Monitoring Fuel Pressure: Accessport Guide

Monitoring fuel pressure is an important aspect of maintaining a well-functioning fuel system and ensuring optimal performance from your vehicle. This is especially true if you have upgraded injectors or are experiencing issues with your fuel pump. By keeping an eye on fuel pressure, you can identify problems early on and prevent potential damage to your engine.

One tool that can assist in monitoring fuel pressure is the Cobb Accessport, which offers a range of data monitors for Subaru vehicles. The Accessport allows you to view and log critical parameters such as fuel rail pressure, fuel pressure target, and fuel pump duty ratio. Additionally, it provides the ability to make real-time adjustments to the tune based on fuel pressure differentials.

To get the most out of the Cobb Accessport, it is recommended to install a fuel pressure sensor. This will provide more accurate data and help you fine-tune your vehicle's performance. However, it is important to note that not all monitors are available for all vehicles, as it depends on the specific hardware and engine control unit (ECU) capabilities.

Installing a fuel pressure sensor

Park your vehicle on a flat, hard surface and ensure that the transmission is in park for automatics or in first gear for manual vehicles. This step is crucial for safety and stability during the installation process.

Install a nine-volt battery saver into the cigarette lighter to keep your computer live and retain your vehicle's current settings. If you don't have a nine-volt battery saver, that's okay; simply move on to the next step.

Open the vehicle's hood and disconnect the battery. Remove the ground cable from the battery's negative post to disable the power to the ignition and fuel system. This is an essential safety precaution.

Remove the engine cover if there is one on top of the engine. Also, take off any brackets or components that may obstruct access to the fuel rail sensor. In some cases, you may need to remove the intake if it overlaps the fuel rail sensor.

Locate the Schrader valve or test port on the fuel rail. Put on safety gear, including safety glasses and protective clothing. Place a small drip pan under the rail and cover the port with a towel. Use a small flat-tip screwdriver to open the valve and release the pressure in the fuel rail. If there is no test port or Schrader valve, you will need to remove the supply fuel hose to the fuel rail using a drip pan and a fuel hose quick-disconnect tool.

Remove the harness and mounting hardware from the fuel rail sensor. Carefully take out the sensor from the fuel rail. Clean the fuel rail with a lint-free cloth, and use electrical cleaner to remove any debris from the fuel rail sensor harness.

Install the new fuel pressure sensor onto the fuel rail. First, screw in the mounting hardware finger-tight, and then tighten it to the specified torque with an inch-pound torque wrench. This will secure the sensor in place.

Connect the fuel rail sensor harness to the sensor. Reinstall any brackets or components that you had to remove earlier. If you had to detach the pressure fuel line to the fuel rail, be sure to reconnect it now.

Install the engine cover by snapping it back into place. Ensure that all components are securely fastened.

Reconnect the ground cable to the battery's negative post and remove the nine-volt battery saver from the cigarette lighter, if you used one. Tighten the battery clamp securely and ensure a good connection. If you didn't use a battery saver, you will need to reset your vehicle's settings, such as radio presets, electric seat positions, and electric mirror adjustments.

Turn the ignition key on and off several times to ensure the entire fuel rail is full and pressurized. Use a combustible gas detector to check all connections for leaks and sniff for any fuel odours. Remove the wheel chocks, if applicable.

Finally, take the vehicle for a test drive. Listen for any unusual engine sounds or vibrations and monitor the dashboard for the fuel level and any warning lights. If the check engine light comes on, further diagnosis of the fuel system or a possible electrical issue may be required.

It is important to note that this guide provides a general overview of the process, and specific steps may vary depending on your vehicle's unique characteristics and the sensor kit you are using. Always refer to the manufacturer's instructions and consult a qualified mechanic if you have any doubts or concerns during the installation process.

Understanding the Accessport's fuel pressure monitor

The Accessport's fuel pressure monitor is an important tool for anyone looking to tune their car and ensure optimal performance. By monitoring fuel pressure, you can identify issues with your fuel pump and make sure your engine is getting the correct amount of fuel. This is especially critical if you have upgraded injectors, as it's common for the fuel pump to also need an upgrade.

How Fuel Pressure Monitoring Works

The fuel pressure monitor works by measuring the pressure in your fuel rail, which is the tube that carries fuel to your engine's injectors. This pressure needs to be carefully regulated because if it's too low, your engine can run lean, leading to potential damage. On the other hand, if the pressure is too high, your engine can run rich, wasting fuel and reducing performance.

Understanding the Monitors and Parameters

The Accessport provides a range of data monitors that you can view and log, including the fuel rail pressure. Here are some of the key monitors and parameters related to fuel pressure that you should understand:

• Fuel Rail Pressure (Fuel Pressure): This monitor displays the current fuel rail pressure, which is measured in pounds per square inch (psi).
• Fuel Rail Pressure Target (Fuel Pressure Target): This monitor shows the target fuel rail pressure as determined by the Engine Control Unit (ECU).
• Fuel Injector Trim Fuel Pressure MULTIPLIER TABLE (Fuel Inj FP Mult Table): This table contains values that are used to trim or adjust the fuel injector pulse width based on fuel pressure.
• Fuel Injector Trim Fuel Pressure OFFSET TABLE (Fuel Inj FP Offset Table): This table contains offset values that are applied to the fuel injector trim based on fuel pressure.
• Fuel Pump Duty (Fuel Pump Duty): This monitor shows the necessary fuel pump duty ratio, which is the percentage of time the fuel pump needs to run to maintain the target fuel pressure.

