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Old 10-23-2015, 09:36 AM   #1
paulca
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Unstable idle when A/C cuts out

My car has developed an unstable idle when the aircon cuts out. Here are the symptoms.

AirCon OFF: 650rpm stable as a rock.
AirCon ON:
- Compressor pump off: 650rpm stable as a rock
- Compressor kicks in: 800rpm stable
- Compressor knocks off: idle drops to 500rpm or below, engine labours, shakes, then stablizes again to 650rpm. Sometimes the load sensor panics and revs the car to 1000rpm in response to the idle drop, but usually it doesn't. Has caused me to stall the car twice now and it's annoying.

If I rev the car with the Air Con off to 800rpm and the release the pedal it settles down normally to 650rpm. It is only when the A/C pump stops that the idle falls too low.

Any ideas? It's really, really annoying as without the aircon the windows steam up pretty quickly here at this time of year, but with it on, the pump cycles on for 2 or 3 seconds 4 or 5 times a minute. I have taken to turning it OFF while waiting in traffic, but that is not a solution.
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Old 10-23-2015, 10:12 AM   #2
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Quote:
Originally Posted by paulca View Post
My car has developed an unstable idle when the aircon cuts out. Here are the symptoms.

AirCon OFF: 650rpm stable as a rock.
AirCon ON:
- Compressor pump off: 650rpm stable as a rock
- Compressor kicks in: 800rpm stable
- Compressor knocks off: idle drops to 500rpm or below, engine labours, shakes, then stablizes again to 650rpm. Sometimes the load sensor panics and revs the car to 1000rpm in response to the idle drop, but usually it doesn't. Has caused me to stall the car twice now and it's annoying.

If I rev the car with the Air Con off to 800rpm and the release the pedal it settles down normally to 650rpm. It is only when the A/C pump stops that the idle falls too low.

Any ideas? It's really, really annoying as without the aircon the windows steam up pretty quickly here at this time of year, but with it on, the pump cycles on for 2 or 3 seconds 4 or 5 times a minute. I have taken to turning it OFF while waiting in traffic, but that is not a solution.
This is not unusual for a small displacement engine since the compressor does take a fair bit of power to kick in. The surprising part is that you just noticed it. I have had several cars that did this and it is never an issue and there is nothing that can be done about it.
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Old 10-23-2015, 10:23 AM   #3
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Yea, but it's not when the compressor kicks IN, the load sensor covers that nicely and the engine revs up to 800rpm. It is when it cut out that the revs fall too far and the engine shakes.

It used to do it a little bit on the odd occasion in the dead of winter when the window heater, lights, fan and everything else was on.

Recently its doing it all the time now.
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Old 10-23-2015, 10:39 AM   #4
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Quote:
Originally Posted by paulca View Post
My car has developed an unstable idle when the aircon cuts out. Here are the symptoms.

AirCon OFF: 650rpm stable as a rock.
AirCon ON:
- Compressor pump off: 650rpm stable as a rock
- Compressor kicks in: 800rpm stable
- Compressor knocks off: idle drops to 500rpm or below, engine labours, shakes, then stablizes again to 650rpm. Sometimes the load sensor panics and revs the car to 1000rpm in response to the idle drop, but usually it doesn't. Has caused me to stall the car twice now and it's annoying.

If I rev the car with the Air Con off to 800rpm and the release the pedal it settles down normally to 650rpm. It is only when the A/C pump stops that the idle falls too low.

Any ideas? It's really, really annoying as without the aircon the windows steam up pretty quickly here at this time of year, but with it on, the pump cycles on for 2 or 3 seconds 4 or 5 times a minute. I have taken to turning it OFF while waiting in traffic, but that is not a solution.

What is your rom calid hopefully its A01G or A02G if not see if dealer will update ecu rom if your on older 700G or 900G rom (assuming manual transmission)

if you have access to a tuning device it can be fixed. better than stock rom update by increasing base idle to 700 rpm and adjusting the port/direct injection tables to run full direct injection at idle instead of a port/direct split.

