How to Check Glow Plugs?

Last Updated on December 4, 2023 by Jaxon Mike

Glow plugs are an important part of a diesel engine’s starting system. They help heat up the combustion chambers to aid starting in cold weather.

With time and use, glow plugs can fail, which can cause starting issues, rough running, and white or blue exhaust smoke. That’s why it’s important to check your glow plugs regularly and replace any that are worn out.

Checking glow plugs is a relatively simple task that you can perform yourself at home with basic tools. In this article, we’ll walk through the complete process of how to check glow plugs in a diesel engine.

We’ll cover how to do a quick check by listening for the glow plug activation sound, testing the operation with a multimeter, and how to perform a physical inspection if you suspect issues. Let’s get started!

What are Glow Plugs?

Before jumping into testing, let’s do a quick overview of what exactly glow plugs are and how they work.

Glow plugs are small heating elements that screw into the tops of the combustion chambers in a diesel engine. Their purpose is to heat up the compressed air inside the cylinders to aid in starting.

Diesel fuel doesn’t ignite through spark plugs like gasoline. It relies solely on heat to combust the fuel-air mixture.

In cold temperatures, the air inside the cylinders can be too cold to ignite the fuel properly. Glow plugs help warm up the air so that combustion can begin. They heat up for a few seconds when you turn the ignition key to the “on” position before cranking the starter. Once the engine starts and warms up, the glow plugs turn off.

The heating element inside a glow plug is a thin wire coil made from materials like silicon nitride, silicon carbide, or metal alloy. When electrified, the coil quickly heats up through resistance heating.

The housing around the coil helps transfer the heat into the combustion chamber. The tip of the housing where the heat is concentrated is called the hot tip.

Preliminary Checks

Before testing your glow plugs, do a quick inspection of wiring connections and check for any engine codes that could indicate issues.

First, check the glow plug control module and inspect the wiring harness. Look for damaged or corroded wiring that could cause circuit problems. Verify the connectors are fully plugged in. Check that the supply voltage is getting to the control module as expected.

Next, turn the ignition on and see if the glow plug indicator light on the dash comes on. The light should illuminate and stay on for several seconds when you first turn the key on. No light means there could be an electrical problem.

Finally, connect a diagnostic scan tool and check for any engine codes related to the glow plug system. Codes for things like control circuit voltage or heater circuit problems could indicate bad wiring, connections, or failed plugs. Clearing any codes before testing will give you a clean slate.

If everything looks good so far, move on to testing each glow plug.

Listen for Glow Plug Activation Sound

One quick way to check if your glow plugs are operating is to listen for the distinct clicking sound they make when turning on.

To test, have a helper sit in the vehicle and turn the key to the “on” position without cranking the starter. Activate the glow plugs 2-3 times to hear the sound consistently.

Then, hood open, place your ear down near each glow plug location on the cylinder head. You should hear a rapid ticking or clicking noise coming from each glow plug as it heats up. It may be faint but should be audible.

If any glow plug is silent when activated, it could be burned out or have a circuit problem. Make note of which locations are quiet.

This method provides a fast way to identify any plugs not turning on properly before moving to electronic testing. But keep in mind it’s not foolproof – some failed glow plugs may still click but not actually heat up fully.

Use a Multimeter to Test Operation

For definite testing of each glow plugs, you’ll need an automotive multimeter. Testing with a multimeter checks if the right voltage is getting to the glow plug for heating.

Here are the steps to test the glow plug operation using a multimeter:

Materials Needed:

• Automotive Multimeter
• Long socket extension
• Sockets to remove glow plugs

Step 1: Remove all the glow plugs from the cylinder head. Be very careful not to break the fragile heating elements.

Step 2: Set the multimeter to voltage mode and set the range to 12V.

Step 3: Have a helper sit in the vehicle and turn the key to the “on” position to activate the glow plugs.

Step 4: Touch one multimeter probe to the body of the glow plug. Then touch the other probe to the electrical contact at the bottom of the glow plug.

Step 5: Check the multimeter reading. You should see about 1-3 volts if the glow plug is activating properly. No voltage likely means a bad glow plug that needs replacement.

Step 6: Have your helper cycle the glow plugs on and off a few times while monitoring the voltage. Check all plugs in this manner.

Step 7: Note any glow plug showing a lower than expected voltage for replacement. Reinstall the functioning plugs once done.

Using a multimeter provides reliable electrical testing to verify if each glow plug is in fact receiving power to operate when activated. This method provides more definitive results than just listening to sounds.

Inspect Glow Plugs Physically

The final way to check the glow plug condition is to physically inspect each plug. You’ll want to look for a few key signs of wear:

Check the tip: Look at the hot tip of each glow plug. It should be smooth and shiny, not excessively corroded. White corrosion deposits can form on the tip over time. If heavy corrosion is present, the plug may not transfer heat well.

Check for cracks: Carefully inspect the entire metal housing of each plug for any fine cracks. Small cracks in the body allow coolant seepage into the heating element, reducing heating performance.

Check the seats: Make sure the seating threads are in good condition and screw in properly. The plug tip must sit firmly against the combustion chamber to transfer heat effectively.

