Excessive rpm increase when AC on?

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  #1  
Old 07-31-07, 08:49 PM
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Excessive rpm increase when AC on?

If idling rpm goes up by 300 or 400 when I turn on AC, what should I suspect?
Thanks.

97 Checy Lumina, 6 cyl, 140k miles
 
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  #2  
Old 08-01-07, 08:02 AM
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Some increase is normal because the A/C compressor needs engine power to run. Unless it's affecting the cars ability to drive, I wouldn't worry about it.

If it gets worse, it could be your IAC (idle air controle valve) or a fast idle solenoid.

Does the A/C work ok? If the compressor/pulley is failing, it may require more engine power to turn the normal.
 
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Old 08-01-07, 09:12 AM
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Thanks for the reply. I think I need to explain more about the problem.

Idling rpm (at Park) was going up to 2000 for no apparent reason. Each time, I restarted the engine and idling returned to normal. It was becoming more frequent. No error code on OBDII reader. Used TBI spray, gas additive. No use. Tips from this forum were to replace IAC and/or TPS. I replaced IAC.

The first day (yesterday) was rough. At traffic light stop, idling was 1400 (at Park). I cut off AC. Dropped to 900. This persisted whole day. (I didn't have this problem before replacing IAC) Also, just once, with ac off at traffic light stop, idling went up from 900 to 1400 for no reason.

Today is the second day. In the morning, I had no problem with AC running. I will see what happens on the way back home. Each way is 11 miles, including 3 miles on superhighway.

Since there is no error code, it's no use to take the car to dealer. Is that right?
Thanks.

PS: Compressor was replaced with a brand new unit 2 years ago.
 
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Old 08-01-07, 09:22 AM
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Did you do a IAC learn? I know on my Maxima, when you replace the IAC, you have to do a learn procedure.
 
  #5  
Old 08-01-07, 09:24 AM
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I don't know the IAC learn procedure. On the sheet inside the package I read that 40 mph at normal temperature will set IAC. I will check shop manual this evening. Thanks.
 
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Old 08-01-07, 04:03 PM
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Just the other day, as sort of some anomoly, MY engine rpm showed it went up about 4-500 rpm above normal while idling. It only did it that one time. I rolled down my window to listen. I am convinced my engine was not really running any faster than normal. Somehow the gauge was reading or giving false information.

That said, does YOUR engine speed get faster or are you JUST going by what the tach says?
 
  #7  
Old 08-19-07, 10:18 AM
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Engine speed is really getting faster. I can tell it at traffic light. When I release brake pedal on green, the car jerks forward. And a few other signs.

By the way, it has gotten much better by now after the new IAC was given a couple of weeks of learning time. But it is still not perfect. I think there is something wrong with the system. I did some internet reading. It can be throttle position sensor, oxygen sensor, EGR, maf, etc. Or a vacuum leak. I have given up on this until an error code registers.

Thanks.
 
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