how to tell if a spark plug is bad

The Ultimate Guide on How to Test a Spark Plug

When you experience a slow-acting engine, there’s a high chance that you could have a bad spark plug. Trouble starting, low momentum while running, or poor performance in your motor can all mean a spark plug problems. The best way is to take your issue to mechanic for a complete diagnosis, but there are ways you can test the spark plugs yourself. Read on, and we will provide you with the answers.

Bad Spark Plug Symptoms

For optimal performance, your engine’s spark plugs should be clean with no damage to the electrodes. If your plugs become fouled or dirty, it can affect how your motor runs. A bad or fouled spark plug is a device that has become covered with a substance like fuel, oil or carbon or one that’s blistered from running too hot. Driving with poor spark plugs can cause plenty of problems for your engine. Signs of bad spark plugs can include:

  • Check engine light comes on – Worn spark plugs can cause your check engine light to come on. If a plug begins to fail, then the most obvious event should be the check engine light coming on;
  • Hard starting – When you first start a car, if you feel that the engine struggles to get going like it needs a moment before it engages, that can be an issue with the plugs. This happens due to the combustion reaction not occurring exactly right when you turn the key in the ignition. If the spark doesn’t work immediately, the motor’s second cylinder may have to take over and get things started. That causes a hesitation, jerking and lurching motions when you try to start the vehicle;
  • Pre-ignition damage – If the side electrode seems like it’s been burned away, that means the engine is running too hot. The spark plug is probably firing too soon, or there’s not enough fuel being burned. This may be linked to an issue with your fuel injectors and the timing of the motor. The spark plugs are likely to die promptly after you experience this type of damage;
  • Engine misfires – One of the most common problems with a spark plug is that your engine will misfire. If a plug can’t produce a spark, that motor’s cylinder won’t have a combustion reaction. If it occurs on a routine basis, you can almost guarantee that it’s a spark plug inducing it and will also lead to some severe damage during the time;
testing spark plug
  • Poor gas mileage – When plugs aren’t working, then the gas injectors are spraying in the engine is essentially being wasted. That will force the engine to push a little harder and utilize more gas to try to compensate for what’s missing; therefore, you’re going to be heading to the gas station more often;
  • Carbon coating – If the top of the spark plug and the sides are black like they’ve been on the BBQ, that’s a sign you’ve got carbon build-up. This comes from running with too much fuel, leaking injectors, bad wiring, and other potential combustion issues. The carbon coating makes it quite tricky for the plug to spark adequately. It’s possible to clean this coating off and get the spark plug working again;
  • Shiny coatingIf the plug’s tip and side are shiny, that might be a sign of a rich fuel mixture. This could also point out problems with ignition or the heat range. In any case, it’s a sign of fuel fouling the spark plug. That could potentially be repaired by adjusting the fuel injector or inspecting the heat range of the spark plugs;
  • Rough idling – If your car’s engine is running rough or noisy when idling, you may have a spark plug problem. Heavy rumbling when waiting in a drive-thru or at a stoplight may indicate your vehicle is overdue for maintenance. Check the spark plugs and replace them accordingly. Replacing the plugs, and potentially also the ignition wires and coils, may solve this;
  • Oil ash – If engine oil is getting on the spark plugs, it could be coming from worn piston rings and damaged valve guides or seals. That’s an indicator of a much bigger issue in your engine that’s going to need to be fixed very quickly before your engine fails on you entirely.
signs of bad spark plugs

How to Tell if a Spark Plug is Bad

To check if your spark plug has gone bad, you’ll need to check your plug for common signs that suggest that it might be damaged or fouled. Follow the steps below, which outline how to determine why your spark plug is no longer working properly.

  1. Disconnect the plug lead. Clean the area around the plug to skip getting debris in the combustion chamber when you remove the plug;
  2. Remove the plug with a spark plug socket;
  3. Check the spark plug for very stubborn deposits or for cracked porcelain or electrodes that have been burned away. If any of these conditions occurs, replace the plug;
  4. Check the plug gap and adjust as necessary;
  5. If it seems in working order, replace the spark plug, taking care not to over-tighten (15 ft. lbs. maximum) and re-attach the plug lead.

If the motor tries to turn over and dies, then you have a spark. This would indicate the problem probably lies somewhere else (carburetor, fuel, valves, etc.). If the engine doesn’t turn over at all, then there may be an issue with your ignition system. The first thing to do is to change the spark plug.

How to Test Spark Plug Without a Tester

Conduct the following steps if you want to know how to check spark plugs without a tester.

  • Step 1 – Start by disconnecting each plug wire on the engine one at a time while the motor is running. If the engine drops speed or starts running rough when disconnected, you know that spark plug is suitable. If you disconnect a plug and no major change occurs in the engine, you have found a bad spark plug;
  • Step 2 – Test the spark plug ignition by disconnecting the plug wire from the spark plug. Hold the end of the wire close to a metal surface. If the plug is suitable, you’ll see a spark, or you’ll hear a crackling noise. As a result, the voltage is getting through the wire to the plug;
  • Step 3 – See if there is a spark on each spark plug wire when the engine is cranked. If compression is right, you’ll have spark on each one of your spark plug cylinders. No spark would mean that a plug wire is dead;
  • Step 4 – Remember that each link connected to the spark plugs has to be hooked up securely. Links include ignition wires, coil wires, and battery cable, not just the spark plug wires;
  • Step 5 – Wiggle the link connections from the plugs. Then test again. Ensure that each of the plugs’ end is clean and free of any oil, dirt, or grease deposits. Also, you can clean them thoroughly, and they’ll pass the test. But if spark plugs still don’t test well after cleaning, you’ll have to replace them.
bad spark plug symptoms

How to Test a Spark Plug with a Tester

We’ll show how you can use a multimeter to test a spark plug successfully in six steps.

  • Step 1 – Connect test leads to the multimeter (multitester) by plugging them into the jacks.
  • Step 2 – Set the (multitester) to the Ohms
  • Step 3 – Touch the test leads together. The meter should show zero Ohms.
  • Step 4 – Move the test leads apart. The meter should read infinite. Write the reading.
  • Step 5 – Remove the ignition wire from the plug.
  • Step 6 – Touch one meter lead to the plug’s terminal, and the other lead to engine ground. Press the points of the tester leads firmly to the metal to provide a good connection. If the meter reading is different from what you read in Step 4, the plug is bad.

What's a Spark Plug Tester?

As the name implies, this tests the spark plug. More precisely, it tests whether there is an electrical current capable of generating a spark. The device is very simple and safe to use. You remove the HT (high-tension) lead and plug the tester in-line, so it connects to the end of the lead and the spark plug’s base. Operate the engine as usual, and the device will illuminate if it’s receiving a strong electrical input.

Culver City Spark Plug Experts

For preventive maintenance service, including your spark plugs, Automed Car Care is your best choice. Our mechanics are either ASE or Master-certified, and our Culver City auto repair shop is equipped with state-of-the-art tools and technology. Our automotive specialists will check and replace your spark plugs quickly and efficiently. For all your spark plug repair or replacement needs, call (424) 372-9120 or visit us today.