Type: Glitch
When my combustion engine was red (about to explode/die), I turned off the power to it, but when I did it kept running, cooling down, producing small amounts of energy, and not changing back to Yellow -> Green -> Blue.
(Sorry for bad snapshot of the red engine, but you can see it through the pipe)
The engines in the background I disconnected a while ago, the power that the hot engine is the only power in the pipe.

EDIT: It went away after I exited and re-loaded the world

Installed mods:

• Carpenter's Blocks 3.3.6
• Code Chicken Core 1.0.4.29
  • Not Enough Items 1.0.3.74
• CoFH Core 3.0.2
  • MineFactory Reloaded 2.8.0
  • Redstone Arsenal 1.1.0
  • Thermal Foundation 1.0.0
    • Thermal Expansion 4.0.1
  • Nether Ores 2.3.0
• Micdoodle Core 3.0.11.306
  • Galacticraft Core 3.0.11.306
    • Galacticraft Planets 3.0.11.306
• Added crafting recipes 1.0 Final (Self made, private, only workbench recipes)

The only mod that was within the energy production process was Galacticraft

Buildcraft 6.4.14

Forge version 1.7.10-Forge10.13.2.1291

From what I understood, I think this is not a bug, this is intended.

Engine and emerald pipe have energy buffer.