Final answer:
Excessive ethanol in fuel can lead to rough idle or stall, and sometimes high idle speed, but is less likely to cause late transmission shifting or poor AC performance. These issues arise from ethanol's lower energy density compared to gasoline, affecting engines not designed for high ethanol content.
Step-by-step explanation:
Excessive ethanol in fuel can cause various symptoms due to its different physical and chemical properties compared to gasoline. Ethanol has a lower energy density than gasoline, which means that it provides less energy per unit of volume. When an engine is not specifically calibrated or equipped to handle ethanol-rich fuel, this difference can lead to issues, including rough idle or stall, and potentially high idle speed.
However, excessive ethanol in fuel is unlikely to cause late transmission shifting or poor AC performance since these are typically not directly related to the properties of the fuel. Vehicles that are not "flex-fuel" vehicles might experience lower energy performance and other related symptoms due to incompatibility with higher percentages of ethanol, such as E85.