Fuel injection systems run at a fairly high pressure, typically 43.5 PSI. While this seems high compared with atmospheric pressure, but it's about the same as common household water supply. It's not as scary as it might seem. However, generating this pressure requires a serious pump. Most EFI systems use a return type system in which a mechanical regulator limits pressure and routes excess fuel back to the fuel tank.

There were a number of challenges associated with typical mechanical fuel pressure regulators with return lines. I decided to design my own electronic system to meet these challenges. Here's a high level block diagram:

Fuel Rail Pressure Controller

Details on the controller board and connections are described below. Although the controller circuit is still somewhat experimental, it has gained a fair amount of interest by DIY fuel injection enthusiasts. If you are interested in obtaining a fully assembled and tested board, send email to madhu@madhu.com

It turns out that many cars use electronic FRP sensors to control fuel system pressure.

There are several advantages to this system:

• It's returnless
• It requires less power (about 2.5 Amps max)
• A standard motorcycle petcock/inline filter can be used
• Fuel pressure is adjustable

Bosch makes FRP sensors that are used in many Ford cars and trucks. You can find them on eBay or at a junkyard. It appears to have a transfer function that is identical to Freescale five bar pressure sensors. I don't think this is a coincidence.

Here's a picture of the FRP sensor mounted on the bike and the control circuit:

The AVR microcontroller adjusts pump motor speed using a digital proportional-integral (PI) control algorithm. The AVR samples system pressure at approximately 8kHz and adjusts pump voltage using pulse width modulation. It maintains constant pressure with only a few percent ripple. It's actually more accurate than a mechanical regulator under certain conditions. Given that injector flow rate is proportional to the square root of pressure, the flow rate does not vary much, if at all.

The pump itself was removed from a 2006 Suzuki GSX-R600 in-tank pump assembly. It's a Mitsubishi UC-T30 that requires roughly 4 amps at 12 Volts. This pump is widely used on motorcycle and some smaller cars. Here's a picture of the original pump mounted on the bike:

Fuel Pump

One of the ports on the end cap is the low pressure supply from the fuel tank through a standard on-off petcock. The other port is a breather line to vent any trapped air within the end cap.

In practice, the pump controller works quite well. So far, the only issue is that on hot days (above 90 F), the pump proper can get fairly warm after about 10 mins. This primarily occurs at idle or low RPM when insufficient fuel is flowing through the pump to cool it off. EFI fuel pumps tend to be very inefficient as only a very small fraction of the power they require is used to push fuel. The pump itself can take plenty of heat, it's the gas that can't. Above 120 F, gas near the surface of the pump inlet can boil, which causes the pump to cavitate and lose pressure. I have some heat sink fins made from thin copper sheet that I plan to attach to the pump housing to remove some of the heat. I made some simulations of natural convection and forced air heat sink efficiency. Given modest air flow through the cooling fins, it should be fine.

Bosch 4-pin FRP sensors

Bosch 4-pin FRP sensors are used on some later Ford cars, the 2006 Ford Focus for example.

Several Ford part numbers exist:

• 3F2E-9G756-AD
• 3F2Z-9G756-AC
• 5C3Z-9G756-AA

Ford part 3F2E-9G756-AD has been tested and known to work. The others appear to be identical, but have not been tested. These sensor often appear on eBay with the description INJECTOR PRESSURE SENSOR. They are usually available for low cost.

• Bosch 4-pin FRP sensor
• Pin connections are 1234 left to right looking at sensor pins, fuel port down
  • 1 - fuel pressure signal
  • 2 - +5v
  • 3 - fuel temperature signal
  • 4 - ground

• Estimated pressure
  • 0.50 volts – 0 PSI
  • 1.2 volts – 10 PSI
  • 1.65 volts – 20 PSI
  • 2.2 volts – 30 PSI
  • 2.75 volts – 40 PSI
  • 2.89 volts - 43.5 PSI
  • 3.45 volts – 50 PSI
  • 3.9 volts – 60 PSI
  • 4.6 volts – 70 PSI

Ford 3-pin sensors

Earlier Ford cars (2000 Ford Contour, Mercury Mystique, Cougar etc.) use 3-pin FRP sensors. These are functionally similar to the 4-pin Bosch sensors, but do not produce a temperature signal. These are typical part numbers that are known to work:

• F8CF-9F972-FF
• 9E26C-84FWC
• 9D29b-830K9

These sensor often appear on eBay with the description Sensor - Fuel Injector Pressure. They are usually available for low cost.

These are the pin connections:

AEM Sensor

The regulator circuit is known to work with AEM sensor part number 30-2131-100. It has a 1/8" NPT male thread:

Pumps

Just about any 12 Volt pump that requires less than 10 Amps maximum should work.

The controller has been tested with Mitsubishi pump model # UC-T30 and pumps from Suzuki LT-R450. The UC-T30 is used in Suzuki GSX-R600 in-tank pumps, as well as other vehicles. For very low power installations, the LT-R450 is a good choice. It requires under 2 Amps max. Even less current will be required when used with the pump controller based on fuel demand. Pumps from the Honda Rincon has also been suggested as another low power alternative.

Pump controller wiring

There are two sets of wires. The larger, 20 AWG wires connect to +12 Volt power and the motor. The colored Cat-5 cable connects to the pressure sensor.

Power/Motor wiring:

Both red wires are connected together, the second one is for convenience.

Sensor wiring:

Normal operation

When properly connected, the LED will flash twice indicating proper operation. If there is no fuel flow out of the system, e.g. injectors are closed, the pump will pressurize briefly (100 ms or less). If the pump continues to run, check for leaks, FRP sensor connections, or low battery voltage. If the pump does not run at all, check pump connections and FRP sensor connections.

Pressure adjustment

There are two buttons, one for pressure increase the other for pressure decrease. Each press changes pressure by a little less than 1/4 PSI. It is initially set to 43.5 PSI. To reset pressure to 43.5, hold the UP button for a few seconds. The LED will blink three times indicating it's back to 43.5 PSI.

Success stories

You read can about fuel pump controller success stories of other installations.