Fuel Pressure And High Idle: Is There A Link?

High fuel pressure at idle can be caused by a number of factors, including a faulty fuel pressure regulator, a vacuum leak, or a faulty temperature sensor. A faulty fuel pressure regulator can cause a rich fuel mixture, leading to an increase in idle speed. A vacuum leak can also affect the air-fuel mixture, resulting in higher idle RPM. Additionally, a faulty temperature sensor can provide incorrect engine temperature readings to the ECU, leading to adjustments in the fuel mixture and impacting idle RPM. Other potential causes of high idle include issues with the throttle cable or throttle body, a faulty ECU, or a broken alternator. Diagnosing and resolving these issues may involve inspections, repairs, or replacements of specific components by a professional mechanic.

Vacuum leaks

A vacuum leak can cause the engine to hesitate when you press down on the accelerator to pick up speed, or once you've reached the desired speed. You may feel the engine hiccup or sputter, indicating that your vehicle isn't getting enough fuel. In severe cases, the engine might stall, especially if it's unable to maintain power during an idle.

If the engine doesn't stall during an idle, it may do the opposite and idle higher than it should. As the vacuum leaks out of the hose(s), the engine's idle increases. In some cases, the air will come out of the leaking hose(s) in spurts, causing the engine to idle sporadically.

If you suspect a vacuum leak, park your car in a safe place, engage the parking brake, and listen carefully with the engine running and the hood open. A hissing or suction sound may indicate a vacuum leak.

Other potential causes of high idle include:

• Ignition timing is too advanced
• Misadjusted or sticking accelerator cable
• Faulty accelerator pedal sensor
• Malfunctioning ECU (Engine Control Unit)
• Broken alternator
• Issues with the idle air control valve
• Open-circuit fuse
• Leaky hoses, tubes, gaskets, or housings
• Faulty temperature sensor
• Faulty fuel pressure regulator

Faulty throttle blades

A faulty throttle blade is a potential cause of high idle. The throttle blade controls the flow of air and fuel into the engine, and if it is not fully closed when idling, it can cause the engine to rev higher than normal. This can be due to a number of factors, including:

• A dirty or malfunctioning carburetor, which can cause an incorrect fuel-air mixture and result in a higher idle speed.
• A faulty throttle position sensor, which can cause the throttle blade to remain open further than necessary, increasing the idle speed.
• A stuck or damaged throttle linkage, which can prevent the throttle blade from closing fully and cause a higher idle speed.
• An issue with the throttle body, such as a buildup of dirt and debris, which can disrupt the airflow and cause the throttle blade to stick open.
• An incorrect throttle adjustment, which can cause the throttle blade to be set too high, resulting in a higher idle speed.

To resolve a high idle caused by a faulty throttle blade, it is important to identify the exact cause and make the necessary repairs or adjustments. This may involve cleaning or replacing the carburetor, adjusting the throttle position sensor, repairing or replacing the throttle linkage, cleaning the throttle body, or adjusting the throttle to the correct position.

Malfunctioning ECU

A malfunctioning ECU can cause a high idle. The ECU, or Engine Control Unit, is the vehicle's "brain", regulating almost every important system and function. A faulty ECU can cause a range of issues, from rough idling to stalling, misfires, and poor fuel efficiency.

There are several reasons why an ECU may malfunction. One common issue is a dying car battery. The ECU relies on the battery's electronic cells to function, and if these cells die, the ECU will fail. Corrosion is another potential problem. Over time, the seals around the ECU can wear out, allowing moisture to enter and cause corrosion. Voltage issues can also lead to ECU failure. The ECU operates within a specific voltage range, typically between 9 and 16 volts, and prolonged exposure to low or fluctuating voltage can cause erratic behaviour or failure.

Other potential causes of ECU malfunction include incorrect jump-starting, electrical system overload, software issues, manufacturing defects, and problems with the starter motor or its sensors.

Diagnosing a malfunctioning ECU can be a complex process. Some common steps include checking the "Check Engine Light", using an OBD-II scanner to read error codes, inspecting the battery and connections, testing battery voltage, checking fuses and relays, and visually inspecting the ECU and its wiring harness for signs of damage. If these basic steps don't identify the issue, more advanced diagnostics may be required.

Fuel pressure regulator issues

A faulty fuel pressure regulator can cause a host of issues with your engine, and is a relatively common problem. The fuel pressure regulator controls the fuel pressure in your car's fuel rail, so a fault here will disturb the air-fuel mixture and cause performance issues.

Symptoms of a Faulty Fuel Pressure Regulator

• One of the most common symptoms is an engine misfire when idling or during acceleration.
• A loss of acceleration, or a reduction in engine performance.
• A check engine light on your dashboard.
• Fuel leaks, which can cause bad smells and are a serious safety risk.
• Black smoke from the exhaust pipe.
• Black debris on spark plugs.
• A gasoline smell, or the presence of gasoline, on the dipstick or in the vacuum hose.
• A whirring noise from the fuel pump.
• Reduced fuel efficiency.
• Weak or slow acceleration.
• Problems when decelerating, such as engine backfire.
• A rough, misfiring engine when idling.
• The engine does not start, or struggles to start.

Diagnosis and Repair

It is recommended to check the vacuum line to the fuel pressure regulator before replacing it, as it is not common for this part to fail. A proper diagnosis should be conducted before replacing parts.

The best way to check the function of the fuel pressure regulator is with a fuel pressure gauge and by disconnecting the vacuum hose. If the fuel pressure is within the correct specifications, the regulator is likely fine.

The average cost of a fuel pressure regulator replacement is between $80 and $500, with the part costing $30 to $200, and labor costs of $50 to $300. However, this can be done at home, without a mechanic, to save on labor.

Damaged fuses

While high fuel pressure can cause issues with your vehicle, it is unlikely to be the direct cause of a high idle. A high idle, where your engine is revving even after it has sufficiently warmed up, can be caused by a number of issues, some more serious than others.

A blown fuse is a common cause of a high idle. Fuses are a safeguard to prevent a circuit from carrying more current than it should, and they can burn out. When this happens, the system it protects won't work, and you may need to replace the fuse to diagnose the problem properly.

There are a few ways to identify a blown fuse:

• Perform a visual check. Most common fuses are transparent, so you can see if the filament is intact or if there are any burn marks.
• Use a circuit tester or test light. This will indicate if there is continuity from one side of the fuse to the other.
• Replace with a known good fuse. If the system starts working again, you know the fuse was the issue.

If the fuse blows again, especially immediately, there is likely another problem that needs to be diagnosed and corrected on that circuit. Always replace a fuse with one of the exact same rating, as using an oversized fuse can result in permanent damage to electrical components or burnt wiring.

A high idle could be caused by a bad fuse in many different electronic systems in your car. The idle air control (IAC) motor, which controls the idle speed in modern vehicles, can be impacted by a blown fuse.

Frequently asked questions