1997 V70r really rough idle and surge on startup

[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?

[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.

[Yog Sothoth]

Brilliant. I shall check that this weekend. Thanks.

[Luxobarge]

As above, but the braking problem sounds like loss of vacuum to the brake servo, so I'd be checking that pipework too.

[Yog Sothoth]

Quick question: are the vacuum pipes shaped or formed, specific to the car? Or will generic pipes be OK if needed?

[Luxobarge]

Originally Posted by Yog Sothoth

Quick question: are the vacuum pipes shaped or formed, specific to the car? Or will generic pipes be OK if needed?

The small bore ones that go to the TCV for example are generic straight piping, can't tell you about the Servo pipes, I'd guess straight generic but not sure offhand.

[richmac]

Hi,

Your TCV lines won't cause those symptoms. If your car is otherwise driving and boosting ok, I would leave them well alone.

I would, however, suggest the following possibilities:

1. The O2 sensors just fitted.

2. The air idle valve/ throttle valve, (clean both with carb cleaner).

3. The MAF sensor, (clean with contact cleaner - spray only, don't touch).

4. The vacuum lines around the manifold/ vacuum tree (under your black plastic throttle cover).

These are all easy DIY novice checks/ jobs which a quick search will reveal a guide for.

Best regards,

Rich.

[Alpine]

mine has been doing this, and turns out the plastic pipe to the throttle body was lose, so was letting in air. I needed to tighten the metal clamp, access is difficult as its under the manifold on mine

[Yog Sothoth]

The TCV body has a ‘pen style’ clip on the back suggesting it should be clipped to something nearby. It’s just hanging free.
Two of the hoses are opposed to each other like a through-flow. The other comes off at 90° to that axis. The two aligned pipes are perished and one is actually not attached to anything other than the TCV. Where does the other end go?
It’s the ‘fresh air hose’ from the diagram above. As it’s “fresh air” does it even connect to anything?

I’ll replace them but try connecting them for now and see if it changes anything.

[Yog Sothoth]

Found something else. Just above the manifold there is a stiff narrow black pipe running across the top of the manifold. It bends downward at the RH side of the manifold and a very short length of rubber hose connects it to a nipple in the manifold. Or should connect it - it’s completely perished and detached.



Update: nicked a couple of inches off the hose next door and used that to remake the connection. Idling at a steady 800rpm now so I think that was the problem!

BUT the lambda night has come on! 😩
I guess after trying to compensate for all that extra oxygen, the ‘proper’ mixture is now throwing the system out of whack. Hopefully it’ll ‘relearn’ and settle down. 🤞