Check the resistance: Using the multimeter probes, you can also check the resistance reading for each glow plug. Refer to factory specs, but readings should typically be around 1-5 ohms depending on the plug design. Higher than-expected readings indicate worn-out elements.

Any plugs with excessive corrosion, cracks, bad threads, or incorrect resistance need replacement. Install new plugs and retest operation.

Removing and Replacing Glow Plugs

If testing revealed any faulty or worn-out glow plugs, you’ll need to remove and replace them. Let’s go through the full process.

Materials Needed

• New replacement glow plugs
• Rubber insulated pliers
• Long socket extension
• Proper size sockets
• Anti-seize compound
• Torque wrench

Disable the Glow Plug System

Before removing the glow plugs, you’ll want to disable the glow plug controller to avoid accidentally energizing the plugs while disconnected.

Locate the glow plug control module under the hood and unplug the wiring harness connector. This opens the electrical circuit so voltage can’t reach the glow plugs.

If unplugging the module is not possible, remove the fuse for the glow plug controller circuit. Check your vehicle repair manual to find the correct fuse. Removing the fuse cuts off power, which also disables the system.

Clean the Glow Plug Areas

Once the system is safely disabled, clean the areas around the glow plugs before attempting removal. You want to prevent any dirt or debris from getting into the cylinder.

Use brake parts cleaner sprayed on a rag to carefully wipe down the glow plug bodies and surrounding cylinder head surface. Remove any oil deposits or built-up grime.

You can use an air hose with a blow gun nozzle to blow out the threaded holes as well. This clears any dust or particles so it doesn’t contaminate the engine interior.

Remove the Old Glow Plugs

With clean areas around the plugs, you can now begin removing the bad glow plugs marked during testing.

Insert a long socket extension – Due to the deep recess of the glow plug holes, you need a long extension to reach the plug socket. Slide a 6-inch or longer extension onto your ratchet or breaker bar.

Use proper size socket – The socket must fit snugly on the glow plug. Metric sizes around 14 or 16mm are common. Make sure the socket is in good condition to avoid rounding the plug.

Heat up the cylinder head – Heating up the cylinder head with a heat gun for 5-10 minutes helps avoid snapping off the glow plugs during removal. The aluminum engine warms up and expands, loosening the threaded connection.

Turn counterclockwise to remove – Place the socket over the glow plug and slowly turn counterclockwise. Don’t force it. The plug should unscrew and come out after breaking loose. Remove all the worn plugs marked earlier.

Install New Glow Plugs

Once the old plugs are out, it’s time to install the new replacements.

Apply anti-seize compound – This helps prevent the new plugs from seizing up or corroding over time. Place a small amount on the threaded portion of each plug.

Screw in finger tight – Carefully thread in each new glow plug by hand until snug. This prevents cross-threading before final tightening.

Torque to spec – Using your torque wrench with the extension and socket, torque each plug to the factory spec. This is usually around 7-15 ft-lbs for ceramic-style plugs. Don’t over tighten.

Reconnect controller – Finally, plug the glow plug controller harness back in or reinstall the fuse. The new plugs are now ready to operate.

Be extremely careful during the installation process not to damage the fragile glow plug heating elements. Getting the plugs properly seated and torqued avoids leaks or failed operations later on.

Verify Proper Operation

Once new glow plugs are installed, verify that they cycle on and off as expected before reassembling the engine.

Turn the key on and note the indicator light comes on for a few seconds. Carefully recheck the operation using the multimeter voltage testing method. Or listen for the activation ticking sound with your ear near the cylinder head.

If the new plugs operate correctly, you can button up the engine. Be sure to refill any lost coolant first. Dispose of used glow plugs, then wash them up.

That completes the glow plug replacement process. You’ll notice much better starting and idling now that your diesel has fresh, working plugs!

FAQ

Here are some common questions about testing and replacing glow plugs:

How often should I check glow plugs?

Inspect glow plugs annually before winter or any time you have starting difficulties. They wear over time, so periodic checks prevent getting stranded.

What causes glow plugs to fail?

Frequent cold starting, oil leaks, combustion blowby, and normal wear over miles can all damage glow plugs. Corrosion and carbon buildup contribute to failure too.

Why won’t my diesel start in cold weather?

If your diesel won’t start when cold, the first thing to check is the glow plug system. Bad glow plugs can’t warm the cylinders properly for ignition.

Can I drive with a bad glow plug?

You can limp home if just one plug fails, but performance will suffer. Allowing continued operation with dead glow plugs will cause misfires, white exhaust smoke, stalling, and poor combustion.

Why do my glow plugs break during removal?

Failing to heat up the cylinder head, using excessive force, or corroded threads can all cause plugs to snap off. Prevent this by heating the area first and turning slowly.

Conclusion

Checking and replacing faulty glow plugs using the techniques described in this article will keep your diesel vehicle starting and running optimally.

Bad glow plugs are a common cause of starting issues in cold weather especially. Following the testing procedures and visual inspections provides a proactive way to identify and address problems before they leave you stranded.

While testing glow plugs may sound complicated, it just takes some basic tools and mechanical know-how. Diagnose issues accurately, then replace worn plugs promptly. Your diesel engine will fire up quicker and run better than ever!