7M-GE Idle and Power Issues from Running Pure Ethanol on Stock ECU

1987 Supra non-turbo (7M-GE) was experiencing idle and power issues after the tank was drained of old gas and refilled with 4 gallons of 100% ethanol, which the owner mistakenly believed was the correct fuel.

The stock 7M-GE ECU has no way to detect or compensate for ethanol fuel. Ethanol is significantly less energy-dense than gasoline and requires 30-50% more fuel volume to produce the same power. The stock injectors cannot deliver enough ethanol to maintain proper air/fuel ratios, causing the engine to run extremely lean chemically. This results in rough idle and reduced power.

Moderate confidence — Original poster confirmed this resolved the issue