Fuel Tank Pressure: A Necessary Evil For Miatas?

The topic of pressure in Miata fuel tanks is an important one for owners of these vehicles, with some reporting issues with pressurisation and others seeking advice on fuel pressure tuning. It is normal to experience some pressure release when opening the gas cap, and this can be attributed to the vapor pressure of gasoline, which can build up in the tank, especially in warm weather. This pressure can be positive or negative, and it is important to ensure the fuel tank is venting correctly to avoid problems with the engine performance. There are various methods and tools suggested to test for pressure and vacuum issues, and it is recommended to seek expert advice for tuning and troubleshooting.

Fuel tank pressurisation is normal

The fuel tank of a Miata is designed to be pressurised. The fuel tank is always under positive pressure, even when the gasoline is being consumed and air is entering the tank to fill the void. This pressure also allows the fuel to expand with temperature changes.

The fuel tank's vent line has both positive and negative pressure. The positive pressure is created when air enters the tank to fill the void left by the fuel being consumed, and the negative pressure is created when the fuel expands with temperature changes. This vent line also includes a rollover valve, which prevents fuel from spilling out in the event of an accident.

It is important to note that excessive fuel tank pressure can be an issue. This can be caused by a faulty vent in the gas cap or a blocked vent line. In some cases, it may be necessary to modify the vent line or upgrade the fuel pump to address this issue.

Additionally, it is worth mentioning that the fuel cap plays a crucial role in maintaining the pressure in the fuel tank. A vented gas cap allows for the release of excess pressure while also preventing outside air from entering the tank. This ensures the pressure in the tank remains at a safe level.

Fuel tank vacuum

A fuel tank vacuum is a common issue in cars, and it occurs when there is an excessive vacuum in the fuel tank or Evaporative Emissions Control System (EVAP). This can be caused by a variety of factors, including a blocked EVAP canister or vent solenoid, a defective tank pressure sensor, a collapsed or kinked hose between the canister and fuel tank, or a fuel filler cap that is stuck closed.

When a fuel tank vacuum occurs, the only symptom is typically the illumination of the check engine light. This can be problematic because it may be the only indication of a problem, and many drivers may not even notice it. In some cases, a fuel tank vacuum can also result in the uncontrolled release of hydrocarbons into the atmosphere, which could cause a failed emissions test.

To diagnose a fuel tank vacuum, a qualified mechanic will typically start by inspecting the EVAP canister or vent solenoid for any blockages. They will then check the hose between the canister and the fuel tank for any signs of collapse or kinking that may be blocking the passage. Next, they will inspect the fuel filler cap to ensure it is not stuck closed. Finally, they will check the fuel tank pressure sensor.

In most cases, the issue can be resolved by repairing or replacing the fuel filler cap. However, in some instances, the mechanic may also need to repair or replace damaged or unhooked lines in the EVAP system and ensure that all electrical components are properly connected.

It is important to address a fuel tank vacuum as soon as possible to prevent any potential damage to the vehicle and to ensure the car meets emissions standards. While a fuel tank vacuum may not cause any immediate mechanical problems or affect the driveability of the car, it is crucial to have it properly diagnosed and repaired by a qualified professional.

Fuel injectors and fuel pressure

Fuel injectors are a key component of a gasoline direct injection (GDI) fuel system. GDI fuel systems are expected to grow to over 90% of the share of vehicles sold by 2025. Fuel injectors spray fuel directly into the cylinders. The fuel is pressurised by the pump, which is typically cam-driven, and the pressure is controlled by a fuel pressure regulator or FPR.

There are two types of fuel systems: returnless and return style. Returnless systems do not return fuel to the tank, while return style systems bleed excess fuel back to the tank through the regulator. Return style systems have the advantage of maintaining a constant effective fuel pressure, which can extend the range of fuel injectors and help them function at lower fuel demands.

