On October 4, 2001, at 1000 eastern daylight time, a Cessna 172B, N7591X, was substantially damaged during a collision with terrain, following a loss of power near Phillips, Maine. The certificated private pilot/owner was seriously injured, and the pilot-rated passenger sustained minor injuries. Visual meteorological conditions prevailed for the personal flight conducted under 14 CFR Part 91. According to the pilot, he performed a thorough preflight inspection and departed from the Pittsfield Municipal Airport (2B7), Pittsfield, Maine, around 0945, destined for the Morrisville-Stowe State Airport (MVL), Morrisville, Vermont. About 30 minutes into the flight, while climbing through 3,700 feet to 4,500 feet, the airplane experienced a "complete engine failure." After an unsuccessful attempt to restart the engine, the pilot prepared for a forced landing to a field. While on final approach for the field, the engine regained full power and the pilot decided to perform a go-around. The airplane cleared the trees at the end of the field, and then the engine lost power again. The airplane then entered a "low altitude power off stall," and impacted the ground in a 45-degree nose low attitude. A Federal Aviation Administration (FAA) inspector performed an on-scene examination. He stated that substantial damage was observed to the engine, fuselage, and cockpit. The engine was rotated by hand at the propeller flange, valve train continuity was established and thumb compression was obtained. The left magneto was broken off the engine and was impact damaged. The right magneto remained attached to the engine, and when tested, spark was produced on all towers. The spark plugs were removed from the engine and a "rust-colored" powder was observed on them. Flight control continuity was established from the cockpit to all flight control surfaces. According to the inspector, at least 7 gallons of fuel were observed in each fuel tank. A fuel sample was drained from the tanks and appeared to be a mixture of auto gas, 100 LL aviation fuel, and a fuel additive. Fuel line continuity was established from the fuel tanks to the engine. Examination of the fuel vent lines revealed the left fuel vent line was contaminated with mud near the intake, which completely sealed the line. The left fuel tank was equipped with an unvented cap, and a vented cap was installed on the right fuel tank. The pilot reported that he refueled the airplane with 14 gallons of "Irving gas" on the night prior to the accident. He stated that "Irving gas" was a Canadian brand of auto fuel which he routinely used in his airplane. The pilot had been refueling the airplane from the same supply for about 2-3 weeks prior to the accident. The fuel was stored in 6-gallon containers in which the pilot had also stored a fuel mixture, which he used in high performance cars. This mixture combined Irving gas with a high performance fuel additive. Additionally, a friend of the pilot's reported to the FAA inspector that the pilot routinely used a "fuel cocktail" in his airplane. The "fuel cocktail" consisted of a lead additive, octane booster, aviation fuel, and auto gas. Examination of the airplane logbooks revealed that a Supplemental Type Certificate (STC) was issued on October 22, 1984, which approved the use of unleaded and leaded auto gas in the airplane. Entries were dated in the airplane and engine logbooks on October 18, 1984, referring to the STC approval. The most recent annual inspection was performed on April 17, 2001, with no deficiencies noted. According to the Cessna 172B Pilot Operating Handbook, the fuel selector installed in the airplane could be selected to 'both,' 'left,' 'right,' or 'off.' The FAA inspector reported the fuel selector was observed in the 'off' position during his examination of the airplane, and emergency personnel reported turning it off. The pilot could not recall which tank was selected prior to the accident. The weather reported at the Waterville Airport (WVL), Waterville, Maine, at 0955, included winds from 200 degrees at 5 knots, 9 miles visibility, clear skies, temperature 16 degrees Celsius, dew point 14 degrees Celsius, and altimeter 29.84 in Hg.