Using ScanGauge II to Monitor Your HHO System's Impact on Fuel Economy

Why ScanGauge II for HHO?

The ScanGauge II is a $150 OBD-II monitor that displays up to four live data parameters simultaneously on a small screen that mounts to the dashboard. For HHO owners, it provides direct, real-time feedback on whether the system is helping or being canceled by the ECU.

Key Parameters to Monitor

• INST MPG (Instant MPG): Shows real-time fuel economy — watch how it changes with HHO on vs off
• STFT (Short-Term Fuel Trim): Immediate ECU fuel correction — should be near 0% with HHO + EFIE properly tuned
• LTFT (Long-Term Fuel Trim): Learned ECU correction — should be slightly negative (−5% to −15%) after HHO installation to indicate fuel savings are being maintained
• O2S1 (Upstream O2 voltage): Watch for voltage patterns to confirm EFIE adjustment effect

Establishing a Baseline

Before installing HHO, spend one week recording Trip MPG from the ScanGauge on your normal commute routes. Average several trips for each route type (city, highway). This baseline is your comparison point — accurate to ±1% over multiple fills.

EFIE Tuning with ScanGauge

With HHO running and EFIE installed, increase EFIE adjustment while watching LTFT. Target LTFT of −8% to −15% at cruise. If LTFT goes beyond −20%, back off the EFIE adjustment — excessive lean correction risks catalyst damage and engine stress.

Trip vs Tank MPG

ScanGauge Trip MPG is calculated from fuel injector duty cycle and is highly consistent trip-to-trip. Compare it to fill-up-based calculations for a cross-check. A 10% Trip MPG improvement that's not reflected in fill-up calculations suggests the OBD fuel consumption calculation has a small calibration offset.