How to test and replace glow plugs in the diesel engine of my ship or boat

What are glow plugs?

A marine diesel engine has electric glow plugs that help it start when cold. Diesel engines use spontaneous combustion by compressing oxygen and fuel. Just before the highest pressure is reached and the piston is in the top position, fuel is injected through a vaporiser and the combustion process begins. As the engine can be very cold after not running for a while, it may need a little help. That is where the glow plugs come in. Glow plugs are located in the combustion chamber of the engine and are made of high-grade materials.

Testing a glow plug

To test the glow plug, measure the resistance using a multifunction meter or ohmmeter The wires are to be disconnected before measuring. The exact value of each plug varies, but a defective plug can easily be distinguished from a working one. If a current passes through and the ohmmeter shows a value, it would be fair to assume that the plug is in working order. If it reads infinite, there is an interruption and it would be fair to assume that the plug is defective.

Two types of glow plugs

Glow plugs come in two different types:

1. Parallel glow plugs

   Parallel glow plugs are connected to earth (engine block) via the entry. The current flows back to the battery via the screw thread. They have one entry and are connected to one another. These are usually the Quick Glow Systems that run on the direct battery voltage.

2. Serial glow plugs

   Serial glow plugs are working like a light bulb. These are often seen in old systems or spiral filaments. They have an incoming and outgoing wire or connection to the next one. The first plug is the entry and the last one is connected to earth. They work on reduced voltage with pre-heating resistors and a glow eye.

DIY replacement of glow plugs on a marine engine.

Remove any defective glow plugs. NB: they can be very tight, so be very careful, especially when dealing with thin-walled glow plugs. Spray generously with penetrating oil before removal if corrosion has built up around the plugs. Thicker plugs are less particular because of their different diameter and plug ratio.
Keep the area around the opening of the motor clean. Watch the maximum loosening torques (Nm = Newton metre = unit of torque) and the thread size (M).

Tip: also use a torque spanner for loosening.

Maximum loosening torques

• M8 – 20 Nm
• M9 – 22 Nm
• M10 – 35 Nm
• M12 – 45 Nm

Fitting new glow plugs

When fitting new glow plugs, it would be best to use the tightening moment as prescribed by the engine manufacturer. A little copper grease on the cutting edge and the thread is recommended. Tightening is also done with a tightening moment. They only need to seal. Indication of tightening moments for electrical connections:

• M8 – 10 Nm
• M9 – 12 Nm
• M10 – 15 Nm
• M12 – 22 Nm

Tightening moment for electrical connections

• M4 – 2 Nm
• M5 – 3 Nm

Removal of jammed glow plugs

When the maximum removal torque has been reached, discontinue the loosening effort and do not overstrain. That is because the glow plug may break. And that means dismantling the entire cylinder head. Try to loosen the glow plugs in the following three steps:

1. Loosening: Spray with penetrating or synthetic oil. Repeat several times during the day and leave it to soak in overnight.
2. Heat the plug: Expanding and contracting materials may create more play. Tune up the engine or use a power cable to warm up the self-regulating glow plugs and proceed a little longer than usual. Repeat this procedure. Please note: This procedure works only on glow plugs with 11 – 12V operating voltage.
3. Loosening: Try to unscrew the glow plug once again. Using a suitable tool, carefully unscrew the glow plug in the cylinder head. A long, extended socket wrench often works better than a spanner. This will distribute the force more evenly without the need for straining it sideways. Use a tap wrench to work with both hands. If no luck, repeat the above three steps.