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Hi Guys,

Quite irritating problem, when I start the car, activate AC and pull gear to D then rpm's start hunting by 100-200 rpm's. As soon I switch AC off rpm's stay at ~800 rpm, I'm still on D with gear. If i have AC on and gear is in P then rpm's are normal (800) and not hunting. Does anyone know what can cause this kind of behavior? I have cleaned MAF's with CRC MAF cleaner but this didn't help. Also I added Nulon fuel system cleaner into fuel tank, so far with 250km it hasn't help. I also checked that MAF's are correctly seated and there is no air leaks (with carby cleaner). I'm getting my vag cable tomorrow then I can check codes if there is any. Any advice what to check would be much appreciated.

Regards,
Tom

HVAC is weird, good luck connecting to it.
The AC shouldn't tax the engine like that, wait for a scan. That will tell more. It's particularly hot in Oz at the moment, innit?

...off topic: how do they fill the box oil with extreme temperatures? at 35c the fill plug has to be closed and stop filiing it...

Does the A/C actually work well?
Normally this is the A/C condenser switching on and off and the idle control falling a little behind.
If you scan then look for "A/C Pressure Switch (F129)" as this can easily cause the condenser to flick on and off.

Very odd that it doesn't happen in park. It may be that the system is less enthusiastic about putting the A/C if the car isn't going anywhere.

Most places will offer a free A/C gas check - might just be a bit low.

mavada wrote:...off topic: how do they fill the box oil with extreme temperatures? at 35c the fill plug has to be closed and stop filiing it...

climate controlled garage but the ZF procedure does state that the filling temperature can be as high as 45C in hot climate countries. I did my gearbox oil change a few weeks back and the first time i filled it up T range was 40 to 45C as in 9 hours it didn't have time to cool down below, outside temperature being cca 30C.

A week later, i checked the oil again as i wasn't too happy about filling it at 40-45C, so after leaving the car overnight to cool down, started to fill it when it was at 30C, stopped at around 34C and got an extra 500ml in. So the 10C difference would translate into 500ml less in the gearbox, i don't think this would lead to a failure on a long run. During that week i used the car and the GB it didn't act weird at all, but i didn't push it hard as i was indeed afraid that it might not have enough oil . I was however surprised that it only got those extra 500ml and not more.

Currently temperature is around 30, so it's not hot.
AC works actually really well, cools down cabin quickly without a problem.
I use AC when i do longer trips as during driving there is no problem with rpm's. When coming to stop then I can see how rpm's slowly (small steps) start pumping up and down until it's reached to 100-200rpm's (like it's collecting speed to go up and down).
I'l do scan as soon I receive vag cable. Thank you.

So I think I have found my hunting idle problem when AC is working. Strange that AC works good.
I received my Ross tech cable and did fault scan test which came back with radiator fan control unit 2 -J671- failure.
I did also engine out put test where fan 1 what cools engine starts working, but fan 2 what cools AC doesn't.
I'm wondering can I be sure that control unit is failed and not the fan.
Is there possibility to wire fan directly to battery? Where this connector should be?

Thank you,
Tom

you can wire it directly to the battery, from personal experience with 2 RS6 it is usually the fan that fails, try and spin it and see if spins freely. You can take off the plastic cover which hides the steering fluid bottle, and you'll see there a bunch of wires, then take it in reverse from the fan to the wires and you'll find your 2 fan connectors so you can plug it to a battery.

But as i said, on both my RS's it was the fan that failed, and so it was for 2 of my friends as well on their RS6. However do test the fan by plugging it directly to the battery for peace of mind.

The fan might be a red herring. I can't think why it would affect the idle. Unless it's pulling so much current that the alternator is making the engine strain. Even then, the ECU should be able to compensate the idle.

I suspect you've got a vac leak you haven't found yet.

Shoppinit wrote: I suspect you've got a vac leak you don't know about.

FIXED.

What do your revs sit at in neutral with air con off?
As above but with air con ?
How about in drive with air con off?
As above with air con on?
I'm with Shopp. You have an issue, but air con will drag the engine down a little and probably too low thus making it hunt.

Hi Chunky79, revs in first 3 versions sit at little under 800rpm
When I start AC and gear is in D rpms start jumping between 600 and 800.
I noticed that I don't have this problem when engine is cold.
Is there possibility to check vac leaks with vac pump or smoke test?
I know that MAF's are installed correctly so if there is leak it must be something small.

Thank you very much guys

Is the car mapped ? . could just be a weak mixture on tickover . With the car standing still in drive its pulling the tickover down slightly anyway , and the AC is adding to that demand.

Lots of places air could be leaking in. I keep meaning to buy some of that ignition retardant spray that Audi are supposed to use to find leaks.

When cold the engine runs rich so would compensate a small air leak.

Got a vajcum?

I have wired 280mm fan directly to battery and it's not working. So far has been offered $1000 to get new fan there.
I also have done one log and maybe someone can look this and comment. It's simple drive nothing on WOT as in streets can't really do that.
As far i understand myself fuel trim stays 0 or goes slightly - (- 0.8 ) what should say that I don't have air leaks? is this correct?
Also seems that one of the MAF's is with really low reading, I will get that changed after this holiday season is over.
I'm not sure if car is mapped or not. Specified boost pressure is 1000Mbar and achieved is 1010Mbar. Specified torque was 780nm - is this stock?
But it shows that it has been mapped 119 times, successful at 109 times. There was no Sticker on top of ECU.
Car has Miltek straight pipes with added 100cpi cats, Wagner coolers and upgraded DV.

Cheers,
Tom