Last edited by steve99; 10-23-2015 at 10:56 AM.
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Old 10-23-2015, 11:33 AM   #5
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Originally Posted by paulca View Post
Yea, but it's not when the compressor kicks IN, the load sensor covers that nicely and the engine revs up to 800rpm. It is when it cut out that the revs fall too far and the engine shakes.

It used to do it a little bit on the odd occasion in the dead of winter when the window heater, lights, fan and everything else was on.

Recently its doing it all the time now.
Ahhh I didn't catch that. Reading impaired this morning I guess.
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Old 10-23-2015, 02:11 PM   #6
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Sometimes when the ac is on and im idling the revs keep bouncing between 650 and 800 over and over and over again. Same issue as you?

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Old 10-23-2015, 02:17 PM   #7
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I asked about this in my local dealer and they said to me: " we dont have such tools for fix it"
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Old 10-24-2015, 03:38 AM   #8
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Sometimes when the ac is on and im idling the revs keep bouncing between 650 and 800 over and over and over again. Same issue as you?
That part is normal. The A/C compressor requires quite a lot of electrical power, so the engine load sensor adds a tiny notch of throttle to pick the revs up, otherwise the drain on the alternator would stall the engine, potentially.

My issue is when the A/C cuts out and the revs return to 650rpm they dip much lower causing the engine to vibrate.

On my way home last night it had a tffy fit at a set of lights. Revs dropped to below 500, then the car lurched and the revs jumped to 1000, then fell back to 500, then jumped to 1000 again, did it three or four times in a row before settling again, just before the A/C kicked in again and it all started over. Thankfully it's not normally that bad.
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Old 10-24-2015, 03:41 AM   #9
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Quote:
Originally Posted by steve99 View Post
What is your rom calid hopefully its A01G or A02G if not see if dealer will update ecu rom if your on older 700G or 900G rom (assuming manual transmission)
If it was a firmware issue it would have done it from the start, wouldn't it? I recall it doing it once or twice before when I had a lot of electrical equipment on; A/C, window heaters, lights, wipers, HiFi, but it started doing this consistently since I tracked the car.

One easy fix might be to have the ECU reset. Maybe it has learned some bad habits on the track.

The other thing I'm considering, as the symptoms are very close, is clutch drag... but then again, it does this when it's in neutral, so that's out.
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Old 10-24-2015, 08:22 AM   #10
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Originally Posted by paulca View Post
If it was a firmware issue it would have done it from the start, wouldn't it? I recall it doing it once or twice before when I had a lot of electrical equipment on; A/C, window heaters, lights, wipers, HiFi, but it started doing this consistently since I tracked the car.

One easy fix might be to have the ECU reset. Maybe it has learned some bad habits on the track.

The other thing I'm considering, as the symptoms are very close, is clutch drag... but then again, it does this when it's in neutral, so that's out.
Yes you would assume if it was early firmware problem would be more consistent, but maybe it only show up under certian circumstances. like ambient temperature conditions , certain fuels, A/C and heavy electrical loads lights on ect.

I dont think ECU reset will help as ECU continually learns .
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Old 10-24-2015, 08:51 AM   #11
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Did you try cleaning your MAF sensor?
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Old 10-24-2015, 10:08 AM   #12
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Happened to me with the car entirely stock and still does it after turbo. Just crack the windows and drive fast with the AC off.
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Old 10-24-2015, 07:39 PM   #13
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Happened to me with the car entirely stock and still does it after turbo. Just crack the windows and drive fast with the AC off.
I like your style. Doesn't work in rush hour traffic unfortunately. Crack windows = cold. Closed windows = steam up. If the A/C is off.

It was going so well too, I'd figured out if you leave the climate control in AUTO mode with A/C on, it takes care of everything really well.

Now I have to turn the A/C off in slow traffic, if I want peace without laboring, and remember to turn it back on, before it steams up again. Not very auto really. But workable.
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Old 10-24-2015, 08:20 PM   #14
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I was having this issue a while back before I had the dealer service a cam chain cover seal leak. I've notive the acc belt sometimes will slip on the compressor pulley while idling with AC on. After getting my car back from having the leak fixed it has stopped doing that completely. I'm assuming the belt tension was tighter after having been serviced than before.

Checking and tightening the belt tensioner may help.
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