How to diagnose a bad spark plug?

Question

What I always did up to now to check if my spark plug was doing fire :

• Remove it.
• Keep it plugged into the spark plug cable.
• Ground it on the frame.
• Start and check if sparks happen.
• If they are blue, the spark plug is ok. Else if the sparks are yellow or there is no spark at all then the spark plug is bad.

While this method may work in most of the case, it does assume that the wiring/spark plug cable etc. are doing their job properly.

Basically, I'm looking for a way that would confirm that I get no fire because the spark plug are bad and not because there is any other background problem such as bad spark plug cable etc.

Is there any other way to check if a spark plug is working properly so I can eliminate the case of bad spark plug when I get no fire ?

Answer 1

First of all, check the image in this link to diagnose the state of the spark plug. Look for cracks in the porcelain of your spark plugs, as this is a sure sign it is bad (electricity leakage will occur). Even if it's working now, it will soon fail.

Next, to check the spark plug lead, you can check it by ohming it (checking the resistance of the wire). A conventional plug wire has a resistance of 10,000 to 15,000 ohms per foot of length: if it's measurably higher, the wire probably is bad. An absolutely failed wire will have a hairline break somewhere, and the resistance will be infinity.

Also, if you have a known good spark plug (good tan color), you can swap it into the position of the possible bad plug and see if it miss follows the possible spark plug or if the known good spark plug is now not firing.

Something else to consider when testing the spark plug outside of the combustion chamber (your described method) is that the spark doesn't travel as well under pressure (as you'd see during compression inside the combustion chamber) as it does under no pressure (in normal atmosphere as you'd see outside the combustion chamber). Just because you see spark when testing as you did, does not prove the spark plug isn't having issues during normal running operation.