Spark Plug Failure Under Compression

Sun Dec 23, 2012 12:47 am

Years ago I watched a technical training video featuring a spark plug installed in a cylinder with a sight glass near the plug tip for spark observation.

A power source was energized producing a visible consistent spark at the plug's electrode. As air pressure in the cylinder was increased, the spark was eventually snuffed out. How many psi I don't recall. As the pressure was released, the spark became visible again.

The focal point of this video was to make you aware that it is possible to visually observe good spark at the electrode of a grounded spark plug during troubleshooting, yet when the plug is reinstalled and energized under compression, it can fail due to power supply issues or if the plug itself is defective.

If the spark is being put out because of a single defective sparkplug, not a big problem as only an engine miss is realized.

However if all the plugs are being snuffed by a weak power supply system, a no start scenario is created.

My theory is the threshold of spark failure under compression from a marginal power source may differ by brand. This may be the reason an engine will not start with one brand of plugs yet will readily fire with a different brand installed, especially after overhaul when compression is the highest.

I have had individual spark plugs that sparked outside the cylinder during trouble shooting yet missed when reinstalled and continued to miss until the plug was replaced. With no way to test the plug under compression I could only assume this was the problem.

I searched YouTube for the original video or a similar one unsuccessfully. I did find a link to a company that markets a tester to ferret out this problem. ... &Itemid=63

Re: Spark Plug Failure Under Compression

Sun Dec 23, 2012 10:15 am

That is the reason gas fouled plugs will not start an engine, in the old days you could clean plugs. I do not have great success with cleaning them now. I think it is a combination of lead free gas[ no lead deposits to clean] and spark plugs are not designed for cleaning now. Who knows?

Re: Spark Plug Failure Under Compression

Sun Dec 23, 2012 2:48 pm

And this is part of the reason that modern engines actually modulate spark voltage with throttle position - more throttle, more voltage. I had a coil fail on my Aurora that was just fine at idle and part throttle, but mash the throttle and it missed like mad. Coil would not supply sufficient voltage at WOT.