Car question

[tw]

Originally Posted by glatt

The car started misfiring and the check engine light came on! Misfire in cylinder 4. The idle dropped down to around 500 like it had been doing. So what I think happened was that I made whatever the problem was even worse by pulling the plugs and reinstalling them. That makes me think it's a spark plug wire.

It could be a spark plug wire or even electronics that drive that wire. Or a cracked conductor inside one plug.

Remove that spark plug. Leave it connected to its wire. Somehow mount it to the engine. Crank the engine. Observe spark on that plug. A defective spark will clearly be evident and diminished. A weak spark drives an engine normally most of the time. But that spark could have always been defective.

To appreciate a good spark, remove another plug to perform the same test. That spark will be fully formed or clearly diminished even when only cranking its starter.

If a clearly diminished spark does not exist, then move things to make a defective (weak) spark. Even freezing the spark plug before connecting it to test may expose a cracked conductor inside the spark plug.

Keep hands clear. Pre-electronic ignitions created only 20,000 volts. That hurt. Don't learn like I did. Electronic ignitions create higher voltages and greater current. Best is to use something like vise gripping pliers to hold or prop that spark plug against its engine block. A spark can even penetrate through heavy insulation on a spark plug wire. Do not even touch the wire.

Normal is for a computer to see a weak spark and not report it until after multiple engine restarts or some other conditions. Since reporting too many intermittents every time causes many mechanics ignore all error codes.