Checking Fuel Pump Pressure: A Step-By-Step Guide

If you're experiencing issues with your car, it may be due to a faulty mechanical fuel pump. A mechanical fuel pump siphons fuel from the gas tank and pushes it to the carburetor when the engine is cranking or running. The output pressure of a mechanical fuel pump is typically quite low, ranging from 4 to 10 psi. However, if there is a leak in the diaphragm or a one-way valve inside the pump, it can cause a loss of fuel pressure, leading to engine problems such as misfiring or stalling. There are several ways to check the pressure of your mechanical fuel pump and identify potential issues. Firstly, you can remove the air cleaner and pump the throttle linkage while looking into the throat of the carburetor. If the pump is working, you should see fuel squirting into the carburetor. Another method is to visually inspect the pump for any fuel dripping, which indicates a failed diaphragm. Additionally, you can disconnect the fuel line at the carburetor, place the end into a container, and crank the engine to observe if the pump is pushing fuel through the line. Finally, you can connect a fuel pressure gauge to the pump outlet or tee a gauge into the fuel line at the carburetor to measure the pressure directly. If there is no pressure or the pressure is below specifications, the pump may need to be replaced.

Check for carburettor faults

Checking for carburettor faults is an important step in troubleshooting fuel supply and engine problems. Here are some detailed steps and signs to look out for when checking for carburettor faults:

Signs of a Faulty Carburettor:

• Reduced engine performance: A faulty carburettor can cause a reduction in engine performance, including sluggish acceleration, reduced power, and decreased fuel efficiency.
• Black smoke from the exhaust: If the carburettor delivers too much fuel, it can lead to black smoke from the exhaust, indicating excessive fuel usage and increased emissions.
• Engine backfiring or overheating: Issues with the carburettor can result in a lean mixture (insufficient fuel), leading to engine backfiring or overheating.
• Hard starting: Problems with the carburettor can disrupt the air-fuel ratio, making it difficult to start the engine, especially during cold starts.
• Engine periodically increases and decreases RPMs after being started: This could indicate a restriction in fuel flow, requiring the carburettor to be cleaned or replaced.
• Dry spark plug tip: If the spark plug tip is dry, it suggests a break in the fuel transfer, which could be due to a faulty carburettor.
• Engine will not keep running with the choke turned down: If the engine only runs with the choke halfway open or more, it could indicate a blockage in the jet or passageways, requiring carburettor cleaning or repair.

Checking for Carburettor Faults:

• Check the fuel level in the carburettor: If the car stops running but still has petrol, disconnect the fuel line to the carburettor. If fuel spurts out, there is a carburettor fault.
• Inspect the mechanical-pump drive: While rare, signs of wear may appear on the cam and operating lever, linkages, and pins.
• Check for leaks: Diaphragms, gaskets, fuel pipes, and seals can develop leaks. Leaking diaphragms can be especially dangerous as petrol can drip into the camshaft area and the sump, posing an explosion risk.
• Examine valves: Valves are generally durable but can become blocked by dirt, affecting their function.
• Check for loose mountings: The pump bolts may work loose, leading to severe oil leaks.
• Test pump operation: Seal the outlet and inlet pipes, as well as the vent hole, and push the lever. You should feel resistance, and the diaphragm should not move if the valves are seating properly.

Inspect for fuel leaks

To inspect your car for fuel leaks, you must first make sure that your vehicle is supported by jack stands. This will ensure that you can safely access the undercarriage of your car. Once your car is securely propped up, grab a flashlight and locate the fuel line. Start at the fuel tank and carefully trace the line to the engine and back, looking for any signs of leaks. Be careful not to confuse leaks with accumulated road grime and dust. If you spot any dripping fuel, try to further diagnose the problem. Keep in mind that environmental factors such as wind, rain, and gravity can impact the fuel's path, so use these factors to help determine the source of the leak.

Another method to help locate a fuel leak is to use a dye product that is safe for your gasoline or diesel tank. These dyes glow under ultraviolet and fluorescent light, making it easier to spot leaks. Simply add the dye to your fuel tank, run the engine, and inspect the undercarriage for any signs of glowing dye. This method can help you pinpoint the exact source of the leak, making it easier to repair.

