My 2001 Pathfinder has 210k miles and is running very rough. Idling is so so, but the larger issue is that when the RPM ranges from 2k to 2.5k, the vehicle shakes very badly. Yesterday I put in new plugs and it seemed to improve a bit, but still not anywhere good enough for me to feel comfortable going anywhere. The check engine light does not come on. In this forum, I found where the issue may be a bad ignition coil. How can i find out if I have a bad ignition coil? I am not a car guy at all, but can probably figure out how to do things. Just looking for a bit of expert advice. If the check engine light is not on, would codes still show up? Are the code readers at, say AutoZone, good enough to find something like a bad ignition coil? I've never used an ohmmeter, but was wondering if that might be a good investment and could tell me if one or all of them need to be replaced?
One more question: When I replaced the plugs, I noticed in the bottom of one of the cylinders, there was a small amount of liquid. I believe I noticed it after removing the ignition coil, but prior to pulling out the plug. Is something like this normal, or does it mean something else?
Sorry for all these questions at once, but I figured this is a great place to get answers. Thanks for any help.