variable idle speed why!

[tianaroja]

since my last mot and the the nice mechanic claiming it failed on emmissions but cleaned the lamda sensor ( which the haynes manual says it shouldnt be touched)my 8v ticks over at about 1500 then gradually over a period of minutes drops to 800  may be a little blip up on the way,

does anybody know if the lamda sensor affects tick over or is it something else ie idle stabilisation valve or a air leak it does seem to stink of petrol now and again

any clues would be nice

[golfvr6]

The lamda sensor will affect the idle, it sends an electrical signal to the ECU which is based on the oxygen content in the burnt gases.
Try replacing the sensor first, if no change then start looking elsewhere, ie. cleaning the idle stabilisation valve.

[tianaroja]

how much is a sensor and are they difficult to fit? do they just thread in exhaust manifold?

[golfvr6]

The plug for the sensor is by the rear engine mount and the sensor itself screws into the exhaust, into the cat converter i think.
The sensor itself will probably not come out easily, although you can buy a special socket to do the job.

[Grovit]

VR6 - It's a possibility that my 8v may have a lamda fault - emissions were high at the last MOT (Feb '04) and the idle is very very lumpy - constantly swings from 900 - 400 rpm - slightly more pronounced when engine is warm.  

Driving the car is fine once on the go - no jerkiness or anything - just v. lumpy on idle -

What do you reakon lamda sensor being a probably culprit....?

Any idea how much a replacement one is....?

Cheers