in closing the AIC valve and the idle will drop too much. Then, because the idle is too low, the ECU will compensate by opening the AIC valve ... but, because the control loop isn't tuned for a system with a vacuum leak, it'll overshoot opening the AIC valve and the idle will rise too much. Repeat the cycle until you press on the throttle and the ECU is out of idle mode.
In the controls biz, that repeated up and down cycling is called "porpoising" and its one mark of a badly-tuned control loop.
Compound that with the fact that the ingress of air through the vacuum leak source (or sources) isn't constant because the intake manifold pressure varies due to the engine speed change and changing amount of backpressure through the constantly-adjusted AIC valve, and you have a system that's pretty much impossible to control to maintain a steady idle.
(Among other things, I design aeration control systems for wastewater plants. Air flow control is a difficult thing, partially because it's compressible. Water flow control is a lot easier because its not compressible under conditions that any of us is likely to experience.)
A happy 900T is one that doesn't have any vacuum leaks (or rust holes ... but that's a subject for a different discussion :-)
->Posting last edited on Fri, 3 Jan 2014 16:51:50.
, Mon, 30 Dec 2013 12:44:12, Tue, 31 Dec 2013 10:01:29, Fri, 3 Jan 2014 16:06:14, Fri, 3 Jan 2014 16:40:42 <-- Viewing This Message, Sat, 4 Jan 2014 12:29:15
, Tue, 31 Dec 2013 22:25:26, Wed, 1 Jan 2014 05:36:21
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