How to troubleshoot poor engine performance in an Audi 80?

I recently noticed that my Audi 80 is not running as smoothly as it used to. The engine seems to lack power and there is a noticeable decrease in fuel efficiency. What could be causing this issue?

First, check the air filter and clean or replace if necessary. A dirty air filter can significantly impact engine performance. Also, make sure the spark plugs are functioning properly as they play a crucial role in the engine's combustion process.

Another potential culprit could be a clogged fuel filter. If this is the case, it can cause a decrease in fuel flow to the engine, resulting in poor performance. Consider replacing the fuel filter every 20,000 miles to prevent this issue.