How to build a simple DIY fuel injection ECU controller

by braniac

This is a follow up to the HHO electrolyser article. It can be used to take advantage of the increased lean burn limit provided by HHO gas injection systems. It can also increase mileage slightly by simply making your car run slightly leaner/more efficient

hole alignments for mounting components

After obtaining materials, and organizing your work area. Drill a 3/8" hole in the top of your project box (or 2 if you decide on the dual control option explained later). The hole needs to be centered for 1 pot. (potentiometer) or aligned equally for 2 pots.(see pic) Then drill a 1/4" hole, or whatever size is suitable for the switch(es) you chose, along the top margin of the project box. The top can be arbitrarily assigned by wherever you drill the holes, as the project box usually has no set orientation. Now mount your pot./pots. and switch/switches in these holes. Note: you will use 2 switches, and pots. for dual control option.

After mounting your parts in the box, orient the pot., leads down, away from the top switch to make wiring easier. Solder the 33K 0.5watt resistor to the ground lead(far right lead if looking at the pot. with adjustment stem facing you). Then solder you black wire onto this. For dual option, repeat on second pot.. The ground wire can be wired common(together). The resistor acts as a "trim" to give more range of adjust ability.

Next, solder the brown wire to the middle lead, and green wire to the last (left) lead, on both pots. if using dual design. Now it's time to wire in the switch(es). For single control, the green "in" wire from the MAP sensor will have a jumper to the left pole of the switch, and run common to the pot.. the center pole is output to ECU, and the right switch pole will be the modified signal from the pot. so when the switch is in one state the circuit is modified and in the other state the circuit is original. In dual control it's just a little more complex. The 1st switch, switches from modified to unmodified, and the 2nd switches between the 2 pots.(see diagram). The first switch still runs the green wire "in" to the right switch terminal, and runs common with both pots.. the second switch "chooses" which pot. output to let through. It is wired with center terminal connected to the left terminal of first switch, left terminal to brown "out" wire from left pot, right terminal to brown "out" wire from right pot.. Leave enough wire coming out of your project box to go from your engine compartment, through the firewall, and to your console or dash board where you will mount it.

install diagram

Find the MAP signal wire on your car. Using a multi tester set for about 20 Volts DC range use the point terminals through the wire sheathing to find the wire that varies voltage with engine load. This is the signal wire. There will be one wire that is 5 volts constant, and one that is ground, DO NOT cut them. Only cut the signal wire, and connect the sensor side to your green "in" wire for the controller, and the other side going to the ECU to the brown "out" wire. Mount the unit in a location handy to adjust while driving.

This step has been added after additional research. In many cases it may be better to install this device in it's simpler single configuration to the 5 Volt reference wire of the MAP sensor. The reason being that the added resistance to the signal side may cause premature failure of the sensor in some cases due to a small voltage feedback. Changing the 5 volt Vref does not pose this risk, and is a more accurate way to manage the tuning. The device should be built with a 15 turn multi turn pot. when used in this configuration, and the fixed resistor for trim can be eliminated, but the signal will still be slightly modified even if the resistance is turned all the way down, because the pot. has a small baseline resistance that cannot be changed. To find the Vref wire simply use your DVOM(multi meter) to find the wire with 5 volts constant, and wire the device in series into it.

To tune the MAP adjuster it is best to use a vacuum gauge put to manifold vacuum with enough vacuum tubing to have the gauge, and the adjuster in the cabin with you. Get on a straight level section of road with little or no traffic. Set the cruise. Make sure you are holding a constant speed, and RPM. Note the inches of vacuum on the gauge. Now turn the pot. counter clockwise until you achieve maximum vacuum. Turning past this point will not gain you any better results as you are also affecting ignition timing curve with this device. The highest vacuum setting will get you the best volumetric efficiency. In other words you are maintaining a set speed with the least amount of air, and therefore fuel. Some people disconnect the O2 sensor when using a MAP adjust. This allows you a broader range of adjustment. However your vehicle will illuminate the check engine lamp when you do this, and in some areas the vehicle will not pass inspection if the MIL lamp is on. Disconnecting the O2 sensor causes the vehicle to operate in open loop mode all the time, and makes the ECU more dependent on the MAP sensor to reference it's fuel tables for operation. The MAP adjust makes the computer look up a leaner set of fuel tables. If adjusted correctly as described above this will not harm your engine, but it is not legal in some areas to operate in this mode, and obtain state inspection.


  • check The reason for dual control is to have 2 preset adjustments, for cruise/pass, city/highway etc.. or whatever you find you need.
  • check Once you fine tune the controller, only small occasional adjustments are usually needed with dual control.
  • check This is only 1 device used in tandem with many others in a professional system to address engine control while using a HHO generator.


  • close DO NOT ATTEMPT IF YOU ARE NOT FAMILIAR with your car, and general mechanical know how. The wiring for every make of car is different so know what is what before modifying anything. Always find the variable voltage wire to connect the controller to.
  • close Don't run the engine too lean, this will cause overheat, and detonation, which can be harmful to the engine.
  • close This device is recommended for use with an HHO gas injection system that greatly increases the lean burn limit on the engine.
  • close Improper installation will cause the engine to run poorly, or not at all.
  • close Improper tuning will cause "check engine" or fault lights, which can be reset by disconnecting the battery for about 15 seconds.
  • close This device is designed for a MAP sensor, and will not work on many MAF sensors.
  • close This device does not provide complete control of your cars ECU it only modifies the computer's barometer. For adequate control of an ECU to utilize HHO gas from a generator, all engine sensors must be addressed, and the tools, and techniques to do so get very complex. Email me at for details on professional systems, and for classes on HHO conversion. The classes are with, and without professional certification.

Items you will need