Is Water-Methanol Injection Legal in the US?

Conditional

Water-methanol injection is legal in most of the US when it supplements a healthy, emissions-intact engine. In California and CARB states it counts as an emissions-related modification, so street use is only clean with a CARB-recognized setup.

Why?

WMI sprays a water-methanol mist into the intake to cool the charge and suppress knock. On a gasoline engine with the full emissions system working, the federal picture is benign: nothing is removed or defeated, so the Clean Air Act's tampering rule is not triggered by the injection itself. The kits are widely sold and run on pump-gas street cars across the country.

CARB states are stricter by category. California treats add-ons that affect engine operation as emissions-related, which means a WMI kit without CARB recognition sits in gray territory at a smog check, and a visible tank and nozzles can prompt questions. The practical CARB-state route is a kit and tune with the paperwork to match, or keeping the system for track use.

Diesels carry the sharpest history: WMI marketed alongside delete tunes put several diesel-performance sellers into EPA enforcement, so on a truck the question is always what the tune does, not the water itself. Everywhere, the engineering rule doubles as the legal one: failsafes so a dry tank or clogged nozzle cannot melt the engine, and a tune that stays clean with or without injection.

What decides if it's legal

WMI must not replace or defeat any emissions control.
In CARB states an emissions-related add-on generally needs a CARB EO.
On diesels, WMI paired with delete tuning has drawn EPA enforcement.
The system must be plumbed and failsafed so a fault cannot cause damage.

Does it depend on your state?

Yes. Non-CARB states have little to say about a supplemental kit on an emissions-intact car; CARB states treat it as an emissions-related mod with the paperwork expectations that follow. Diesel applications get more scrutiny than gasoline ones everywhere.

Related legality guides

Find your car in the garage

Sources

This page is general guidance, not legal advice, on the rules in the US for water-methanol injection. The detail varies by exact vehicle and changes over time - confirm with your insurer and the EPA, NHTSA and your state DMV before modifying.