I noticed that usually on idle, my Octavia 2.0 tdi running around 1000 rpm. It is not regeneration of dpf. Sometimes it is normal at 750rpm, but not often. I was thinking that it is because of AC and another big electricity consumers, but when I turn Ac off it still running high. After turning AC off, sometimes I press acceleration pedal to the metal (quick press) and then revs come to normal around 750rpm. AnBody have the same issue?

I've only ever seen this when the car's regenerating the DPF - at least once a week on my car.  

 

Are you 100% certain it's not that (i.e. is the Start/Stop function disabled as displayed by an 'A' in a circle on the dash)?

Edited 10 June, 201412 yr by Smeghead

1k rpm is almost certainly a regen attempt. Should happen every few hundred miles but not be like it all the time.

They do regen alot when new and it calms down over the passage of time to every 3-400 miles.

Is your gearbox manual or DSG?  I can replicate what you describe very easily on my DSG by lightly pressing the brake pedal while it's in Drive.  Press harder and the revs drop back.

My Superb II did this as did my Mk 5 Golf GT TDi 170. If you have the heating on, air con, heated seats / windscreen etc and the battery is not fully up to the job of powering them, the car raises the rpm to assist (& protect) the battery by generating additional juice to run these "Covenience Consumers."

Try running the aircon immediately after you start the car on a hot day, the car will raise the rpm straight away as the aircon blower fan draws a lot of current which is above that sustainable by the battery alone. The car will idle at 1000rpm continuously until the desired temp is reached &/or the fan speed is reduced considerably or turned off. If the revs dont drop after 1-2mins after switching Aircon off, a small blip of the throttle should do it.

My 2006 Golf would idle for between 2 & 5mins (to recharge current lost) before dropping the revs back to norm. The Superb was the same'ish & in both (& my current VRS) almost always a press of the throttle would bring it back down. During a DPF regen this doesn't work unless the car aborts it.

The science behind all this:

The battery current/charge used to start a car is considerable but short in duration. It is manageable and equates to you sprinting 10m as fast as you can once. You would barely notice it, but running the aircon or heated seats is more of an endurance test and at a difficult but less intense running pace.

As you start to tire, the battery (your body) is loosing charge as the work load is too high and unsustainable. The ECU recognises this as it continuously monitors all the cars vitals & instructs the engine to raises the revs to help. It turns your flat road endurance test into a gentler downhill run as the alternator is now producing more of the electrical load. It eases/balances the load on you/your cars battery and either maintains current battery levels or can even recharge your energy/battery levels.

In the cars handbook it states under start stop that one of the factors that will cancel that feature is if battery voltage levels have dropped too low. If the engine is constantly starting & stopping every 10-50m, the battery will not recover enough charge during the very short periods when the engine is running to keep it up.

Try sprinting max effort for 10m, pausing for 5 seconds and then repeating it again. You'd be blowing hard within 5mins. If you stop for a 5 min break to recover, its the same as the car cancelling start stop to prevent the battery's charge dropping too low.

Start stop is a great idea, but not if you are commuting in long periods of start stop traffic, then throw in aircon & heated seats on top, along with ever smaller lighter (less powerful) batteries, it shows what a juggling act your car has to do just to keep everything within comfortable limits of operation.

If I am on a short journey with lots of T-lights etc, I often turn SStop off to protect the battery and save fuel (only on occasions where I will be stationary less than 30secs).

Hope my essay explains things and prevents worry. DPF regens totally diff kettle of fish & covered else where in great detail.

Cheers & good night.

Blahde2, you are completely right. I think that I had high idle because of AC. These days are very hot in my country, around 34C, and because of that AC was working full. But as you said, after some time revs come to normal 750rpm.