My friend has a '93 Honda Civic. When he is driving, his RPMs involuntarily increase. This happens when the vehicle is in drive and he is not accelerating (his foot is not on the accelerator). The problem goes away or lessens as the vehicle’s engine warms.
That’s all he told me.
A dirty Mass Air Flow Sensor (MAF) could cause this.
Does this car by any chance have one of those aftermarket K & N air filters?
I ask because over-oiling of those filters can lead to all sorts of MAF-related problems.