My car randomly idles at 1000 rpm - with no extra load like air con etc - just radio and a heater blower on at 4.
 

Removing all electrical load doesn’t solve it. 
 

Restarting the car returns the idle to “normal” which is just over 800 revs. I say normal but I’m sure it’s supposed to be just under 800 rpm. 
 

It’s recently had a new alternator and battery but was doing this before they were changed. 
 

It went a while since changing those without doing it but now the issue has returned. 
 

No codes are returned via Carista. 
 

I’m stumped. 
 

 

Could it be the GPF regen ? I've no experience of the petrol regen but the Diesel DPF regen, when happening, has the car idle at ~1000rpm, it's one f the ways of knowing it's happening.

On the Diesel if you interrupt the regen by turning off the engine the fans stay on to cool the DPF back down, does this happen on the petrol versions ?

1 hour ago, gm73 said:

It’s recently had a new alternator and battery but was doing this before they were changed. 

Was the battery management coding updated when the new battery was fitted? If not it will still be assuming the old battery is fitted.

1 hour ago, PetrolDave said:

Was the battery management coding updated when the new battery was fitted? If not it will still be assuming the old battery is fitted.

Yes. It was coded in at the garage. 

1 hour ago, lway said:

Could it be the GPF regen ? I've no experience of the petrol regen but the Diesel DPF regen, when happening, has the car idle at ~1000rpm, it's one f the ways of knowing it's happening.

On the Diesel if you interrupt the regen by turning off the engine the fans stay on to cool the DPF back down, does this happen on the petrol versions ?

This is what I was thinking. But it only started doing this during the summer. Then it stopped when I replaced the alternator and then the battery. It’s only recently started to do it again. No fault codes but live data does keep listing misfires across cylinders. 
 

I took it for a drive down the motorway and then came back over country roads in lower gears and high revs. It was still

idling high. 

Edited 18 November, 20241 yr by gm73

Normal is starting the car and it is at 1,100 rpm and drops to 800 rpm in 40 seconds or so. 

That is a manual or DSG TSI in P or N. & stationary. 

 

As to High Idle because of the GPF there are threads on that in the Fabia mk3 section.

 

1 hour ago, Ootohere said:

Normal is starting the car and it is at 1,100 rpm and drops to 800 rpm in 40 seconds or so. 

That is a manual or DSG TSI in P or N. & stationary. 

 

As to High Idle because of the GPF there are threads on that in the Fabia mk3 section.

 

I’m not sure it’s the GPF at all. A lengthy motorway then B roads run in low gears/high revs didn’t sort it at all. Restarting the car did however and all seems well again. I’m beginning to think it’s a charging issue as this started during the summer before the alternator packed in. 

That is not how you clean a GPF. 

 

But the Light does not have to be on for them to be running at a higher idle speed.

Edited 18 November, 20241 yr by Ootohere

It’s the alternator again. 
 

Replaced it in September and it’s stopped working again. 
 

It was over compensating for not charging by running at 1k rpm. 
 

Completely packed in this evening. 
 

Strangely though the battery light hasn’t came on.   

Car was fine this morning, no issues. Alternator charging the battery, no high idle and most importantly the stale exhaust smell from yesterday had vanished. So the mechanic is erring on the side of the GPF being regenerated again. 
 

Apparently these cars have a smart charging system so when the battery doesn’t need charged any longer the alternator stops charging and that’s what caused my confusion as the multimeter was reading 12.5v whilst it was idling high. 

11 minutes ago, gm73 said:

Apparently these cars have a smart charging system so when the battery doesn’t need charged any longer the alternator stops charging and that’s what caused my confusion as the multimeter was reading 12.5v whilst it was idling high. 

Lots of discussion on here about the micro-regenerative charging system on these cars - even one recently who said his alternator was faulty because the voltage varied between just over 12V and over 14V when driving, which is perfectly correct and normal for this system.

 

I have a USB device from Amazon that I plug into the 12V socket in the centre console that shows the voltage, as an engineer it's quite enlightening to see how the voltage changes depending on time since starting, whether on trailing throttle, etc.

 

https://www.amazon.co.uk/gp/product/B0878Z3BMX

So an update. 
 

We have a U code that isn’t triggering the EML. It shows a lack of communication between the ECU and alternator. So it looks like a wiring issue although further investigation will have to be carried out.