The two pressures that need to be considered are rail pressure and effective pressure. Rail pressure is the pressure inside the rail, while effective pressure is the actual applied pressure for the injector and is the pressure differential across the injector. Effective pressure is what injector flow rate is based on. When an engine is idling, there is a vacuum in the intake manifold, which pulls fuel out of the injectors and increases the effective pressure. When a turbocharged vehicle is in boost, the pressure inside the manifold pushes fuel back into the injector, reducing the effective fuel pressure.

The FPR mechanically or electronically controls fuel pressure. It raises or drops the fuel pressure from a preset point by the same amount the pressure in the intake manifold changes. For example, if the FPR is set to a base pressure of 43.5 PSI and the manifold pressure increases by 10 PSI, the FPR will control the fuel pressure at 54.5 PSI.

To maintain advertised flow rates, the fuel pressure should be raised or dropped by the same amount the pressure in the intake manifold raises or drops. If the static fuel pressure is set to 43.5 PSI and the manifold pressure increases to 30 PSI, the fuel pressure should be raised to 73.5 PSI.

Fuel tank venting

Unvented fuel tanks pose a serious safety risk. As fuel is consumed, a vacuum is created, which can cause the tank to collapse and potentially leak. Additionally, without venting, pressure can build up inside the tank as the fuel expands due to temperature changes, leading to a possible rupture.

There are two main types of fuel tank vents: passive and active. Passive vents are simple devices located at the highest point of the tank, relying on atmospheric pressure differences to function. They are low maintenance and commonly used in small fuel tanks with minimal temperature or pressure changes. Active vents, on the other hand, use powered mechanisms to control airflow and are often used in larger tanks or those with significant temperature or pressure fluctuations.

It is important to ensure proper design and maintenance of fuel tank venting systems to prevent accidents and ensure the safety of vehicles and their passengers.

Fuel pump

The fuel pump is an important component of the Miata's fuel system, responsible for delivering fuel from the tank to the engine. Here's a detailed overview of the Miata's fuel pump:

Miata Fuel Pump Function

The fuel pump ensures a consistent flow of fuel from the tank to the engine, where it's injected into the combustion chamber. The pump needs to supply enough fuel to match the engine's demand, especially when the car is under load or accelerating.

Miata fuel pumps are rated in Liters Per Hour (LPH), which indicates the volume of fuel the pump can deliver. The stock pump typically provides sufficient flow for naturally aspirated engines, but for forced induction setups, an upgrade may be necessary. The maximum pressure available from the stock fuel pump is around 78 to 85 PSI, which is adequate for mild boost levels.

When choosing a fuel pump for your Miata, there are several models to consider:

• OEM Replacement: If you're looking for a direct replacement for your stock fuel pump, you can opt for the OEM pump or a mild upgrade from brands like Deatschwerks DW100 Series.
• Track Racing or Boosted Applications: If you plan on using your Miata for track racing or have a forced induction setup, a higher-flow pump may be required. However, be cautious as a pump with too high a flow rate can overpower the stock fuel pressure regulator, leading to a rich air/fuel mixture and potential power loss. In such cases, an aftermarket fuel pressure regulator is recommended.
• Walbro 190lph High-Pressure: This is a popular choice for applications that need more fuel flow. It comes in standard and high-flow varieties, with the latter being more common.
• Deatschwerks 200 Series: Offering a flow rate of 165lph, this pump falls between the Walbro 190 and stock replacement options. It's a good choice for Spec Miata racing.
• Walbro 255lph: This is a strong pump designed for engines with larger injectors. It will likely overpower the stock fuel pressure regulator, so an upgrade may be necessary.
• Deatschwerks 300 Series: With a flow rate of 320lph, this pump offers more than what a typical Miata will ever need.

Fuel Tank Pressure and Venting

It's normal to experience some pressure release when opening the fuel cap, similar to opening a carbonated drink. This pressure is caused by the gasoline's vapour pressure, especially when the fuel warms up. To manage this pressure, the Miata's fuel system includes a vent line with rollover and check valves. However, some owners choose to modify this system by looping the vent line or deleting the charcoal canister, which is designed to absorb vapours.

Frequently asked questions