Help needed - Engine revs drop when just started and running at idle

[BlizzardX]

Hi,

I have a 1997 R Volvo S40 1.9 Turbodiesel. I've just come back of holiday and its just started to do something rather strange. As soon as I start the engine, it sits at idle maybe 900 revs and the will suddenly dip down to maybe 500, and then rise again. It does this quite often until its warmed up and then its fine. It even does this when running at low rpms, again about 1500 until its warmed up.

Does anyone know possible causes? I wondered whether the symptoms may be due to the filters being clogged, possibly the air filter...

Thanks

 

[BlizzardX]

Dirty or faulty idle speed control valve?

Excuse my ignorance, but if it was working fine before I went away (I was away for 4 weeks though) could something like that be the cause perhaps? Or could the warmer weather have hidden it from me.