I've got a 97 Chevy Blazer with the 4.3 and 281k miles. It might be of-note that a shop replaced my Lower Intake Manifold gasket about 18 months ago.
At the very end of a long trip this weekend it started to overheat while idling. I shut it off shortly before the temp gauge read 210 (the red starts about 240/250). When it cooled somewhat I discovered it was low on coolant ..even though plenty was in the overflow reservoir. So I topped it off and replaced the old radiator cap, which I think was the source of a slow leak. But when I started it back up, no more than 30 minutes later) I had a new engine code, p0303, which is "Cylinder 3 Misfire Detected". Also, it seemed to have a rough, unusual, pulsing rhythm at idle, not exaggerated, but I thought I noticed something different about the idle.
It was late, so I cleared the engine codes and let it sit over night. Now today on my way to work, it started up great just like usual, and I didn't notice anything weird at first--however it was probably getting extra fuel then due to being cold. But once it warmed up, I thought the idle seemed rough and heavy, kinda shaking the cab, and definitely different from normal. My drive to work is only about 10 miles/20 minutes, and it hasn't kicked a new engine code, but I'm expecting one any moment with it idling like this.
Looking into symptoms/solutions, I found this thread where similar symptoms were fixed by addressing JUST the #3 cylinder fuel injector. Also, right here on mechanics.se, a different user emphasizes the potential for the SCPI (fuel injection) system to influence misfiring. Meanwhile, over on YouTube, a different guy fixed his misfire by just replacing the distributor cap. And closer to the worse-case scenario, someone suggested the potential for bent valves on an unresolved thread over at BlazerForum.
Tonight, if I can get to it, I'm going to check the #3 spark plug for fouling evidence as well as the condition of its plug wire for wear/melting/shorting. But I don't suspect the electrical components because wouldn't the problem be more aggressive, regular, and obvious with near immediate error codes?
I suppose my question is: How can I rule out an issue with bent valves following overheating? And would bent valves feel smooth again at high RPMs (basically anything above idle?)
Secondary question related to the scenario: Is it realistic that a mild overheating might finish off the old OEM fuel injector spider, or an individual cylinder injector poppet? What is the best way to rule it out?
I'm optimistic that rough idle but smooth running at RPMs above idle suggests a fuel delivery fuel mixture issue to the problem cylinder, rather than bent valves. So someone with a strong opinion on that would carry a lot of weight.