How to Test Spark Plugs

by Contributor

When you experience a sluggish engine, there's a good chance that you could have a bad spark plug. Difficulty cranking, low momentum while running or sluggish performance in your engine can all mean spark plug failure. You can take your problem to a professional mechanic for a complete diagnosis, but there's a way you can test the spark plugs yourself.

Start by disconnecting each spark plug wire on your engine one at a time while the engine is running. If the engine drops speed or starts running rough when one is disconnected, you know that spark plug is good. If you disconnect a spark plug and no significant change happens in the engine, you have found a bad spark plug.

Test the spark plug ignition by disconnecting the spark plug wire from the spark plug. Hold the end of the spark plug wire close to a metal surface. If the spark plug is good, you will see a spark or you'll hear a crackling noise. This means that voltage is getting through the wire to the spark plug.

See if there is a spark on each one of your spark plug wires when the engine is cranked. If compression is good, you will have spark on each one of your spark plug cylinders. No spark would mean that a spark plug wire is dead.

Remember that each link connected to your spark plugs needs to be hooked up securely. Links include battery cable, ignition wires and coil wires, not just the spark plug wires.

Wiggle the link connections from your spark plugs. Then test again. Sometimes it's just a matter of a line connection being loose.

Check to make sure that the end of each of the spark plugs is clean and free of any dirt, oil or grease deposits. Sometimes you can just clean them thoroughly and they'll pass the test. But if they still don't test well after cleaning, you'll need to replace them.

Warning

  • close Protect yourself against the risk of shock while testing spark plugs. Wear rubber gloves and don't lean against any metal part of your vehicle while the engine is running.

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