Additionally, you can check for fuel leaks by inspecting the mechanical fuel pump. Disconnect the fuel line to the carburettor, using a plastic bag to catch any fuel inside. Then, turn the engine over with the starter. If fuel spurts out, there is a carburettor fault. However, if no fuel or only a small amount emerges, there could be a blockage or leak in the fuel line or the fuel pump itself may not be functioning properly. Gaskets, loose fuel pipes, and the seal between the diaphragm and the pump body can all be potential sources of leaks.

Check for fuel line blockages

Checking for blockages in the fuel line is an important step in troubleshooting mechanical fuel pump issues. Blockages can prevent fuel from reaching the engine, leading to problems such as difficulty starting the car, engine stuttering, and the engine cutting out.

To check for blockages, start by inspecting the fuel line for any signs of damage or leaks. Look for any cracks, abrasion, or corrosion on the fuel line, especially if it is made of metal or plastic. Corrosion in steel pipes and cracks in plastic pipes are common causes of leaks. Check both the single and twin fuel-pipe systems, as well as the flexible hoses and rigid pipes. Disconnect the battery to prevent any accidents and use a torch for better visibility.

Next, check the carburettor, which is the most common source of leaks. Disconnect the fuel line to the carburettor and use a plastic bag to catch any fuel. Turn the engine over with the starter. If fuel spurts out, the issue is with the carburettor. If there is no fuel or only a small amount, there may be a blockage or leak in the fuel line or the fuel pump may not be working properly.

Another way to check for blockages is to free both ends of the fuel pipe and blow through it gently using your mouth. Do not use a high-pressure air line as it may cause damage. If you are unable to blow through the pipe, there is likely a blockage, which could be at a filter or caused by kinks and dents in the line.

If you suspect a blockage in the fuel filter, you may need to remove and clean or replace it. Additionally, check the fuel pump for any signs of wear or damage, as this could also be causing the issue.

Check for fuel pump faults

If your car engine stops suddenly, the first thing to do is to check if you have run out of petrol. If not, there is likely a fault in the fuel system.

If the engine won't start, begin by verifying if the problem is fuel-related. Listen for the fuel pump by putting your ear near the fuel tank and turning the ignition key to the "on" position. If the pump is working properly, you should hear an audible noise. If you don't hear anything, ask an assistant to crank the engine while you hit the fuel tank with a rubber mallet. If the vehicle starts, it indicates a problem with the electric motor inside the pump.

You can also try using starter fluid, but this procedure comes with a warning as it can cause personal injury and damage to your engine. If you decide to go ahead, ensure you have safety equipment such as a fire extinguisher and safety glasses. Try to start the vehicle by removing the air intake and spraying starter fluid into the throttle body. If the engine starts and runs momentarily, it confirms a fuel-related problem, which may indicate a bad pump.

If your engine exhibits performance problems, check for diagnostic trouble codes (DTCs) with a scanner or code reader. Many auto parts stores will pull these codes for free. If there are codes, the next step is to check the fuel pressure.

Checking the fuel pressure and/or volume will indicate whether enough fuel is making it from the tank to the engine. However, this doesn't necessarily mean the pump is faulty. You can rule out fuel pump circuit issues and other problems, such as a clogged fuel filter or bad pressure regulator, by doing further testing.

Before connecting a fuel pressure gauge, be sure to relieve the fuel system pressure as outlined in your repair manual. Always wear safety glasses and keep a fire extinguisher nearby.

For a static fuel pressure test, simply connect the gauge as outlined in your repair manual. Turn the ignition to the "on" position (key on, engine off), note the reading on the gauge, and compare it to the specification in your repair manual. If the reading varies significantly, you may have a bad fuel pump.

When performing a static test, check if the fuel pressure holds. The factory repair information will tell you how long the pressure should hold once the pump is turned off. If the pressure drops too quickly, there is a fuel-related problem, such as a bad pump check valve or faulty regulator.

For a more in-depth assessment, you can check the fuel pressure at idle or under load. Additionally, continuous fuel systems can be checked for "dead head" pressure (pressure with the return line pinched off). However, this test doesn't work on newer returnless systems with the regulator in the tank.

You can also check the fuel volume. A pump may produce adequate pressure yet not create enough volume. Start by connecting the gauge and placing the bleed end of its hose into a clean container. Without starting the engine, activate the pump for 15 seconds while holding down the relief valve on the tool. Typically, the pump should deliver about a pint of fuel during this time. Consult your repair information for the exact procedure and specifications for your vehicle.

