On October 18, 2002, about 0830 eastern daylight time, a Piper PA-28-236, N4295R, sustained minor damaged during a forced landing near the Altoona-Blair County Airport (AOO), Altoona, Pennsylvania, following a partial loss of engine power. The certificated commercial pilot and passenger were not injured. Visual meteorological conditions prevailed for the local maintenance flight. No flight plan was filed, and the flight was conducted under 14 CFR Part 91. According to the pilot, the purpose of the flight was to check the autopilot system on the airplane. He noticed no anomalies with the airplane during preflight, taxi, or the engine runup checks. The pilot taxied the airplane onto the runway, advanced the throttle, and departed. After reaching 300 feet agl, the engine started to run "very rough." He switched from the right fuel tank to the left, and noticed that the vertical speed (VSI) was indicating a 700 fpm climb. After completing the emergency checklist, the pilot noticed that the climb rate had slowed to 200 fpm. The climb rate reduced further, and the airplane started to descend. During the descent, the pilot reduced the throttle and the roughness decreased. He then reapplied the throttle, and the roughness increased. Unable to maintain altitude, the pilot selected a forced landing area, secured the engine, and selected full flaps. The airplane touched down in a field, and began to rollout. The pilot maintained backpressure on the control yoke until the airplane impacted an embankment. The airplane came to rest, the pilot retracted the flaps, and shutdown the remaining aircraft systems before exiting. The pilot stated that an engine run was performed after the accident, and that power was increased to 1,500 RPM. The left and right fuel tanks were selected, and no anomalies were identified. Because of propeller damage, engine power was not increased above 1,500 RPM. The engine was then removed, and sent to the manufacturer for overhaul. During the overhaul process, no preimpact failures or malfunctions were identified. The engine was then reinstalled, and the airplane painted. While taxing the airplane after being painted, the engine lost power. The pilot noted that the engine would run normally when the right tank was selected, but not the left. When the left was selected, fuel pressure would drop to zero, and the engine would lose power. The pilot had the fuel selector removed, and examined. No anomalies were identified, and no obstructions were found in the fuel line that ran from the left fuel tank to the fuel selector. The pilot elected to change the fuel selector, and the problem did not reoccur. In addition, the pilot remembered that the fuel pressure gauge indicated zero when he was performing the forced landing for the original loss of power.