Hi and thanks for using 2CarPros.
First, how high is the idle speed? Often times, this is caused by an engine vacuum leak. On this model vehicle, it is common for the intake manifold gaskets to leak. If unmetered air leaks into the engine, it can cause a high idle. Now, as the engine warms up (five minutes), the steel will expand on the engine and it could cause the leak to stop and the idle to return to normal.
With that in mind, the first thing I suggest is to check for a vacuum leak. Here is a link that shows how it is done:
https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge
If you decide to try the test and use a carburetor cleaner to check, remember it is flammable. Avoid spraying it directly on any component which is extremely hot such as an exhaust manifold. I always keep a fire extinguisher close by.
Try this and let me know what you find.
Take care,
Joe
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Tuesday, February 19th, 2019 AT 6:44 PM