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      04-21-2010, 10:21 PM   #45
Grease Monkey
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Drives: 2011 E90 M3,1994 Euro E36 M3/4
Join Date: Apr 2006
Location: Fort McMurray, Alberta, Canada

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Originally Posted by elp_jc View Post
You got it backwards buddy . Closed-loop is when O2 sensors are on board, since they're 'in the loop'.

It's definitely NOT the A/C compressor (I also always turn the HVAC completely off before shutting engine down). Whatever the heck happens is loading the engine A LOT more than the compressor. I engaged A/C once that happened, and as always, couldn't even feel/hear it (and it doesn't feel like it cycles either, due to using swash plates, rather than pistons). When that loading takes place, you can hear the darn engine laboring, like if we had hooked up a TIG welder or something. It has to be either that air injection pump (like a 2nd mode?), or the alternator getting a load from hell. Can't see anything else being the culprit. I'd like to know though.

Hey MBRLVR, please let your car idle until this happens so you can see what we're talking about, and then try to guess what the heck is happening . One thing I can hear is like an exhaust leak, which I know it's not. And just noticed the other day while driving with the window down (rarely happens) it's the same 'leaky' noise when I blip the throttle to rev-match. Maybe it has something to do with that air injection valve, no? It has to be connected to the exhaust, right? We might be getting somewhere .
Well, I didn't post this info asking a question I posted trying to answer a question.

An automotive AC compressor is not Variable displacement with a swash plate.... You are thinking of a hydraulic pump, where a swash plate is used in conjunction with pistons to increase or decrease displacment of the pump. Changing the angle of the swash plate increases or decreases the stroke of the pistons in the pump varying the amount of oil that the pump moves. An AC compressor is constant displacement and cycles of and on based on system pressure. There is a switch that tells the compressor to turn off and on based on the preset low and high pressure levels.

Let me give you some basics of how an AC system works. An AC system cools the air through dehumidification of the air. By dehumidifying/drying the air it's temperature drops. An AC system has a high pressure side and a low pressure side. Low pressure refridgerant in which is cool and in a gas state (R134a in our case) enters the compressor which compresses the low pressure gas and makes it a high pressure gas which is hot in tempeature. The HP hot gas flows through the condensor ( a large heat exchanger mounted in front of the rad) where it condenses into a much cooler liquid state. This cool HP liquid then flows to the expansion valve where is slowly bleeds through and changes state again to a very cool gas which then flows into the evaporator (a heat exchange just in front of the heater core). When the outside air flows over the evaporator it cools and dries the air that is entering the cabin. Since the evaporator sits in front of the heater core the AC can be running and drying the air even with warm air comming out of the vents in the cabin.
Every time the defrost is on the AC is operating to dry the air before it is blown on the windows to prevent further fogging. After the evaporator the refridgerant goes back to the compressor to start the cycle again.

Finally since the oil is mixed with the refridgerant the compressor is constantly cycled so that the seals don't dry out and the compressor remains lubricated. The electronics of all modern vehicles work in this way to extend the life of the AC compressor. I will re-iterate, the only way to stop the AC compressor from cycling is to disconnect the clutch on the AC compressor or pull the fuse.

The "Leaky" sound of your exhaust is just the sound of the engine under load, load from the AC compressor. Please unplug your clutch on the AC compressor or pull the fuse to confirm what I say.

By the way, the alternator would not load the engine like you say.... a voltage regulator on an alternator is infinetly variable.
2011 E90 M3 ZCP - Individual Moonstone/Individual Amarone Extended/Individual Piano Black With Inlay:LINK!!!
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Last edited by BMRLVR; 04-21-2010 at 10:32 PM.
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