Not sure why your car struggles to start? Is your engine lagging, hard starting, or stalling? Do you suspect the fuel pressure is low? If you’re unsure if you have low fuel pressure or want to know the cause and you know it’s not the fuel pump, this article covers the signs and symptoms of low fuel pressure and how to diagnose it yourself. Watch our video to learn more.
What Causes Low Fuel Pressure When Your Car Struggles to Start?
Running the vehicle on low fuel often can damage the fuel pump over time, which affects the fuel pressure. Fuel cools the fuel pump, and less fuel means there’s a greater chance fuel pump components can overheat or wear out. Low fuel can also draw air into the pump, preventing a consistent flow of fuel. Because of this, fuel pumps tend to break on fuel tanks on vehicles driven with these kinds of habits.
Low fuel pressure can also occur from fuel line leaks, leaking fuel injectors, or a defective fuel pressure regulator.
What Are the Different Types of Fuel Pressure?
The optimum mixture of air and fuel is 14.7 parts of air to 1 part of fuel (i.e. 14.7:1).
A lean condition means the engine has too much air coming in or not enough fuel.
A rich condition means you have too much fuel coming in compared to the air.
Signs of a rich condition include:
Signs and Symptoms of Low Fuel Pressure
If the engine cranks for a long time after trying to start the vehicle or the engine is slow to start, it could be from low fuel pressure inside the system. If the fuel system loses fuel over time, it could be from a faulty fuel pressure regulator or a leaky fuel injector.
If the engine is stalling while running, there is a fuel delivery issue. This can be from a malfunctioning fuel pump, clogged fuel filter, or sticky fuel injector. There may be a little or a lot of hesitation depending on how hard the accelerator pedal is pressed. This symptom may feel like an engine misfire, but if it’s a fuel pressure issue it will manifest as uncontrolled combustion in the engine.
Lagging Throttle Response
If the engine is lagging after pressing the accelerator pedal, almost as it it’s chugging to get moving, it could be from a fuel pump having low fuel pressure, or there is a clog in the system like from a fuel filter.
Check Engine Light (Codes P0087 and P0190)
A check engine light is more likely to appear on newer vehicles than older vehicles, but a check engine light could appear. If it does, the CPU could read codes P0087 (low fuel pressure) and P0190 (low fuel pressure inside the fuel rail) could indicate a low fuel pressure issue.
A mid to high-range scanner can direct you to the issue. If you don’t have access to a scanner, the fuel pressure can also be measured with a fuel pressure gauge.
Although vehicles can vary, on most vehicles normal fuel pressure is usually 55-65 pounds per square inch (psi) to start the vehicle. If the pressure is below this, vehicles can have issues starting, experience stumbling starting, or stalling conditions. With direct injection, fuel pressures are typically higher. On vehicles with carburetors, typical fuel pressure can be 4-5 psi lower.
Melted or Prematurely Worn Spark Plugs
Low fuel pressure means you have a lean condition and too much air is mixing with the amount of fuel in the combustion chamber, causing them to run hot. This can melt the spark plugs. If you notice you are replacing the spark plugs too often, test the fuel pressure.
How to Diagnose Low Fuel Pressure When Your Car Struggles to Start
1. Check the Battery Voltage
Test the battery’s voltage with a multimeter. It should measure at 12.4 volts or a little above.
2. Check the Engine Codes
If you vehicle shows a check engine light, pull the codes to help diagnose the cause. On newer vehicles, you might be able to check the fuel pressure with a scanner and graph it.
3. Test the Fuel Pressure with a Fuel Pressure Gauge
- Check your repair manual or the internet to find your vehicle’s fuel pressure range
- Test the fuel pressure with a fuel pressure gauge by connecting the gauge to the valve on the fuel rail if it has one, or connect it to where your kit requires.
- Turn the key to the ON position without starting the vehicle. Notice its initial measurement and watch it for 10-15 min. If the fuel pressure gauge drops to 5 or 0, there is a leak with the fuel pressure regulator.
- Once the fuel gauge is connected, turn the key to the ON position (without starting the engine) to build up pressure and force fuel into the fuel line.
- Release the pressure into a drain pan
- Reposition the fuel gauge the take a proper measurement
- Turn the key to the ON position (without starting the engine)
- Watch the gauge to see if the needle drops over a span of 20-30 min. If the needle significantly drops (like below 30, 20, or 10 psi) this means the fuel pressure is too low and the vehicle will have to spend time building the pressure back up so it can run.
Causes of Low Fuel Pressure
With dropping fuel pressure, if the problem is not the fuel pump, it is either:
- Leaking fuel injector
- Broken fuel pressure regulator (located on the fuel rail or inside the gas tank)
- Leaking fuel lines
If the fuel gauge slowly rises upon testing, it could be:
- Defective fuel filter
The fuel filter is usually located underneath the vehicle and is usually the easiest component of the fuel system to change, but sometimes it is connected to the fuel pump.
4. Check the Fuel Pressure with the Vehicle Running
With the vehicle running, observe the fuel pressure gauge needle for excessive movement. If the fuel pressure gauge needle fluctuates and bounces, there is an issue with the fuel system.
Fuel Pressure and Driving Under Load
If driving under load on low fuel pressure, problems can appear as
- Engine Hesitation
- Lean Fuel Condition
Replacing the Fuel Pump when your car struggles to start
If the fuel pump is the cause, check out this video below to get a feel for the steps.
Tips for Replacing the Fuel Pump
- When replacing the fuel pump, replace the fuel filter
- Disconnect the negative battery terminal
- Have enough space to lower the fuel tank to access the top and remove it
- Have the fuel level as low as possible
- If you replace the fuel pump and experience a long crank at start up, it could be from air in the fuel system from disconnecting the fuel lines during the repair. To prevent this, before start-up turn the key to the ON position and wait for the fuel pump to activate. After 3-5 seconds, turn the key to the OFF position. Then turn the vehicle on, listen for the fuel pump, and then crank the engine.
Shop parts and tools featured in this article:
- Fuel Pump
- Fuel Tank
- Fuel Lines
- Fuel Injectors
- Fuel Pressure Regulator
- Tail Pipe
- Catalytic Convertor
- Spark Plugs
- Fuel Filter
- Safety Goggles