Determining The Cause Of Glow Plug Tip Failures

There are basically four causes for complete glow plug failures:

… Counterfeit or imitation glow plugs do not have the life or strength of a genuine Caterpillar glow plug.
… Using a voltage that is too high – for example a 12-volt plug in a 24-volt system, or jump starting with a portable DC welder.
… Energizing the glow plugs too long – observe the instructions in the Operation and Maintenance Guide.
… Fuel system problems can cause one or more plugs to fail.
a. primary pressure is too high
b. defective nozzles
c. fuel timing advanced too far.
Genuine Caterpillar glow plugs can be recognized by:

… Manufacturer’s name STAMPED into the barrel: either “GE” or “Wellman”.
… And Caterpillar part numbers, for example “3T9562”, STAMPED into the barrel.
… And manufacturing date code STAMPED into the barrel such as “JX” or “3Y”.
Counterfeit or imitation glow plugs often show a Caterpillar part number, but the Caterpillar part number is only INK-STAMPED on the barrel.

Glow plug failures due to engine related problems are covered in the following topic.

Examples And Causes Of Tip Failures

The piston, valve and turbocharger in Illustrations 1, 2 and 3 were destroyed by a glow plug tip that broke off. When an engine has piston, valve or turbocharger damage, many servicemen do not think of the glow plug as a possible cause. They often are the cause.

Always check the glow plugs of cylinders which have damage done to the valves and/or piston, and when there is no other clear cause for a turbocharger failure. The following paragraphs tell how to make correct identifications of the causes of failure of glow plug tips.

Illustration 2. Valve damage from a glow plug failure.
Illustration 3. Turbocharger damage from a glow plug failure. Note that the tips of the blades are gone.

Glow plug (1) in Illustration 4 is a new part. Use it for comparisons.

Glow plug (2) has part of the tip broken off. This failure was the result of rough combustion. The rough combustion was caused by the timing being approximately 20° early. If only one glow plug is damaged, check the lifter setting for that one cylinder. If all glow plugs are damaged, check the timing of the injection pump camshaft.

Illustration 4. Types of glow plug failures. Plug (1) is a new part for comparison.

Glow plug (3) is similar to plug (2). Its failure is the result of the same problem. In this case, the engine was operated longer with an early timing for the injection pumps. Note that the tip is broken off even with the case.

Broken tips can also be caused by slobbering fuel nozzles and/or primary fuel pressure being too high. High primary fuel pressure can be caused by a transfer pump with an incorrect or damaged spring, or a relief valve that is stuck closed. High primary pressure can also be caused by a plugged or pinched return line between the fuel pump and tank, especially on 1693, 1674 or 3306 Truck Engines.

Illustration 5. A failure from an electric short circuit to the case.

Glow plug (4) has the tip broken off inside its case. This was caused by an electric short circuit inside the element. Oil came in around the terminal and caused the heating element to make a short circuit to the case. The tip of this plug fell into the valve and piston area. This failure is found only in engines which have the glow plugs under the valve cover where they are covered with oil. Note the deformation of the case as indicated by the large arrow. This is the location of the short circuit.

Illustration 6. Plugs (4) and (5) were damaged through electric short circuits. The tips broke off inside the case.

Glow plug (5) had a similar failure to plug (4), but it does not have the deformation on the outside of the case. Note that both plugs (4) and (5) have tips broken off inside the case.

Illustration 7. Plug (6) had an electric short circuit inside the heating element. Note the hole in the tip.

Glow plug (6) had a short circuit in the heating element which burned a hole in the tip. This was caused by too much voltage, such as is made when a direct current (DC) welder is used to start machines in cold conditions, or using 12-volt plugs in a 24-volt system.

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