How to Check & Replace Spark Plugs: A Complete DIY Guide
Spark plug maintenance is one of the easier car repair jobs you can handle yourself. However, you have to do it right to avoid damaging your engine. But no worries, that’s why you’re here. We’ll take you through all the basics and give you step-by-step guidance, so you can get the job done like a pro.
Did you know? If you have an electric vehicle, you’re in luck. EVs don’t have spark plugs. So you can stop reading and move on to the next thing on your to-do list, like rotating your tires!
Bad Spark Plug Symptoms: 5 Signs of Trouble
- Signs of wear
The tip of the spark plug or the electrode may show excess wear or fuel and oil contamination. A great way to tell is to compare your spark plug to a brand-new one. - Reduced MPG
If you notice your miles per gallon starting to drop, bad spark plugs may be the cause. In fact, faulty spark plugs can cause an MPG decline of up to 30%. - Difficulty with cold starts
Spark plugs collect residue over time. This can cause longer crank times, often during colder days. - Noisy idling
You may notice your engine is very noisy when you’re stopped and idling. There could be a variety of causes, but a spark plug inspection is a good place to begin. - Engine misfiring
Your engine is running, but it’s shaking and the whole car is vibrating.
5 steps to remove spark plugs
Prepare your car and remove the spark plugs. Here’s how:
Step 1: Cut off the fuel supply
Fuel-injected cars: pull the fuse for the fuel pump (refer to your owner’s manual for guidance).Carburetor engines: disconnect the tubes to the fuel pump, then run the engine until it burns off all the fuel.
Step 2: Turn off the engine and wait for it to cool down
Safety first. Remember, engine parts can get extremely hot.
Step 3: Detach the wire from each spark plug you are checking
Step 4: Use your socket wrench to loosen each spark plug
Once it’s loose, you can unscrew the rest by hand. You can visually inspect the spark plugs to see if any are clearly bad. Some of the signs may include:
- electrode wear
- cracked porcelain housing
- black sooty or oily deposits
- greenish or whitish deposits
Step 5: Use your multimeter spark plug tester
How to test a spark plug tester
If you’ve never used one before, it’s pretty simple.
Step 1: Set your multimeter to Ohms
You should see an indicator marked “OHM” or the Greek letter omega Ω.
Step 2: Test the multimeter probes
Touch the metal tips of your black and red probes together. You should get a reading on the multimeter, then it will zero out.
Step 3: Test the spark plug
Touch one lead to the end of the spark plug, then touch the other lead to the tip of the black electrode at the other end of the spark plug. An acceptable reading should be between 4,000 and 8,000 Ohms.
Pro tip: If you touch the probes to any other parts of your spark plug and you get a reading, you know the spark plug is bad.
Here’s some additional info that’s good to know.
Testing Spark Plug Wires
You can use the multimeter to check your wires. But the easiest way is to trust your eyes. If you see excessive wear, fraying or rotting, it’s time to replace your wires.
Okay, so you’ve determined that one or more spark plugs are bad. So now what?
Installing the new spark plugs is the easy part. Follow these steps:
Step 1: Compare the part number on your new spark plugs to make sure it matches the old ones.
Step 2: Carefully install the new spark plug(s), being mindful to not cross-thread it. Use your wrench and tighten to the proper torque as indicated in your owner’s manual or spark plug box.
Step 3: Apply a small amount of plug wire grease to the plug wire boot. Then reconnect each plug wire to the correct spark plug.
Step 4: Reconnect the fuel supply.
Step 5: Start your engine. If there are no unusual engine lights or warnings, you’re good to go!
Pro tip: If your check engine light flashes, there’s probably a misfire. Double check your installation. If you can’t diagnose an issue, you might want to have it checked out by a repair shop to rule out any other issues.
Some final thoughts…
Recommended spark plug maintenance varies by car and spark plug manufacturer. Typical replacement ranges from 30,000 – 100,000 miles. Always follow the recommendations in your owner’s manual.
