Hi Is the battery full. If you have a full battery the engine will sound like its going to red line when in fact the generator is spinning and no fuel is being used.
Your statement is very confusing. For clarity, I monitor rpm of ICE, the generator, and the motor all the time with ForScan. When ICE is off (rpm = 0), the motor and generator spin at the same rpm whether the battery is full or not in my 2013 and assume the same in all C-Maxs. That's the way the planetary gear system works. One does not hear the generator spin when the HVB is full especially if it sounds like ICE going to redline.
ICE will spin when coasting if the HVB is full just like ICE in a "normal" car will do. The purpose of spinning ICE is to provide normal engine braking because engine braking can no longer be done with the traction motor (via regenerative braking) since the HVB is full. One would have to be going very fast to have the engine near red line.
If you have grade assist on going down a hill, ICE will spin if speed begins to increase and cannot be controlled by regenerative braking (either max regeneration is reached or HVB is full). ICE rpm will vary (increase / decrease or shut down) to help maintain speed. This is where ICE rpm can increase significantly if the hill is steep and can be heard. I have never seen grade assist use the friction brake to maintain speed. The driver assumes such responsibility.
Bottom line: I'm not sure what you are trying to say with repect to the sound the OP is hearing. The only time ICE spins more than what one would expect and may approach redline is when grade assist is being used. The HVB does not have to be full for ICE to increase rpm to assist in maintaining speed (not speeding up) when going down hill using grade assist.
Edited by Plus 3 Golfer, 06 May 2017 - 05:42 AM.