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Old 07-24-2012, 01:17 PM   #21
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because if the computer sees the need to charge more, it controls the iac to adjust idle. When you push the button to request AC, it automatically opens the IAC to a set value. perfect example is in my car that I disabled the ac system on, if I press the AC request it increases idle about 500rpm. Normally the engine will die or come very close to stalling if the IAC is disconnected and you either run a non intrusive running load test on the charging system or if you turn on the AC. since its not doing that with it blocked off, there seems to be an unmetered air leak.

Are you getting any diagnostic codes? maybe P2177, P2187 or even P0300? those are associated with a good vacuum leak.
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Old 07-24-2012, 01:33 PM   #22
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hmm, thanx for your input mate, tho it's not really countering the theory of higher RPMS due to 'higher electrical load', what I said about A/C was just a supporting argument, still tho in reference to what you've said I'm wondering:

does computer use IAC to increase RPM to get better output from alternator?

why does his car run normal exactly when A/C is on, and IAC is completely blocked, you say it should be stalling?

isn't it that computer always control RPM by throttle position and uses IAC just to control amount of air needed in relation to what it thinks is the best mixture at those relatively low RPMs?
(i assume this is not true, but just want someone to confirm it)

Edit: Also, when you come to a stop, the air isn't forced on the radiator anymore, so fan kicks in at high speed to keep the cooling system, which is an additional draw over lowered output from alternator at idle,
and in case of my SPI only taking care of electrical installation fixed the problem of higher revs after coming to a stop, there was nothing else done to it at that time.
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Old 07-24-2012, 02:52 PM   #23
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this is kinda over my head, i just wanted to point out that my alternator and battery are brand new, the IAC valve is blocked (like you guys pointed out) and yes, when i turn the AC on, i don't have any problems

the thing is, i'm not sure if the alternator is delivering correct voltages, i got 15v with the old battery, i only checked once with the new one and it read 14v

it may be unrelated, but whenever i turn the AC on, the big fan at the front of the car (don't know what it's called) automatically starts

anyway, my biggest question here is:
how can the computer increase the RPMs ? if the IACV is blocked, is there another way the ECU can up the revs ??
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Old 07-24-2012, 03:00 PM   #24
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well I don't know about the RPM control, so waiting for others to speak up,


just don't take new alternator as something to scratch electrical power problems from the list, imagine - it's like when you can't drink a Coke at MC Donald because your straw is blocked, and instead of changing straw you go get new cup of Coke but by using the blocked straw you still can't drink it.


If the A/C keeps fan on all the time, it would explain why there is no RMP spike at stop, since radiator fan was already on, and alternator stays at little higher RPM for A/C, vs. when you have A/C off and you come to a stop alternator is working at slower speed and fans kicks in at high speed causing more draw - so computer is trying to compensate.

It may be not the problem in your case, but it definitely was in my SPI Focus.
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Old 07-24-2012, 09:44 PM   #25
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I'll be following this. Your problem sounds very close to mine.
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Old 07-25-2012, 02:06 AM   #26
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so it will idle like it should when the AC is on? if so that makes perfect sense.

ON A NORMALLY operating system with no trouble, if you blocked off the iac valve and turned on the AC, the engine would die.

The fact that it returns to correct idle when the AC is switched on instantly tells me there is a vacuum leak.

there's no "higher load" on the alternator increasing idle speed... the charging voltage of around 15.5 is acceptable because the idle is too high.

Also there is no concern with the fan coming on as soon as the AC is engaged. that's normal operation.

I've not mentioned anything new really from my last post, just made it easier to understand I hope.

look for that vacuum leak.
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Old 07-25-2012, 02:29 AM   #27
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so it will idle like it should when the AC is on? if so that makes perfect sense.

ON A NORMALLY operating system with no trouble, if you blocked off the iac valve and turned on the AC, the engine would die.
In the winter i had to warm up the engine for 1-2 mins so it wouldn't die. The weather is pretty hot now, are you sure the engine should die with the IACV blocked ? It's lowering the RPMs to about ~800 if i start the AC on a "cold" engine, but it's returning to normal after a few minutes.

Anyway, i'll try and do one of those smoke tests, i can't do it on my own, and i'm afraid the car services here have never heard of such a test :)
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Old 07-25-2012, 04:02 AM   #28
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Anyway, i'll try and do one of those smoke tests, i can't do it on my own, and i'm afraid the car services here have never heard of such a test :)

Please do, I think I suggested it to you before, it is truly the only was to find real small leaks. Which is what I have believed this to be from a ways back.
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Old 07-25-2012, 01:18 PM   #29
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the engine idles high when its cold so it can warm up to optimum temperature. that may be why it drops idle when its warm after you've had the AC on. only problem is if you have the iac blocked it shouldn't do that because it lets in more air past the throttle body to increase RPM.
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Old 07-25-2012, 03:56 PM   #30
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I'll be following this as well. My 01 ZX3 does something very similar, when i stop, sometimes it hits around 1500 RPM and then almost bogs out the engine because it goes back down closer to 500 RPM. I'm talking to some buddies at work and checking with my pops today. A vacuum leak does seem like a probable issue for my car and yours. I'll see if i can find a place that can do a smoke test on mine. If you can end up getting it fixed, then post what you needed to do, that would be awesome! Thank you to everyone's input in here
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