Diagnosing For Spark On a Gas Engine

When a gas engine doesn’t start, the spark plug is usually one of the first things tested. You could pull the spark plug wire off and see if you get shocked, or you could unthread the spark plug, ground it to the cylinder head, and have someone crank the engine to see if there’s spark.

Ray Bohacz is Successful Farming’s engineman. He says these are the most common ways that people test for it, but that’s incorrect.

"And why it is incorrect, is that it does not recognize the fact that the ignition coil has to produce enough energy to jump the gap of the spark plug under the cylinder pressure of compression when that engine is trying to start," he says. "And that often bites you when you’re trying to diagnose a no-start condition. You look at the engine, it has fuel, you think it has spark, and yet it doesn’t run."

He says the proper way to diagnose for no spark is to use a spark tester, because the central electrode is deep down inside the porcelain insulator.

"That causes the coil to have to work harder to try to simulate jumping the gap of the spark plug under compression. When you’re just grabbing the wire and seeing if it has a spark or grounding the spark plug, you’re not taxing that coil," says Bohacz. "Often you will have enough energy in the ignition coil to arc the spark plug under no compression, but not to arc it under the cylinder pressure of trying to start the engine."

Spark plug testers are available for small gasoline engines, and for high-energy ignition. They’re inexpensive and easy to use.

Find more tips for working with spark plugs