Fuel Vapor Vent Pressure Sensor: P0174 And P0171 Trouble

P0171 and P0174 are error codes that indicate a lean fuel condition, which can be caused by the engine sucking in too much air or the fuel system not providing enough fuel. This can be the result of a vacuum leak, a dirty airflow sensor, a faulty EGR valve, or issues with the fuel pump, filter, regulator, or injectors.

Vacuum leaks

• Leaky or dry vacuum hoses
• Cracked or loose intake gaskets
• Faulty PCV valve
• Leaks in the EVAP purge valve or unsealed engine intake parts
• Blocked or disconnected air intake hose
• Dirty or faulty mass airflow sensor

To diagnose a vacuum leak, you can perform a smoke test or use carburetor/throttle body cleaner to spray around suspected leak areas while monitoring engine performance and gauge readings. If a leak is detected, repairing or replacing the faulty component is typically necessary to resolve the issue.

Low fuel pressure

Symptoms of Low Fuel Pressure

Another symptom of low fuel pressure is a backfiring engine, which will make gunfire or loud popping sounds and cause a loss of power and motion. Your vehicle may also crank but fail to start if the fuel pressure is not correct.

Causes of Low Fuel Pressure

A faulty engine can also cause low fuel pressure if it does not have the right amount of fuel and air. This can lead to misfires and an uneven drive.

Fixing Low Fuel Pressure

To fix low fuel pressure, it is recommended to have a professional mechanic diagnose the issue. However, if you have experience with vehicle maintenance and repair, you can attempt to fix the issue by following these steps:

• Check your gas tank: Ensure that you have put the correct type of fuel in your tank and that the fuel is of decent quality. Also, look for any wet areas in the tank holding area that could indicate a leak.
• Inspect the fuel line: Check if the fuel line is connected, cracked, broken, or has any leaks.
• Test the battery: Use a multimeter or power probe to test the life of your battery.
• Inspect the fuel pump: Look for any broken parts in the fuel pump and replace them if necessary.
• Check the fuel pressure regulator: Test the fuel pressure regulator using a fuel pressure gauge.
• Consult a professional: If you have checked all of the above components and your vehicle still has low fuel pressure, take it to a professional mechanic for further diagnosis and repair.

Cost of Repair

The cost of repairing low fuel pressure can vary depending on the specific issue and whether you do it yourself or hire a professional. Replacing a fuel pressure regulator can range from $150 to $600, while a fuel pump replacement typically costs between $75 and $600. Fuel line replacements can range from $60 to $1,000, and a new battery will cost around $70 to $700. The most expensive repair would be a new engine, which can cost between $4,000 and $7,000.

Faulty EVAP purge valve

A faulty EVAP purge valve can cause a host of issues with your vehicle's performance and fuel economy. The EVAP purge valve, or canister purge valve, is an important component of your vehicle's Evaporative Emission Control (EVAP) system, which prevents fuel vapours from escaping into the atmosphere. When the purge valve is stuck open or closed, or does not open at the right time, it can lead to the following issues:

• Check Engine Light: The check engine light may illuminate due to higher or lower than expected purging from the valve, indicating potential issues with the valve's function.
• Lower Gas Mileage: A faulty purge valve can affect your gas mileage as the vapours that are usually recycled into the engine are released into the environment, resulting in a loss of fuel.
• Rough Idling: A stuck-open purge valve can cause a vacuum leak, allowing an uncontrolled amount of air into the engine and disrupting the air-to-fuel ratio. This can lead to rough idling and difficulty starting the engine.
• Trouble Starting the Car: A vacuum leak caused by a faulty purge valve can make it challenging to start your vehicle as the unmetered air interferes with the internal combustion process.
• Poor Engine Performance: A faulty purge valve can significantly compromise engine performance, making it feel like your vehicle lacks power even when you press the accelerator pedal firmly.
• Emissions Test Failure: As the purge valve is responsible for redirecting fuel vapours back into the engine, a failing valve can cause an increase in toxic hydrocarbon emissions, leading to a potential failure during emissions testing.

It is important to address these issues promptly and consult a certified mechanic or conduct repairs to ensure the proper functioning of your vehicle and reduce the risk of further complications.

Faulty MAF sensor

A faulty MAF sensor can cause a range of issues with your vehicle, including:

• A check engine light appearing on your dashboard.
• Trouble starting your engine.
• A rough-running engine.
• Trouble with idling or accelerating.
• Misfiring, stalling, or loss of power.
• Poor drivability, such as engine stalling, jerking, or hesitation during acceleration.
• A rich air-fuel ratio, indicated by black smoke exiting the tailpipe and worse fuel efficiency than usual.
• A lean air-fuel ratio, which can lead to engine hesitation and/or surging, and hard engine seizures.
• Surging or hesitation during cruising or acceleration.
• Difficulty starting the engine.

Exhaust leaks

An exhaust leak can cause a P0171 code, but it depends on where the leak is located. If the leak is at the exhaust manifold gasket, right at the head, then it can cause a P0171 code. However, if the leak is further down the exhaust system, it is less likely to be the cause.

In addition to an exhaust leak, there are several other potential causes of a P0171 code, including:

• A disconnected hose or vacuum leak
• A weak fuel system
• A faulty mass air flow (MAF) sensor or dirty/faulty air filter
• Low fuel rail pressure or blocked fuel injectors, filters, or pump
• Bad oxygen sensors
• Faulty coolant temperature sensors or other engine monitoring sensors
• A faulty EVAP purge valve
• Faulty MAP/MAF sensors
• Faulty sensor wirings
• Faulty ECM/PCM (rare)

To diagnose the cause of a P0171 code, it is recommended to use an OBD II scanner to read the trouble codes and check live data for the MAF sensor and temperature sensors. Other diagnostics methods include:

• Checking for disconnected or leaking hoses, including the air intake hose and hoses under the engine cover
• Listening for whistling or rushing air sounds while the engine is running, which could indicate a leak
• Checking for debris or damage in the mass air flow sensor or air filter
• Checking for unsealed, disconnected, or damaged engine intake parts, gaskets, and the purge valve seal
• Testing for leaks by spraying carburetor or throttle body cleaner around suspected areas while the engine is running
• Checking fuel rail pressure and inspecting fuel injectors, filters, or pumps for blockages
• Inspecting exhaust parts for leaks, such as the exhaust manifold, gasket, and flex pipe

Frequently asked questions