idle speed stays same when A/C turned on

My 2002 wrx has a history of misfire codes (e.g. P0302) every 30,000 miles or so and the solution has always been to replace spark plug.
Right now, it has P0302 code when the car is driven for half an hour or more. If I turn off the car for a while, it runs fine again.
Also, the idle speed does not increase when A/C is turned on. I wonder why.
What is the mechanism that increases the idle speed when A/C is turned on? Does the A/C button sends a signal to the ECU to increase idle speed, or does the ECU senses the extra load on the engine and increases idle speed to prevent stall?
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On 9/1/2014 1:04 PM, bob wrote:

Apples and oranges but, out of curiosity, I checked the idle speed on my 2008 Outback with AC on/off: the speed remains constant to within the discernible accuracy of the dash tachometer.
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On 02/09/2014 2:38 PM, John McGaw wrote:

Yeah, I noticed that neither my current 2008 Tribeca, nor my old 2000 OBW had their idle speeds increase when the AC was on. I think the AC is probably running in a very low power mode during idle. Or even perhaps disengaged during that time?
    Yousuf Khan
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On 9/2/2014 5:39 PM, Yousuf Khan wrote:

Well, with mine I can faintly hear the clutch engaging and the air coming out of the vents gets cold so it is certainly doing something. With the clutch engaged there really is no 'low power' mode - it is either pumping Freon or it isn't. It seems likely that the engine computer is simply very good at keeping the idle speed where it wants it to be. This wasn't so in the old carbureted days when there was an idle speed solenoid to keep the engine from stalling.
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wrote:

Correct - the engine speed does not NEED to increase with the AC on. The throttle needs to open, or in the case of a GDI engine, more fuel needs to be injected to increase the power output enough to MAINTAIN engine speed. The ECU on today's cars is extremely powerful and fast.
On carbureted engines, generally there was only on or off for the "idle boost"
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On 02/09/2014 9:06 PM, John McGaw wrote:

Yeah, it's more than likely that the idle speed is already optimized for AC operation.
Interestingly, I found out some time ago, that the AC needs to operate in the winter to defrost the windshield, as it is used to dehumidify the natural moist air coming through the air vents.
    Yousuf Khan
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Well, I like the idea of the AC operating this way. It's a good idea to have the AC system run every once-in-a-while during the winter and other cooler times to keep he system lubricated.
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On 9/2/2014 11:39 AM, Yousuf Khan wrote:

Subaru messed up big time on this one. My Subaru would have a noticeable drop in power whenever I had the misfortune to be compelled to turn on the AC. As they say, they ain't building them like they used to. Subaru could easily program their control system to replicate their cars of the 80s but it's this lack of attention to details that will be the downfall of Subaru.
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On 02/09/2014 9:18 PM, dsi1 wrote:

I used to notice a drop in power on my old 2000 OBW too, but that only started happening after it had gotten very old (on its 11th or 12th year). I think it had more to do with natural engine compression loss reducing the power overall. I used to actually turn off the AC on certain uphill climbs around the end.
    Yousuf Khan
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I had bought an ac cutoff back in the 80s but never got it installed on my datsun. That automatically turned off the ac when you wanted maximum acceleration. That was vacuum operated as well as the ac idle increase. I don't notice much change in a slow steady acceleration in my outback, but it was more pronounced in my cavalier. Yes, I do the test in every car I drive. It's easier to check up hill.
Greg
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