car idles too high when aircon switched on

[gilburton]

Recently bought a 2002 450 in generally good condition. I have been told the engine had new rings etc and indeed it starts/runs fine.
My problem is that it idles at app 1000rpm on the rev counter. If I switch on the aircon it shoots up to 2000rpm and will settle back to app 1500.
This makes the car unusable in traffic conditions(with the aircon on) and I believe it is affecting the gearchange at lower revs as I went to pull away and all I got was lots of revs but no engagement.
Switch off the aircon and it was back to normal.
I have been told to check for intake/vacuum leaks but I can't see anything obvious around the throttle body however I did find that the valve on top of the engine just has a small hose a couple of inches long just dangling there? Is it supposed to be connected to something as putting a finger over the end didn't make any difference? Any other ideas where to look for the problem? What triggers the revs when the a/c is switched on? I'm new to the Smart but not cars in general. Here is a generic pic of the valve.
I do understand that the revs are supposed to rise at low rpm but not this much lol

[Thrumbleux]

The 'valve' is the fuel pressure regulator and it has to be connected to the inlet manifold.

https://www.fq101.co.uk/how-to-guides/fortwo-450/engine/176-identifying-engine-components

[gilburton]

Thanks yeah I did find that out after a bit of study this morning it was finding out where the tube was supposed to connect to.
After looking at various pics I thought it was attached to the throttle body so I removed the larger upper tube and there was the connector underneath the larger one.
All connected up and the difference was immediate. I assume the engine was compensating for the extra air by pumping in more fuel?
Anyway it's a vast improvement

[Thrumbleux]

Good that it's sorted.
What was going wrong was..... the tube you reconnected tells the fuel pressure regulator the manifold pressure. Which it needs as it is calibrated to maintain fuel pressure at X bar (typically 4 bar) above manifold pressure. With the tube disconnected the regulator only sees atmospheric pressure so will over fuel wen the manifold is under vacuum and under fuel when the manifold is at positive (boost) pressure.