How to Test a Spark Plug with a Multimeter (Complete Guide)

Whenever we talk about vehicles and engines in regards to maintenance, we always hear about the spark plug first. It is an essential part of an engine found in all sorts of gas engines. Its main work is to ignite the air and fuel mixture inside the engine with the right timings. Poor quality of fuel and their usage can contribute to a deficient spark plug. Higher fuel consumption and less power than usual are symptoms of a bad spark plug. It is good to test a spark plug before large trips and is part of your yearly maintenance routine.

A spark plug can be tested using a multimeter in which you can use a grounding test. In the grounding test, the fuel supply is cut off from the engine, and the spark plug wire or coil pack is removed. You can remove the spark plug from the cylinder head. In a multimeter test: 1. Set the multimeter to ohms, 2. Test the resistance between probes, 3. Test the plugs, 4. Inspect the reading.

Not enough detail? Don’t worry, we will elaborate on testing spark plugs using the grounding test and multimeter test.

Grounding Test

A grounding test is performed initially to test a spark plug. You can follow the below-mentioned steps:

  1. Cut off the fuel supply to the engine
  2. Remove the spark plug wire and coil pack
  3. Remove the spark plug from the cylinder head

1. Cut Off the Fuel Supply to the Engine

For fuel-injection cars, you should simply pull the fuse for the fuel pump. Disconnect the connection to the fuel pump on the carbureted engines. Run the engine till it burns off all the fuel in the system. (1)

2. Remove the Spark Plug Wire or Coil Pack

Undo the mounting bolt and pull the coil from the plug, especially for cars with coil packs. If you have an older engine then pull the plug wire free from the spark plug. You can use the help of spark plug pliers to make this process easy for you.

3. Remove the Spark Plug from the Cylinder Head

You should remove the spark plug from the cylinder head of the engine to check it using a multimeter.

You may want to check more here for ground testing.

Multimeter Test

Do the steps mentioned above and use a multimeter to test the spark plug. Perform the following steps mentioned below:

  1. Set the multimeter to ohms
  2. Test the resistance between probes
  3. Test the plugs
  4. Inspect the reading

1. Set the Multimeter to Ohms

Ohms is a unit for measuring the resistance and other associated calculations. You should set up the multimeter to ohms to check the spark plug for the best results.

2. Test the Resistance between Probes

Check for the resistance between probes and make sure there is no resistance in them. This is necessary for getting an accurate reading.

3. Test the Plugs

You can test the plugs by touching the one lead to the terminal end of the plug and the other to the center electrode.

4. Inspect the Reading

Inspect the reading to ensure the resistances that appear on the specs. Readings ranging between 4,000-8,000 ohms are acceptable and also depend on the manufacturer’s specs.

Workings of Spark Plug

technician testing spark plug with multimeter

  • Spark plugs are seen at the top of the cylinder head of almost all sorts of small engines. They have cylinders and cooling fins on their outsides and are considered the largest part of small-gasoline-powered engines.
  • The thick wire and fitting pushed over the end region of the spark plug can deliver electric power.
  • The engine has an ignition system that can send a pulse of very-high-voltage current using this wire. It can further move to the spark plug and typically has 20,000-30,000 volts for a small engine.
  • The tip of the plug is located inside the combustion chamber of the engine in the cylinder head and holds a small gap.
  • It jumps to the air space when high-voltage electricity encounters this gap. The circuit is completed by flowing into the engine block. This jumping leads to the creation of a visible spark igniting the air or fuel mixture inside the engine for running it. (2)
  • All kinds of problems associated with spark plugs are boiled down to a handful of deficiencies that can prevent the electricity from jumping to important spark plug gaps.

Items Required for Testing the Spark Plugs

There are only a few tools required to perform spark plug tests. There are many professional ways of doing this, but here we will mention some of the most important tools for going ahead.

Tools

  • Multimeter for resistance test
  • Spark plug socket
  • Spark plug wire puller for older vehicles having no coil packs

Parts

  • A spark plug
  • Sockets for cars with coil packs

Spark Plug Test Safety

We recommend you follow some safety procedures when testing spark plugs. You will only need a multimeter along with an exposed plug under the hood

Follow these best practices:

  • Throw on a set of safety glasses and gloves.
  • Avoid pulling the plugs when the engine is hot. Let the engine cool first. 
  • Make sure that the cranking of the engine is over and it does not have any moving parts with it. Be mindful of all sorts of moving parts.
  • When the ignition is engaged, avoid touching the spark plug. On average, about 20,000 volts run through the spark plug which is enough to kill you.

Wrapping Up

Evaluating spark plugs and spark plug wires is just as important as testing every other component of an engine, especially in vehicles before a lengthy trip. Nobody likes to find themselves stranded in the middle of nowhere. Make sure you follow our guide and you’ll be in the clear.

You may check other multimeter guides below;






References
(1) fuel supply – https://www.sciencedirect.com/topics/engineering/fuel-supply
(2) electricity – https://www.britannica.com/science/electricity

Video Reference

About Sam Orlovsky

b1d87d2ee85af3e51479df87928bdc88?s=90&d=mm&r=gI realized early on carpentry was a huge passion for me and I’ve stayed in the industry for over 20 years now. This gives me a unique ability to really be able to tell you what the best tools and recommendations are. I’m not only a carpenter but I also like machinery and anything to do with electrics. One of my career paths starting off was as an apprentice electrician so I also have a lot of experience with electrical products and anything related.