How Much Dirt Is Too Much?
With regular scheduled maintenance, most manufacturers recommend changing the engine air filter every 10,000 to 20,000 miles, mostly as a preventative step. Really, the idea is to change a dirty air filter before it’s too far gone. On carbureted engines, a dirty air filter can impact performance and fuel economy, foul spark plugs and eventually cause misfires. On fuel-injected engines, it can impact performance, but efficiency is usually unaffected.
Either way, a degraded air filter may allow unfiltered air into the engine.