Simple and Cheap Spark Plug Tester

[MolaKule]

While not a professional spark plug tester, one can make a simple tester using a piezoelectric ignitor

such as this one:

Ignitor

I used one that I pulled out of my old water heater and connected the ground of the piezo transducer to the ground of the spark plug at the shell or grounding electrode tip, and the other to the spark plugs top terminal. Use those insulated 18" jumper wires from Radio Shack.

These transducers produce about 28,000 volts, enough to jump the electrode gap of just about any modern spark plug.

 

[Andy636]

Not being able to reproduce the hi pressure and temps the spark plug is subject too in the combustion chamber may throw you off.

I had a brand new NGK spark plug once in my bike that would work great when pulled out but not so great when put back into the engine

 

[4IsWEm]

Exactly. Had bosch spark plugs do that to me too.

Re the tester, isn't it simpler to just hold the wire cap, put the spark plug in and press it against grounded metal in the engine compartment? That's what I used to do in the old days, when my bike would foul its plugs in very short time.

Anyway, thanks Molakule. Nice thought.

 

[DriveHard]

For me, spark plugs are cheap enough that if I have it out it is being replaced. No sense to take the time to remove it and not replace it. IMHO of course.

 

[MolaKule]

Originally Posted By: Andy636
Not being able to reproduce the hi pressure and temps the spark plug is subject too in the combustion chamber may throw you off.

I had a brand new NGK spark plug once in my bike that would work great when pulled out but not so great when put back into the engine


Quote:
While not a professional spark plug tester, one can make a simple tester using a piezoelectric ignitor.


Take it for what it is, a simple tester made from cheap or scrounged parts.

I once had a Suburban that had one spark plug that kept fouling and missing when cold so I took it to my friends mechanic shop and he put it on his high priced Professional tester. No problem found.

Thinking it might be a thermal problem we then took a torch to it and checked it; checked ok. We let it cool down and sprayed a circuit cooler on it. Bingo, failed spark.

We cut it open using one of his precision grinders and found that the spring was broken. Theory: when the engine got hot, the spring would expand and make contact. When cold, it would pull back and be intermittent.

Yea, I know, I could have just purchased and installed a new plug and called it a day, but investigating why is just plain fun.

 

[TFB1]

The real spark plug testers of old were supplied compressed air and amount of pressure could be adjusted to see exactly at what point the plug could function(I used to play with one of these at a garage as a kid)... Of course those used a ignition that approximated a the points systems of 50-60 years ago, today's ignitions could fire plugs at a much higher pressure than those primitive systems...

If you really want something to test plugs, a ignition transformer from a furnace or bug zapper will will tell you if the plug is capable of firing... Can also connect it to your trash can to keep the stray critters at bay(just don't forget to turn it off before opening the lid)...