Sometimes can even happen under light braking... but rare. the RPM goes as low as 500rpm and the engine might stall. I can rev it a bit, but has to change to N for that. Any idea where to look? Can one of the sensors cause that? Does the same in D and N - so can't be the gearbox then? Surely in Neutral the gearbox is disconnected?

The car drives and STARTS just fine even when cold.

 

Thank you.

Probably throttle position sensor. But could be a vac leak or something else would suggest plugging it in to make sure everything is as it should. you cans test sensors with a multimeter or the backyard way of unplugging said sensor if the problems still there then generally the sensor is at fault. 

If unsure get a mechanic to diagnose it. 

The TPS is brand new, fitted and adjusted by myself -)  ICV is properly cleaned as well.

I mean the engine starts no probs at all, like a dream. MAF sensor is 1 year old, but probably could do with a clean... VAC leak - possible, but why there isn't any indication of a problem when the car is idling during the cold engine start?

The idle is raised to 1.1k ish when on cold start so could easily mask rough idle issues .

Tbh since fitting the bigger turbo mine has stalled a couple of times when warm and suddenly dipping the clutch (heavy braking ) .

I just assumed it was the flutter /turbo stall affecting the idle airflow .

Unfortunately it's only occasionally and it's far to cold to investigate atm .

I intend on cleaning the icv and replacing it's gasket to see if that resolves it and if not mark it's original position and adjust the icv to see if I can solve it .

At cold mines goes to 1700k or even 2K for a few seconds, then drops to around 1500K mark when cold, then to 750 rpm stable - no jumping. Mine is pretty much standard, no bigger turbo or anything like that -) just the exhaust has been replaced.

I personally would look at maf had same symptoms with my classic and p1 it was the maf for p1 and maf and loom for classic  (I cut back as far as I could on the loom about 500 mm  as it hangs over what I would call it's hot side ) what is it your driving 

You can see my car in description -) but also being a FIRST forester, it's pretty much an impreza inside!

AFAIK the brakes use some manifold pressure in some way (very limited knowledge about it but heard of it a while ago) If that is the case its more than possible as me ruined that it's a vac leak.

I would say look at the MAF mine started to do this when it was starting to fail.

Age of them is irrelevant they seem to go just for fun

Well I believe it was the fuel cut or fuel starvation, as such the last time it happened only when the petrol tank was near empty, not at the bottom line but not far from it... With the full tank I was unable to reproduce the problem.

Oh and I cleaned the MAF, it did help for sure, sure MAF cleaner does wonders if you spray it at the headlights soo shiny after that -) The engine start has improved and now the car doesn't jump to 2K RPM during the cold start then settle at 1.5K, now it jumps to 1.6-1.7 and settle at the 1.4K -)

So thank you everyone, I'll monitor the issue.

Havent had the time to investergate this properly but as temporary fix, I marked the position of the icv ,loosened it off and turned it to raise the idle too 900 ish rpm (hot idle) .

Ive only Done about 100 mls of town n b road driving but it's not stalled so far .

I'll still give the icv a clean at some point and let you know if it improves the idle at the original icv position .

I have cleaned the ICV recently, so that is spot on for sure. I mean when the car is warm the idle is fantastic 750 rpm.

Cleaned the MAF sensor - no issues so far (drove about 500 miles at least after the cleaning). That could be the reason for sure.