The Flex-Fuel Sensor: How Excell Reads Your Tank
A 30-second engineering breakdown of the GM/Continental 13577429 sensor that makes plug-and-play E85 conversion possible.
Every Excell kit ships with a flex-fuel composition sensor inline with your fuel feed. It's the unsung hero of the entire system — without it, you're guessing at blend ratios and praying.
Capacitance, Not Chemistry
The sensor doesn't taste the fuel. It measures electrical capacitance between two concentric tubes the fuel flows through. Ethanol and gasoline have radically different dielectric constants — roughly 24 vs 2 — so the capacitance reading scales linearly with ethanol percentage.
Three Signals on One Wire
The sensor outputs a 50-150 Hz square wave where:
- Frequency encodes ethanol percentage (50 Hz = 0%, 150 Hz = 100%)
- Pulse width encodes fuel temperature
- Voltage drop confirms sensor health
Excell's main board parses all three in real-time and uses fuel temperature to compensate the AFR target. Cold ethanol behaves slightly differently than warm ethanol, and the difference matters when you're chasing 0.1 AFR precision.
Why Inline, Not In-Tank
Some OEMs mount their flex sensor in the tank. We don't. Inline mounting after the fuel pump means you read the actual blend hitting your injectors, accounting for any layering in the tank from fresh fill-ups. The trade-off is one more connection point — one we engineered with mil-spec connectors rated to 200°C.