BuildCraft version: 7.99.19
Forge version: 14.23.5.2768
Link to crash report or log: N/A
Singleplayer or multiplayer: Both
Steps to reproduce:
1)Build the Combustion Engine
2) select a fuel source and feed it to the combustion engine (Gaseous Fuel used for testing purposes)
3) apply kinesis pipes and something to consume power (BC Quarry used for testing purposes)
4) Activate engines by applying a redstone signal and observe as engines operate without being supplied with a coolant.

Additional information: According to ingame information, as well as what online documentation is available, the combustion engine is suppose to require both coolant and fuel in order to operate. However, at present the engine only requires that it be supplied with fuel. It is producing the correct MJ per each fuel type (8MJ for Gaseous Fuel, 6MJ for Fuel, 4MJ for Dense Fuel) however it is ignoring the requirement for coolant. I have tested this in a single player instance that contains only Buildcraft 7.99.19 and Forge 14.23.5.2768 to ensure this bug was not the result of some other mod.

PS: In your preformatted bug report form you have misspelled "relevant" and "suggestion". Your form has it spelled "relavant" and "sugestion"

It's not a hard requirement for coolant but as engines get hotter they need to be cooled down or they will overheat.

So I guess the documentation should specify that coolant is only required for preventing overheating rather than normal operation.