What problem can high fuel viscosity cause in a metering system?

High fuel viscosity can significantly affect the performance of a fuel metering system primarily by causing flow issues and degradation in engine performance. When fuel viscosity is high, it means that the fuel is thicker and flows less easily through the metering system. This increased resistance to flow can lead to an insufficient fuel supply to the engine, resulting in inconsistent combustion, misfires, and overall reduced efficiency.

Additionally, the viscosity can impact the ability of fuel pumps and injectors to deliver the correct amount of fuel at the right pressure. If the fuel does not flow smoothly, it can create challenges in maintaining the optimal air-fuel mixture, which is crucial for proper engine operation. Consequently, engine power may be compromised, and emissions can increase as the combustion process becomes less efficient.

In contrast, the other options do not accurately reflect the effects of high fuel viscosity. Increased fuel pressure might occur in some scenarios, but it's typically the flow issues that manifest first. Higher viscosity doesn't lower the risk of engine knocking; it generally increases the chance due to poor atomization of fuel. Lastly, enhanced fuel economy is unlikely, as the engine will struggle to operate efficiently with higher viscosity fuel.