Benefits of Fuel Pressure Monitoring

By monitoring fuel pressure, you can catch issues early on and make real-time adjustments to your tune. For example, if the fuel pressure starts dropping, as indicated by the red lines in some fuel pressure curves, it could be a sign of a failing fuel pump. With this information, you can replace the fuel pump and potentially save your engine from serious damage.

Additionally, fuel pressure monitoring is especially useful if you're running flex fuel or have larger than stock injectors. It allows you to ensure that your engine is always getting the correct amount of fuel, regardless of the fuel type or injector size.

Interpreting fuel pressure readings

• Understanding Fuel Pressure: Fuel pressure readings give you insights into the health of your fuel system. High fuel pressure can cause an engine to run rich, while low fuel pressure can make it run lean or not function at all.
• Identifying Potential Issues: Deviations from the manufacturer's specified fuel pressure can indicate problems. Higher readings may suggest issues with the return line fuel components, while lower readings may point to faults in the pressure line fuel components.
• Fuel Pressure Fluctuations: Minor fluctuations in fuel pressure are normal, but significant variations could indicate a problem. For example, a fuel pressure reading of 40 +/- 1.5 is generally acceptable, but fluctuations of 20-30% may signal an issue.
• Fuel Gauge Symbols: Pay attention to the fuel gauge symbols and indicators on your dashboard. A red or orange fuel gauge icon indicates an empty or reserve fuel warning. A yellow or green check engine symbol with a fuel pump icon highlights low fuel pressure issues or a faulty fuel pressure sensor.
• Fuel Mileage Calculation: To interpret fuel gauge readings accurately, calculate your fuel mileage by dividing the total distance travelled by the total amount of fuel used. This helps you understand your vehicle's fuel efficiency and identify any unusual deviations.
• Factors Affecting Fuel Gauge Accuracy: Be mindful of factors that can impact fuel gauge accuracy, such as driving habits (aggressive driving, frequent braking, rapid acceleration), road conditions (hills, mountains), weather conditions (extreme temperatures, wind, precipitation), and vehicle maintenance status.
• Adjusting and Maintaining Your Fuel System: To optimise fuel gauge readings, ensure proper tyre inflation, regularly check and replace the air filter, perform tune-ups and oil changes, and consider using fuel-efficient driving techniques such as smooth acceleration and coasting to a stop.

Adjusting the fuel pressure

Firstly, determine the type of fuel pressure regulator in your vehicle. There are two main types: Blocking Style (Traditional Style) and Bypass Style (Return Style) Regulators. Blocking Style Regulators are characterised by the absence of a fuel return line from the regulator to the fuel tank, whereas Bypass Style Regulators have this return line.

For Blocking Style Regulators, follow these steps to adjust the fuel pressure:

• Understand "Pressure Creep": This occurs when there is a spike in fuel pressure as the fuel control valve reaches the closed position, leading to inconsistent fuel pressure readings.
• Eliminate "Pressure Creep": To do this, maintain a small amount of fuel flow through the regulator while making adjustments. This can be achieved by idling the engine.
• Adjust Fuel Pressure: With a small amount of fuel flow, make the necessary adjustments to the fuel pressure using a threaded adjustment mechanism.

For Bypass Style Regulators, the process is similar:

• Understand the Basics: Fuel enters through the inlet port, travels past the fuel bypass valve/fuel return line port, and is distributed to the carburettor. The fuel pressure is controlled by the spring-loaded bypass valve.
• Adjust Fuel Pressure: Use the threaded adjustment mechanism to set the desired fuel pressure (psi) for the carburettor.

Additionally, consider using a fuel pressure sensor kit, such as the Cobb kit, which provides instructions for activating fuel pressure monitoring on the AccessPort. This can offer real-time adjustments and ensure the correct fuel pressure differential, maintaining the desired air-fuel ratio (AFR).

It is important to consult a professional or a detailed guide specific to your vehicle's make and model for precise instructions on adjusting fuel pressure.

The importance of fuel pressure monitoring

Monitoring fuel pressure is essential for keeping your car in good working order and can save you time and money. A good working fuel pump is critical to a proper tune, and monitoring fuel pressure can help you identify if your fuel pump is upgraded or failing.

Turbocharged Subarus and Mitsubishis, for example, require a manifold-referenced 1:1 ratio fuel pressure regulator. This means that as the manifold pressure goes up, so too must the fuel pressure, in order to maintain the correct fuel pressure differential and keep the fuel injectors flowing with the correct amount of fuel. If the fuel pressure is higher than the 1:1 ratio, the engine will run rich; if it's lower, it will run lean.

If you are monitoring fuel pressure and notice that it is dropping, this could indicate a failing fuel pump. A new fuel pump can be installed, saving the customer time and money by avoiding the need for a re-tune.

Monitoring fuel pressure is particularly critical when making more than stock power. Too little fuel pressure can cause your fuel injectors to not inject enough fuel, resulting in your engine running lean and potentially causing major damage.

By monitoring fuel pressure, you can make real-time adjustments to the tune based on fuel pressure differential. This will make the fueling model more accurate and keep your AFRs (Air-Fuel Ratios) hitting the desired target.

Frequently asked questions