Bouncy cold rpm gauge (v6)

[Ben82]

So it's getting rather cold over here now, and it seems like the V6 is exhibiting a rather strange "issue".

Two days in a row now, I've got on to the main road here from a cold start, get up to about 80-90kph and then ease off on the accelerator for the approaching 50kph section. And the rpm gauge will start to bounce by about 500rpm a number of times (eg 2000->1500->2000->1500, etc), generally until I start applying pressure on the accelerator again. However I do not notice any change in the engine tone (certainly isn't like cold idle variations at least), and if I wasn't looking at the gauge I probably wouldn't think anything was out of the ordinary, especially with the engine being cold n a all.

Once the engine has warmed up it doesn't do it again!
I'm guessing it's most likely the sensor not taking to kindly to the weather. But I would imagine that would have potentially created other issues..

Any ideas? I might see if I can get my Lexia sorted again and see if that shows the problem on the graphs.
Thanks

[Mandrake]

Maybe the idle control valve (ICV) is sticking a bit ?

It provides a bypass flow of air around the throttle plate to maintain idle speed, but during acceleration when you open the main throttle plate the ICV is also commanded by the ECU to open wide. (You should be able to see this on the Lexia) The reason for this seems to be to help avoid lurch when you take your foot off the throttle abruptly - without the ICV you'd get a distinct lurch. My V6 had this lurch on removing the throttle when I first got it when the ICV was seized.

When the ICV works properly when you take your foot off the throttle suddenly it stays open momentarily providing some air flow and then closes smoothly over a half second or so - thus no lurch. It could be that yours is sticking a bit thus holding the rpm at 2000 when it should fall to 1500.

[Ben82]

Ohh Thanks Simon,

That actually sounds likely. I'm not usually one to abruptly take my foot off the throttle, but I remember recently when I have, there has been a bit of a lurch.

Found this which is for yours: http://www.406coupeclub.org/Files/HowTo ... ne%202.pdf
I would imagine the C5 one isn't much different. Will need some carb cleaner it seems. If I can I might try to get my lexia up and running before the weekend, and see if I can confirm that to be the case.

[xantia_v6]

It is also possible that the behaviour is normal in very cold conditions, maybe to keep the catalytic converter from getting cold?
Check the idle control valve, but it that seems OK, then I would not go looking for problems.

[Ben82]

It is also possible that the behaviour is normal in very cold conditions, maybe to keep the catalytic converter from getting cold?
Check the idle control valve, but it that seems OK, then I would not go looking for problems.

It's possible, but I would have thought I would have noticed it before in the 3+ years I've had it That said we did move in August to the countryside, so we get out on to the fast roads a lot quicker than when we lived in the city.
Though I would have thought the rpm would just stay high to keep the cat from going cold, rather than having it bouncing (and I don't believe the engine's RPM is actually bouncing at the time either).