Fuel Pressure Sensor, Diagnosis, and Engine Protection

Fuel Pressure Sensor, Diagnosis, and Engine Protection

This guide is based on the TI 150 psi pressure sensor.  Other sensors can be used however please check wiring and calibration data to suit the sensor.




About

Haltech Part HT-010904


A fuel pressure sensor is a sensor that can read up to around 150 to 250 psi of pressure and is resistant to fuel and oil.  With fuel pressure being monitored the ECU can be programmed to provide Engine Protection (selected Elite ECU models) whereby the engine can be placed into a safer state when fuel pressure is considered to be at a level where engine damage may occur.

The sensors can be fragile and are not designed to be mounted where there is high vibration, instead, they should be mounted somewhere insulated from this. Many fuel pressure regulators have a port where the sensor can be added, and this is fine to do so long as the regulator is not vibrating with the engine.  Otherwise. an extension hose is ideal for this purpose.





Wiring

Any spare AVI or SPI can be used for a Fuel Pressure Sensor.


Configuration

Wiring

It is best to configure the wiring connection in your ECU first before physically wiring the sensor in.  This ensures that only an input that can be used for Fuel Pressure is used.

To enable a Fuel Pressure Sensor:
1. Go to the Main Setup by clicking on the Gear icon at the top of the screen and then selecting Functions
2. Select Add/Remove Functions
3. Find the Fuel Pressure Sensor (you can use the search for this) and move it to the right hand side and hit OK.
4. Fuel Pressure Sensor will now be in your list of Functions.  Click it to proceed with the configuration.
5. Select Edit Connection to pick an appropriate spare input.  Only inputs that can be used will be in the list.

In this instance, AVI2 has been selected.


6. Ensure that the Pull Up is set to Disabled. This is because the sensor it already powered and does not need a pullup.  Enabling the pullup can skew the readings.
7. Load or manually enter the Calibration data and hit OK.

Calibration



Base Fuel Pressure

The Base Fuel Pressure is the pressure that the system operates at when the engine is not running.  i.e. not under vacuum or boost, and can be found by turning on the fuel pump without the engine running and monitoring the fuel pressure, or it can be tested with the engine running and the vacuum hose disconnected from the Fuel Pressure Regulator.  This can be entered into the "Main Setup / Engine / Fuel"  section shown below.  This setting has no impact on the tuning of the engine, it is only used as a reference point for Engine Protection.

Entering the Base Fuel Pressure setting



With the base pressure now known it is the job of the Fuel Pressure Regulator to mechanically adjust the fuel pressure based on a pressure signal from the intake manifold via a hose connection.  This is required so that the actual fuel pressure at the injector tip remains constant.  If the fuel pressure was not adjusted for manifold pressure the flow of fuel out of the injectors will vary.  The effective fuel pressure (fuel rail pressure minus the Manifold Pressure) is called the Injector Pressure Differential.

Hose connection from Fuel Pressure Regulator to the intake manifold


To explain this further, let's say for example that we have 50psi of fuel pressure.   If we put 20psi of boost into the engine we now have 20psi of pressure trying to push back up the injectors as they open so the Injector Pressure Differential drops to only 30psi (50psi - 20psi = 30psi).  If we increase the boost pressure to 50psi of boost the Injector Pressure Differential is now zero, so no fuel will come out!!  This is why the regulator needs to increase fuel pressure as boost increases, and conversely reduce fuel pressure when there is vacuum in the intake.  With the Fuel Pressure Regulator doing its job, with 20psi of boost the rail pressure should also raise by 20psi to 70psi of pressure to compensate for the manifold pressure trying to push back on the injector tip.

So in the end what you have is a constant 50psi of Injector Pressure Differential and therefore a constant injector flow rate.

Diagnosing Fuel Pressure Problems

With a Fuel Pressure Sensor present in the system it is very easy to monitor if there are any issues with fuel pressure.  The key to this is monitoring the Injector Pressure Differential channel.  As mentioned in the Base Fuel Pressure section, we expect this to be a flat line.  The best way to view this is via a Trace View.

If you see the Injector Pressure Differential drop, especially under high load and rpm, then you have a fuel pressure issue. More likely is the fuel pump cannot flow enough and is losing pressure as the injectors draw more fuel away from the rail under high injector duty conditions.  

Injector Pressure Differential dropping away



The only solution is to increase the fuel flow.  Raising the fuel pressure will not fix this issue, in fact it will make it worse.

It is normal to see small fuel pressure fluctuations when there is rapid manifold pressure changes.  This is because the Fuel Pressure Regulator cannot instantly change the fuel pressure, and this is considered to be normal and not an issue.



Engine Protection

For the purposes of this demonstration the Engine Protection configuration is shown for Elite 1500 and 2500 models.

Enabling Fuel Pressure Fault Detection

The detection of a low fuel pressure condition is handled by going to the Fuel Pressure Sensor function and then going to the Diagnostic tab.  From here the Operating condition should be ticked.

Fuel Pressure Sensor Diagnostics. Clicking on each of the setting names will pop up a help bubble that describes that setting.


In the above example, we set the DTC Severity to 2 because a low fuel pressure condition can result in damage to the engine, but it is not as critical as say a lack of oil pressure.  We have set it to flag the P0191 code for Fuel Pressure Operating when the engine is greater than 10psi of boost, is more than half throttle, and we see the Injector Pressure Differential drop more than 12psi below the Base Fuel Pressure setting.  When that P0191 code is flagged the engine will enter Engine Protection.   The DTC code is helpful in diagnosing what caused the ECU to enter Engine Protection.

Engine Protection Settings

After the detection of a low fuel pressure condition has been set up, we now need to decide what to actually do about the situation.  To be able to react we first need to enable Engine Protection as a Function.  As with adding a Fuel Pressure Sensor we also need to go to the Main Setup / Functions section and add Engine Protection to the list.

The next step is to enable all of the Severity Levels.  We are more interested here in Level 2 as that is what we have set to flag when a fault occurs.




In this section, we set what we want the ECU to do to prevent engine damage.  In the above example, we have told the ECU to run slightly richer by 0.5 AFR, we are lowering the Boost Control by applying a -50% correction, we are retarding ignition timing by 5 degrees to take a little power away, and most importantly we are enabling a fuel cut at 4500 rpm.  If we had a DBW throttle we can also impose a maximum throttle limit.  The intended purpose is to limit the engine so that it is not possible for the driver to unintentionally harm the engine.  You should select more appropriate values to use for your application.

 

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