P0171 and P0174 indicate a LEAN fuel condition (too much air and not enough fuel).
1. The first thing to check for is any vacuum leaks around the intake manifold (the gasket seams, vacuum fittings and hoses). Leaks here are very common.
2. The next most likely possibility would be a dirty mass airflow sensor. The sensor is located in the ductwork between teh air filter and throttle body. Cleaning the sensor element with aerosol electronics cleaner often solves this problem. You already did that, so you can probably rule the MAF sensor out as a cause — unless it is defective and is underreporting airflow.
Yet another possibility is a defective DPFE sensor. This device is part of the EGR system, and it sometimes goes bad and causes too much EGR flow, which creates a lean condition. Replacing the sensor often fixes the codes.
3. Finally, dirty fuel injectors or low fuel pressure can also cause the lean codes.