Hi there all You gurus!

My 2003 C-class has an annoying idle problem. Instantly after start it's normal but after some driving the idle rpm is around 1200. After some more driving it's cycling between 1200..2000 rpm for example when standing still at the street lights. A quick flooring of the gas pedal helps a bit but only temporary. The cycle between 1200 and 2000 will continue shortly afterwards.

There's nothing else wrong with the car as far as I know: the acceleration, full power delivery, the smoothness of the power delivery is exactly as before. No error codes from the ECU or excess fuel consumption.

Car specs:
- bought as new in 2003, M271 engine in 180K version, 6 speed manual.
- has a Kleemann K1 installed some 60.000 km ago (K1 is similar to a ECU remap but done with a separate box and all of it's connections to the car are soldered etc)
- total km driven 105.000
- it's been serviced once a Year by a very trustworthy and indipendent service company
- location Finland

I've planned to do the next steps:
- vacuum leak check near the intake manifold area (is there a vacuum hose under the air filter? Some writers claims this to be the reason in other forums.)
- new iridium spark plugs (They have never been changed until now, but there's been no problems either!)
- if all these fail, then thorough cleaning of the throttle body

So what are my odds, are these steps worth trying?

BR

IHe