Glow plugs help trucks and other diesel vehicles start in cooler weather. These plugs promote faster combustion for faster starts in cold temperatures since compression alone works well enough to start the engine during normal temperatures.
What Are Glow Plugs?
Glow plugs resemble spark plugs found in the engine manifold or cylinders. They are metal parts with long, slim shapes and toaster-like heating elements at the bottom that connect to electrical wires, enabling combustion. The plug body on the cylinder head grounds the bypassing current.
What Do Glow Plugs Do?
Diesel fuel does not ignite easily when cold, so it needs immense pressure to auto-ignite. The high compression creates heated air that combusts when met with diesel fuel, making auto-ignition challenging when the engine is cold.
Glow plugs feature heating elements that glow when heated through electrical energy, reaching up to 1,300 degrees Celcius or 2,372 degrees Fahrenheit. These components promote combustion, aiding the internal compression by igniting the fuel when the heated elements come in contact.
Direct-injected diesel lacks glow plugs, while in-cylinder counterparts preheat an engine for up to five seconds before starting. The plugs spring into action when the ‘wait to start’ warning kicks in, during which the vehicle warms up. This light signals on your dashboard and should be off before starting the engine.
Ignoring this light can damage the cold-start feature since the brief waiting time allows the glow plugs to function.
Types of Glow Plugs
Diesel-operated passenger vehicles, trucks and other heavy-duty non-commercial vehicles use various types of glow plugs:
- Ceramic glow plug: This plug heats low-compression diesel engines up to 1,500 degrees Celcius or 2,732 degrees Fahrenheit in seconds for a quick start.
- Pressure sensor glow plug: This plug includes a sensor that saves the measured pressure data in the combustion chamber and sends it to the engine control module (ECM) to release the appropriate heat for combustion.
- High-tech steel glow plug: GN and GE three-phase technology preheats during ignition, heats as the vehicle starts and heats for roughly three minutes after starting. The GV-type alternative uses two-phase technology and has a preheating time of about five to seven seconds, which is more suitable for aged diesel engines.
Commercial diesel engines use a specific type designed for their unique requirements.
Common Reasons for Glow Plug Failure
Glow plugs generally have an 80,000- to 100,000-mile life span. At the end of their life span, they may start to fail, displaying visible signs that they need replacing:
- Fuel injectors spraying incorrectly: The elements risk burning out if the tips are constantly cooled by fuel patterns squirting irregularly or in large volumes.
- Wear and age: The glow plug’s relay wears out with excessive usage over time, potentially making the vehicle unresponsive or causing it to struggle to start.
- Carbon buildup: Carbon deposits accumulating on glow plugs during incomplete combustion may impact their heating ability and performance.
- Consistent heat stress: Heating continuously may cause the element to degrade over time.
- Overvoltage: Too much voltage from the ECM can burn out the glow plug’s heating element.
- Incompatible fuel additives: Using the wrong additives may impact performance.
The same plug and voltage type must be used when replacing glow plugs for diesel engines. The ECM must be reprogrammed when changing the plug type since it can’t be converted in all vehicles with different voltages.
FAQs About Diesel Glow Plugs
We’ve compiled some frequently asked questions to address potential glow plug issues.
How Do I Test Glow Plugs?
Test your glow plugs by performing a diagnostic inspection. Check for visible damage and compare the plugs using the diagnostic codes in the user manual. Using an infrared thermometer, scan each plug to determine the temperature reading. Low temperatures may indicate faulty plugs.
To further test the glow plugs, you’ll need a few tools, including a multimeter, wrench or socket set, torque wrench for reinstallation, safety gloves and optional goggles.
Follow these steps:
- Turn off and cool the engine, then disconnect the negative battery terminal.
- Locate the individual glow plugs on each cylinder in your engine.
- Inspect each plug’s wiring and connectors for damage, which should be replaced or repaired before you continue testing.
- Use the wrench to remove the plugs, noting which was attached to each corresponding cylinder.
- Test the plug’s resistance by setting the multimeter to ohms and placing a probe on the metal tip and another on the body. The reading should be between 0.5 and 5 ohms, and the glow plugs should be replaced if the readings are significantly lower or higher.
- Place a probe on the plug terminal and another on the body to check for a short circuit. A high reading could indicate a short, meaning the plug must be replaced.
- Reconnect the plug and battery’s negative terminal, then turn on the ignition without starting the engine to see if the plug heats up and glows.
- Clean the threads if the glow plugs are functional, using a torque wrench to reinstall it.
- Reconnect all the parts, including the glow plug’s wiring harness and negative battery terminal.
- Start the engine to see how quickly it responds — if you don’t have issues, the glow plugs are functional.
Always check your user manual if unsure about anything during testing or seek professional advice.
How to Change a Glow Plug?
Change your glow plugs by following these steps:
- After disconnecting the battery’s negative terminal, find the glow plugs on the cylinder head in the engine bay.
- Disconnect the wiring harness without damaging the wiring.
- Use a wrench to remove each plug from its socket and inspect the individual plugs for carbon buildup or other damage.
- Apply dielectric grease to each new glow plug’s thread to help prevent corrosion.
- Thread each new glow plug into the cylinder head and tighten them with a torque wrench, ensuring they’re not too tight.
- Use a wrench to reestablish the electrical connections, attaching the wiring harness to the new glow plugs.
- Reconnect the negative battery terminal after securing the electrical connections.
- Start and idle the engine while checking for warning lights and sounds. If there are none, your glow plugs have been successfully changed.
How Much Are Glow Plugs?
The price of glow plugs varies between vehicle makes and models and whether you buy them individually or as a set. A reputable dealer should indicate how much you’ll pay upon confirming this.
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