Diesel Glow Plug Replacement (OM617)

Step-by-Step Instructions

1. 01

   Disconnect the Battery

   Before working on the glow plug circuit, disconnect the negative battery terminal to prevent accidental short circuits. The glow plug system draws significant current (up to 100 amps total), and accidentally grounding a glow plug wire with the battery connected can cause sparks, burns, or electrical damage. Place the battery cable where it cannot accidentally contact the terminal.

2. 02

   Locate All Five Glow Plugs

   On the OM617 five-cylinder diesel, the glow plugs are located on the passenger side of the cylinder head, one per cylinder. They are threaded into the pre-chamber of each cylinder. You will see a heavy copper bus bar connecting all five glow plugs together. The glow plugs are most easily accessed from above, though the rear two cylinders may require reaching from different angles. Apply penetrating oil to each glow plug base and allow it to soak for at least 15-30 minutes before attempting removal.

3. 03

   Remove the Glow Plug Bus Bar

   The bus bar is a thick copper strap that connects the power supply to all five glow plugs. Remove the nut securing the bus bar to each glow plug terminal. These are typically 8mm nuts. Carefully lift the bus bar away, noting its routing. On high-mileage engines, the bus bar may be corroded or the nuts may be seized. Use penetrating oil if needed and be careful not to round the nuts.

4. 04

   Remove the Glow Plugs with a Deep Socket

   Using a 12mm deep socket on a ratchet or breaker bar, carefully loosen each glow plug. Turn slowly and steadily—glow plugs on older diesel engines are notorious for seizing in the cylinder head due to carbon buildup and corrosion. If a glow plug feels excessively tight, stop and apply more penetrating oil. Forcing a seized glow plug can break it off in the head, requiring expensive professional extraction. Remove each plug and its copper crush washer.

5. 05

   Test the Old Glow Plugs

   Use a multimeter set to resistance (ohms) to test each removed glow plug. A good glow plug should read approximately 0.6-1.0 ohms. An open circuit (infinite resistance) means the heating element is burned out. You can also do a visual test by connecting a glow plug to 12V briefly (using jumper cables with a battery)—the tip should glow bright orange within 5-7 seconds. This helps you understand which plugs had failed and caused your hard-start condition.

6. 06

   Install New Glow Plugs to Torque Specification

   Place a new copper crush washer on each new Bosch glow plug. Apply a thin coat of anti-seize compound to the threads only (not the tip). Thread each glow plug in by hand to avoid cross-threading—they should turn easily for the first several turns. Once hand-tight, torque each glow plug to 15-20 Nm (11-15 ft-lbs). Do not over-torque, as this can damage the pre-chamber or make future removal very difficult.

7. 07

   Reconnect the Bus Bar and Wiring

   Place the bus bar back onto the glow plug terminals and secure each connection with its nut. Tighten firmly but do not over-torque the small nuts. Ensure the bus bar does not contact any other metal surfaces, which would cause a short circuit. Inspect the main power wire to the bus bar for corrosion or damage and clean or replace if needed.

8. 08

   Test Cold Start Performance

   Reconnect the battery. Turn the key to the glow position and wait for the glow plug indicator light on the dashboard to extinguish (typically 8-15 seconds depending on ambient temperature). Then crank the engine. With fresh glow plugs, the engine should start within 2-4 seconds of cranking and idle smoothly. White smoke on initial startup is normal and should clear within 30 seconds. If one cylinder still seems rough, recheck that glow plug connection.

Frequently Asked Questions