[T3] Spark Plug Life...

Jim Adney jadney at vwtype3.org
Mon Nov 14 18:31:59 PST 2016

On 14 Nov 2016 at 23:50, J. Jonik wrote:

> What's rule of thumb about how long spark plugs last?

The official replacement interval for FI cars was 12 kmiles. I tend 
to get much more than that out of them, but that may be partly due to 
the Delta Mark 10 CDI ignition units I use.

> How to know if they are on the way out?

The thing to look at is the end of the center electrode. You're 
looking at the square cut end of a metal rod. The critical factor is 
how sharp the corners at the end of that rod are. As the plug is 
used, the corners erode away and become rounded. The more rounded 
they are the higher is the voltage necessary for a spark to jump the 
gap. At some point, the required voltage becomes higher than your 
ignition system can deliver; then you'll start to experience hard 
starting and missing. These signs are pretty obvious, but it's 
probably a good idea to replace the plugs before they get to that 

15-20 years ago, I had let one of my cars go for a long time without 
checking the plugs. I was busy and didn't need the car often, and it 
started to get harder and harder to start. When I finally had a 
chance to check things out, I eventually found that the center 
electrodes were worn to the point where the ends were hemispherical 
with the lower ends of the hemispheres tucked below the upper ends of 
the insulators. These were extreme cases and they got replaced, but 
now I know what to look for in spark plugs.

> And what's the story on periodically touching up the tips with some
> fine sandpaper, or using one of those shop sand-blaster things for
> this purpose? 

Don't bother; you don't want to take a chance on letting a bit of 
abrasive residue get into the combustion chamber. And never use a 
metal tool in an attempt to scrape something off the insulator; it 
will leave a metallic track on the porcelain that can short the spark 
to ground. In a correctly operating system, the insulators clean 
themselves as you drive. If they don't, there's something wrong (too  
rich or too lean) with the fuel system.   

Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA

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