3406E / C15 Fuel in Coolant, Engine Miss Explained

Caterpillar 3406E, C15, & C15 Acert engines all use the same style cylinder head, injectors, and injector cups (see images 1 & 2). The injector on these engines seats directly against the cylinder head casting, not on the injector cup. The seat is located below the injector cup and should completely seal the cup from combustion pressure and heat. (see images 3 & 4). The injector doesn't actually even touch the injector cup. One issue with this design is that it is possible for the injector not to seat completely against the cylinder head. This may be due imperfections in either the head casting, the injector, or both. If the injector to head seat does not sealing it will let engine cylinder pressure go past the seat and pass in-between the cup and injector. When this happens the the cylinder pressure and heat can now reach both the injector o-ring and the upper injector cup o-ring. These o-rings will then fail as they were not designed to withstand the high cylinder temperatures and pressure.

Some of the signs that an injector seat is not sealing correctly are:

  1. Burnt injector o-rings
  2. Carbon buildup or tracking on the inside of the injector cup or on the injector body above the sealing line (see images 6 & 7).
  3. Traces of coolant or leaking coolant around the injector cup.


The failure of these o-rings can lead to problems such as:

  1. Fuel in the coolant
  2. Engine miss from combustion pressure aerating the fuel
  3. Overheat of the injector tip causing a miss


The cause of this type of failure can be contributed to a few possibilites such as:

  1. The injector body is worn, warped, or damaged.
  2. The cylinder head casting is worn, pitted, or otherwise damaged.
  3. The injector hold down bolts were improperly tighted or stretched.
C15 Injector Image 1. Complete injector for 3406E / C15
C15 Injector Cup Image 2. 3406E / C15 Injector Cup
C15 Injector Seat Image 3. Seating surface of injector tip to cylinder head
C15 Injector and Cup 2 Image 4. Seating surface of injector to head is below the injector cup
C15 Injector and Cup Image 5. C15 Injector shown installed in the injector cup
C15 Injector Cup Carbon Image 6. Start of carbon buildup due to leak at injector seat
C15 Injector Cup Carbon 2 Image 7. Carbon buildup due to leak at injector seat








  • Posted on   11/13/14 at 05:12:34 PM   by Nathan  | 
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Tagged with 3406E, C15 Fuel, Coolant, Engine Miss

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