Building a DIY Water Injection System for Under $100

Components List

• 12V windshield washer pump (from auto parts store): $10–$20
• Brass nozzle (0.5–1.0 GPH flow rating): $5–$15
• Check valve (prevents backflow): $5
• MAP sensor (0–5V output for triggering): often already present in vehicle
• Adjustable voltage relay or Arduino Nano for logic: $15–$25
• 2L reservoir (repurposed washer fluid bottle): free
• 1/4" tubing and fittings: $10
• Wire, connectors, inline fuse: $10

Trigger Logic

The system should inject only above a MAP pressure threshold — typically 5–8 PSI boost (35–55 kPa above atmospheric) for turbocharged applications, or vacuum reduction below −5" Hg for naturally aspirated (indicating high load). An Arduino sketch reading the MAP sensor's 0–5V output and triggering a relay is cleaner than analog voltage thresholds but both work.

Nozzle Installation

Drill and tap a 1/8" NPT fitting into the intake pipe between the throttle body and engine. Position the nozzle so the spray pattern crosses the airflow. Install the check valve in the supply line between the pump and nozzle to prevent intake vacuum from drawing water back into the pump when the system is off.

Reservoir and Pump

Mount the pump and reservoir in the engine bay as close together as possible. The windshield washer pump flows approximately 0.5–2.0 gallons/hour depending on supply voltage and backpressure — adequate for most naturally aspirated applications. For more precise flow control, use a PWM controller on the pump to adjust flow rate.