Print this page

Guide on Checking Spark Plugs

Loose, defective or failing spark plugs, or spark-plug wires can be the reason behind some major engine trouble. You can actually read your spark plugs for helpful indications about how your engine is working. Knowing it's the spark plugs can save you considerable time and money at the auto-repair shop. If your plugs show that something is seriously wrong with your engine, ask a professional for an opinion.

sparkplugBut before you have your mechanic charge you by the hour to dive into your engine compartment, examine your spark-plug wires yourself and see if you can't save a dime. If you think your spark plugs or the wires are causing problems in your engine, start by inspecting the wires.

Find the Wires

The spark plug wires are the black rubbery cords that link your engine block and distributor cap. The wires are different lengths and twist around each other, so it's important to mark them so you know which one is which if you take them off. They have to go back exactly the same way.

Check the Wires

You should examine and make sure the rubber insulation around the spark plug wires is not damaged. If it is, a wire could be sparking in the engine compartment, causing its connected spark plug to misfire. Inspect the length of each wire, bending them to make sure no cracks show up. Replace any wires with insulation damage. Make sure you check the part number on the wire and use the same one.

Check the Connections

After you follow along each wire to make sure the rubber insulation is intact, you need to check the connections to the plugs. Check each wire's connection at the spark-plug end individually by pulling it off the plug, checking it for any tears or cracks in the insulation and then replacing it tightly to make sure the connection is secure. You should also look for any burning or darkening at the end which would indicate arcing.

Check the Spark Plugs

If the wires all look good, check the spark plugs and see if they need changing. You'll want to pull the spark plugs out one at a time. They fire in a specific order, so don't get them mixed up.

Remove the Plugs

To remove a spark plug, pull the plug wire by its boot at the base closest to the engine block to get it off the end of the plug. You don't want to pull the plug wire from its boot, or you'll have to get a new one. When the wire is off, use a spark-plug socket on your ratchet to unscrew the spark plug from its housing. Again, do this one after the other.

Inspect the Plugs

When you have the spark plug out, check to see how dirty it is. It should have a little bit of soot on it, but not a lot. If the plug is white or oily, that could be a sign of other issues, so take note. You should also check the porcelain insulator to see if it's cracked. If the plug looks okay, you can put it back in the housing.

Replace the Plugs

If your spark plugs are blackened with soot or have cracked insulators, you need to replace them. First, make sure you are using the exact same spark plugs, or else they won't fit properly and the wires won't attach. One at a time, carefully put the new plugs in the holes with your socket. Screw them in by hand at first, and then tighten them with the ratchet. Then reattach the spark-plug wire to the end and make sure it's connected well.

James Blanchard

James Blanchard writes for which provides an inexpensive instant car history reports for second-hand cars in the United States. VinAudit is an authorized consumer access service provider for National Motor Vehicle Title Information System(NMVTIS).