1997 V70r really rough idle and surge on startup

[El Padrino]

Originally Posted by Yog Sothoth

My V70r surges at high revs on starting, then settles into a really rough idle at low revs, almost stalling point.
On acceleration or at speed, it's fine, but as soon as I take my foot off the gas or come to a stop, the rough idle returns. It even cut out the other day, going downhill, which was scary.
It started doing this fairly recently but seems to have got worse.
No warning lights on the dash.
Any ideas?
I read something on another forum about vacuum lines causing poor idling but not sure what I'm looking for.
I had the battery out recently and had both O2 sensors replaced last week but I don't think that should affect it(?) - I've had the battery out before with no problems.

EDIT: a couple of times I noticed the brakes felt 'weird' - I had to push on the pedal more than normal to get effective braking. They worked OK but weren't as 'instant' as usual. However, that was on 2 or 3 occasions, most of the time they felt OK. Could be connected to the other issue? Don't the brakes have a vacuum link?

Hi Yog Sothoth, I would say checking the vacuum hoses is a good place to start. Sounds like you have a vacuum leak causing the erratic/rough idling. The vacuum hoses are connected to the turbo control valve (TCV).

The first photo is from my 98 V70R, so it should look like yours. The TCV lives underneath/near the air filter box and turbo intake pipe. My air filter box has been removed so you can see the TCV pointed by the red arrow.
TCV_1.jpg

The second photo is a close up of the TCV, with its three connections and the power socket.
TCV_2.jpg

The third photo/diagram shows the routing from the TCV.
TCV hose connections.JPG

Check for signs of cracking on the three vacuum lines and replace if necessary. I'd also check the turbo intake pipe is in good condition and check the vacuum lines from the TCV to the turbo intake pipe connections are secure too. I'd very much doubt the brakes would relate to the erratic idling, but more experienced forum members can advise on that.

I hope that helps as a start.