Cold Fuel Pressure: Why Engines Start Roughly

There are many reasons why a car may struggle to start when the engine is cold. One of the most common issues is a lack of fuel system pressure, which can be caused by a variety of factors, including a weak fuel pressure regulator, a leaking fuel injector, a clogging fuel filter, or a faulty fuel pump. Other potential causes of a rough cold start include problems with the ignition system, such as worn-out spark plugs or carbon deposits, and issues with the emission components, such as a faulty mass airflow sensor or throttle position sensor. In some cases, a rough cold start may be due to a failing fuel pump check valve, which can cause the system pressure to bleed off when the car sits for a prolonged period.

A failing fuel pump

• Difficulty Starting the Engine: One of the most common signs of a failing fuel pump is difficulty starting the engine. If your vehicle struggles to start or doesn't start at all, it could indicate a faulty fuel pump. The car may crank when you turn the key, but it won't start because fuel isn't reaching the engine.
• Sputtering or Stalling: If your engine sputters or stalls while driving, especially during acceleration or when under stress (such as towing a heavy load or driving uphill), it could be due to low pressure caused by a faulty fuel pump. The engine isn't receiving the required fuel and air mixture for proper combustion.
• Engine Surging: A failing fuel pump can cause the engine to surge, resulting in unexpected speed increases and decreases, even when you're not touching the gas pedal or brake. This is often caused by too much fuel being sent to the engine.
• Whining or Whirring Noise: If you hear a noticeable whining or whirring noise coming from your fuel tank, it could indicate an issue with the fuel pump. Normally, the fuel pump produces a low, barely audible hum. A significant increase in pitch or a whining sound may suggest a problem.
• Lower Gas Mileage: A failing fuel pump can lead to poor fuel efficiency. Worn or damaged components can allow excess fuel to enter the engine, resulting in higher fuel consumption and more frequent trips to the gas station.
• Loss of Power: If your vehicle loses power when driving up steep inclines, carrying heavy cargo, or operating under similar stressful conditions, it could be due to a faulty fuel pump. The engine requires more fuel to perform effectively under these conditions, and a failing fuel pump may not be able to deliver the necessary fuel supply.
• Cold Start Issues: In some cases, a failing fuel pump can cause cold start issues. When a car has been sitting for a while and there is no fuel pressure, a faulty fuel pump may struggle to build up enough pressure to start the engine. This can result in a rough or delayed start.

It's important to note that these symptoms can also be caused by other issues, such as bad fuel, damaged fuel lines, or a clogged fuel filter. Therefore, it's recommended to have a comprehensive inspection and diagnosis performed by a qualified technician to accurately identify the root cause of the problem.

Poor electrical connections to the fuel pump

A car that struggles to start when it has been sitting for a while, and there is no fuel pressure left, could be indicative of poor electrical connections to the fuel pump. This could be due to the fuel pump not receiving enough voltage when in the 'on' position. The fuel pump may also not be priming enough to create adequate pressure to start the car.

To help identify issues of poor electrical connections, a high-quality digital volt/ohm/meter should be used to test for voltage drops and continuity. This can help determine if the issue is indeed with the electrical connections to the fuel pump.

In addition to electrical issues, other factors that can cause low fuel pressure and rough starts include fuel contamination, clogged strainers/filters, and a failing fuel pump. It is important to identify the root cause of the issue to ensure proper diagnosis and repair.

A clogged fuel filter

Finally, a clogged fuel filter can trigger the check engine light to come on, as the low fuel pressure is detected by the vehicle's sensors.

It is recommended that you change your fuel filter every 2 years or 30,000 miles, whichever comes first. However, this can vary depending on your vehicle's make, model, and year, so it is always best to consult your owner's manual for the recommended maintenance schedule.

A failing fuel pressure regulator

• Engine Misfires and Decrease in Power: A faulty regulator can interrupt the vehicle's fuel pressure, causing the engine's air-fuel ratio to be thrown off. This can result in engine misfires, reduced power, and poor acceleration.
• Fuel Leaks: If the regulator's diaphragm or seals fail, it can lead to fuel leaks, which pose a potential safety hazard.
• Black Smoke from the Exhaust: A failing regulator can cause the vehicle to run excessively rich, emitting black smoke from the tailpipe and reducing overall performance.
• Fuel Smell from the Dipstick: During engine checks, if you notice a petrol smell from the oil dipstick, it indicates a failing or bad fuel pressure regulator.
• Engine Malfunction: Fuel leaks from the tailpipe are another sign of a bad regulator. This occurs when excess fuel floods through the fuel lines and overfills the exhaust system.
• Noisy Fuel Pump: A failing regulator can cause the fuel pump to make an irritating noise, especially noticeable in traffic jams.
• Hard Starting and Stalling: A bad regulator can cause issues with starting the vehicle and may result in stalling.
• Illuminated Check Engine Light: A faulty regulator can trigger the check engine light and store a corresponding diagnostic trouble code (DTC).

It is important to note that these issues can also be caused by other mechanical problems, so proper diagnosis by a professional is crucial to identify the exact cause. Additionally, the replacement cost of a fuel pressure regulator can vary depending on the vehicle's specifications, typically ranging from $250 to $400.

A bad check valve

To test for a bad check valve, you can try the following: leave your vehicle sitting overnight without attempting to start it. In the morning, instead of starting the engine, turn the key to the "RUN" position for 2-3 seconds, then turn it off. Repeat this process three times. On the third attempt, try to start the engine. If it starts, a bad check valve is likely the issue. This process works because each time you turn the key, the fuel pump runs for about 2 seconds, pushing fuel up to the engine and building pressure. By the third attempt, the pump usually has enough fuel pressure to start the engine.

If you have a bad check valve, you will need to replace the entire fuel pump as the check valve is built into the pump and is not replaceable as a separate part.

Frequently asked questions