Cleaning Spark Plugs

[BBO]

I know plugs are not that expensive, and I have always just put in new ones and trashed the old ones just to eliminate the question.

But I have been checking some old plugs with a VOM and I suspect that cleaning a plug might be just as good as a new one.

I use carb cleaner, soft brass brush, some small object to clean out the groove up around the ceramic insulator.

From the plug connector to the small ignition tip a new plug will read shorted if it's a non resistor plug.
A resistor plug (normally an "R" in the part number) will read maybe 5000-7500 ohms with the meter.

Any reading from the outside case/ground to the tip is abnormal.
What I think is that carbon builds up down in the groove making the spark less effective.
When I use the feeler guage to check the gap I kinda scrape the metal where the spark actually happens.

I work with 2-cycle engines a lot and it seems that gas mixed too rich, tends to blacken the plugs.

Does anyone else use a VOM to check plugs, and is it a good or bad thing?

 

[k0ua]

Does anyone else use a VOM to check plugs, and is it a good or bad thing?

Why not? You can see if the resistor is still in spec, and if there are any shorts or opens. Can't hurt 'em!

 

[oosik]

I clean the plugs every spring before I put anything back into service. Remove any carbon buildup and resize the gap. I will replace if they quit working. Rather than a VOM - I reconnect the plug wire, lay the plug on the cylinder head and make sure it spark when the engine is turned over.

 

[jenkinsph]

You can use a vom but the real test is whether the spark is compromised by a leak through the insulator to ground. It may read good with the meter but can short through the insulator at higher voltages. The vom is a good way to check the resistor in plugs if you get an open reading then get a new plug. I have cleaned many a plug to save a trip to town for a new one. I would also check the gap before replacing the plug in the engine too.

 

[oldballs]

Never have used a VOM on a plug but have cleaned and checked the gap on plugs for years and seldom replace one has been my experience.

 

[Threepoint]

Interesting discussion, and brings up something I've wondered about. I've heard small engine techs over the years say that, even with a practically new plug, if the ceramic insulator has been soaked with fuel repeatedly from chronic flooding, the plug usually needs to be replaced even if it looks o.k. How does this factor into DIY cleaning and testing? Is it a matter of the soaked insulator creating a ground fault, thus compromising the spark intensity? When you test for spark with the ground electrode held against the head or engine block, can you visually judge intensity well enough to know?

 

[DFB]

All I ever hear about is those crying about sandblasting your plugs :confused2:

 

[RandyT]

The push behind not sandblasting plugs is you can't get all the grit out, and will come loose in the cylinder and wear the wall as if it was dirt ingestion. My understanding is the new plugs are designed for EFI, and no longer have the sealant on the ceramic, which allows the fuel to soak into the ceramic causing fuel fouling and internal shorting.

 

[beenthere]

Add in that many of our plugs now are made in other countries with less quality than we are accustomed to. For a few years, Champion plugs were running bad batches. Now I hear that NGK plugs are found in bad batches, as well as Bosch.
Maybe these tests suggested by the OP (BBO) would sort out the "bad" new plugs as well.

 

[PILOON]

Not all that long ago plugs were always blasted and re used but then with rising labor costs it became more profitable to sell new plugs.
On most cars now they use either platinum or iridium plugs with much longer life expectancy. Also spark is now much hotter which overcomes many problems.
The 'annual tune up' seems to be gone from today's vocabulary.

I still clean my 2 stroke plugs and check the gaps as well. More spark is lost IMHO by old dry plug wires.