Why is the idling irregular when my 2003 Vauxhall Zafira starts up from cold?

After 140,000 miles of reliable use, my wife's 2003 Vauxhall Zafira 2.0 DTi is misbehaving. At temperatures below 8c it starts instantly and fast idle is fine but as it warms up and the idle speed drops to normal there is a period when the idling is lumpy and irregular. As the engine warms up more this stops and the engine behaves normally. No engine management lights illuminate and apart from the idle problem it runs fine. It still uses no oil or water between services, doesn't smoke and the engine seems as powerful and bomb proof as it did 100,000 miles ago. At first I thought it might be the glowplugs but these were changed a few months ago (difficulty cold starting) and it starts immediately at low temperatures now. Any suggestions greatly appreciated.
I would guess that the low temperature increased fuelling is not working, possibly because a wire has dropped off the engine temperature sensor. So what is happening is your glowplugs are glowing, the engine is firing off on the glowplugs, then, once the glowplugs go out, it is having to manage on normal rather than increased fuelling until it gets to temperature.
Answered by Honest John on

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