I have a 2001 mercury sable that has a serious problem. It misfires when accelerating. It does not show any wrong codes when diagnosed., but it usually misfires when it reaches about 4,000 rpm. I have overhauled the transmission because some one told me that the misfire when driving seemed like a stucked valve body.That did not alleviate the problem.
You may want to stop working on your own car. You can’t sue yourself for failing to fix it. Check the owners manual troubleshooting guide. When it says to bring it in to a dealer; it may be trying to save you money. I know it doesn’t say to ask somebody at work.