1998 buick high idle

What would cause my 1998 buick centry 3.1L to have a high idle when I first start the car, once the car warms up and I restart the engine the idle returns back to normal. Sometimes the car will go 35 mph by itself without touching the gas. I replaced the idle air control valve and was going to replace the egr valve. I have no check engine light showing and only had a service engine light on for about 15 mins and then it went out. I don’t hear any vaccume leaks at the intake and checked all the vaccume line and they are fine. Thanks to whoever answers this question.

Get the codes read. A computer in limp mode will cause a high idle speed on some GM cars.

The engine computer takes the signal from the coolant temperature sensor (cts), and other sensors, to control the idle air control valve (iac) and the fuel injectors. It could be getting the wrong temperature from the coolant temperature sensor (colder than actual).
You could take voltage readings of the sensors using a digital multimeter, or a scan tool.
It wouldn’t hurt to clean the oil and carbon out of the intake tract (off the MAF, the iac pintle and passage, the throttle plate and bore). Use MAF cleaner, and carb/throttle body cleaner. The instructions are on the cans.