On July 12, 2004, at 1730 Pacific daylight time, an Ercoupe 415-D, N3837H, collided with a rock during a precautionary landing to a field in Santa Clarita, California. The private pilot, also the registered owner, was operating the airplane under the provisions of 14 CFR Part 91. The pilot, the sole occupant, was not injured. The airplane sustained substantial damage. Visual meteorological conditions prevailed for the flight, and no flight plan had been filed. The airplane departed about 1630 from the Santa Monica Municipal Airport, Santa Monica, California, and was destined for Modesto City County-Harry Sham Field, Modesto, California. According to the pilot, after departure from Santa Monica, he climbed to 1,800 feet mean sea level (msl), which he maintained while crossing through the Van Nuys, California, airspace. As he began his climb northbound, he encountered turbulence. Approximately 2,500 feet msl, the airplane was not climbing and the oil temperature gauge was indicating 220 degrees Fahrenheit (F). The pilot elected to reverse course and return to Whiteman Airport, Pacoima, California; however, during the turn he lost altitude and the airplane would not climb. Assuming that he would not be able to attain the altitude needed to transition through the Newhall Pass, the pilot made a precautionary landing in a field. During the landing roll, the left main landing gear collided with a rock and sheared off, resulting in substantial damage to the main spar. The pilot manually shutdown the engine after the landing. The pilot stated that his engine was running normally, but the temperature was warmer than he expected after his departure. When he departed, the temperature was about 80 degrees F; as he progressed northward, the temperatures exceeded 100 degrees F. Post accident examination and a test-run by a Federal Aviation Administration inspector did not reveal any operating anomalies with the engine. The inspector did note that the air filter was dirty. The density altitude at the accident site was calculated to be in excess of 5,300 feet.