Cold start issues

I have a problem with my 1999 Vauxhall Astra 1.6 8v (X16SZR). On starting the car in the morning, it starts first time and revs at about 1800rpm. If I let it warm up on the driveway, the revs will die down as it warms up. I can then drive off and the car is perfect all day, with a normal tickover.

However, if I start up the car in the morning and drive straight off, the revs shoot straight up to 3000-4000rpm and it will keep revving. But if I stop, the revs will die down to tickover at around 800rpm. It will keep doing this until the engine has warmed up, then it is fine.

I have changed the temperature sensor and fitted a new EGR valve. There are no fault codes stored and the engine is in excellent condition – it has only done about 50,000 miles. The speed sensor is operating correctly. The vehicle has just passed its MOT and the emissions are fine. Do you think there’s a problem with the stepper motor? Ron Beard As you mention that the tickover will stabilise when stationary, but not when coasting, this suggests that the information from the speed sensor is affecting the system. This is not to say that the sensor is faulty, but that movement is a factor in the equation.

As you have already replaced the electronic EGR valve, I would also advise checking for any air leaks in the inlet manifold and pipes. You mention the stepper motor and I am assuming you are referring to the idle control valve, in which case I would try cleaning it and also the throttle body with carb cleaner, which may help.