Before condemning the pump, there are a few other things to check. In most older continuous-style fuel systems, the engine control module (ECM) controls the fuel pump via a relay. Check this relay before assuming the fuel pump is at fault.

In newer, returnless fuel systems, the fuel pump control module (FPCM) directly controls the fuel pump. This system uses information from sensors, such as the fuel pressure and fuel temperature sensors, to determine control of the pump's ground circuit and vary its speed. Ensure everything within the control system is functioning properly before blaming the pump.

With both continuous and returnless systems, check the integrity of the wiring in the pump circuit with a digital multimeter (DMM). You can use this to check the circuit for continuity, power and ground, as well as any unwanted excessive resistance.

Next, check the fuel pressure regulator. Returnless fuel systems don't have an external pressure regulator. However, on continuous systems, you can check the regulator by disconnecting the attached vacuum hose and seeing if the pressure increases. If the pressure doesn't increase, the regulator is either faulty or doesn't have an adequate vacuum supply.

Finally, check the fuel filter. Continuous fuel systems have an external filter that's relatively easy to install. Rather than testing the filter, it's usually easier to install a new one and see if that helps. Remember to relieve the fuel system pressure and have safety equipment on hand before replacing the filter.

Check for fuel pressure

Checking the fuel pressure is an important step in diagnosing issues with a mechanical fuel pump. Here is a detailed guide on how to check the fuel pressure and troubleshoot any problems:

Checking Fuel Pressure:

• Check the Carburetor: Start by checking if there is petrol in the carburetor. Disconnect the fuel line to the carburetor and place a plastic bag over the end to catch any fuel. Secure it with a rubber band, then turn the engine over with the starter. If there is no fuel in the carburetor, there is a fault in the fuel system.
• Inspect for Fuel Leakage: If there is no fuel in the carburetor, the next step is to inspect the fuel pump for any signs of fuel leakage. A fuel leak could be due to a faulty diaphragm or one-way valve, which can cause a loss of fuel pressure and affect engine performance.
• Disconnect Fuel Line: To further diagnose the issue, disconnect the fuel line at the carburetor and place the end into a container. Crank the engine and observe the fuel flow. Strong, steady spurts of fuel indicate a functional pump, while a weak stream or no fuel could mean a faulty pump, clogged fuel filter, fuel line blockage, or an empty fuel tank.
• Use a Fuel Pressure Gauge: For a more accurate measurement, connect a fuel pressure gauge to the pump outlet or tee a gauge into the fuel line at the carburetor. Crank the engine and note the pressure reading. If there is no pressure or the pressure is below specifications, the pump may need to be replaced.
• Check for Blockages: If the fuel pressure is low, there could be a blockage in the fuel line or fuel filter. Check for any restrictions or clogs in the fuel lines going to and from the pump. Replace old or damaged fuel lines to ensure proper fuel flow.
• Inspect for Fuel Leaks: In addition to checking fuel pressure, it is important to inspect the fuel pump and lines for any signs of fuel leaks. Fuel leaks can be dangerous and are often caused by cracks or holes in the diaphragm, loose inlet or outlet fittings, or faulty gaskets. Address any leaks immediately.

Troubleshooting and Maintenance:

• Fuel Pump Replacement: If the fuel pump is faulty, it will need to be replaced. Choose a quality name-brand pump with a warranty to ensure longevity and performance.
• Fuel Filter Replacement: When replacing the fuel pump, it is recommended to also replace the fuel filter. A clogged fuel filter can restrict fuel flow and impact fuel pressure.
• Clean Fuel Tank: If the old fuel filter is severely clogged, consider cleaning or replacing the fuel tank if it is rusty or contaminated.
• Inspect Fuel Lines: Carefully inspect all fuel lines for cracks, leaks, or deterioration. Replace old rubber fuel lines, especially if they are over 10 years old, to prevent leaks and ensure the safety of your vehicle.
• Gasket Maintenance: When reinstalling the fuel pump, use a new gasket and apply gasket sealer to both sides for a secure seal. Also, check the gasket between the diaphragm and the two halves of the pump body for leaks.
• Avoid Safety Hazards: When working on the fuel system, always exercise caution. Do not smoke, and avoid any sources of ignition or sparks that could cause a fire. Wait for the engine to cool down before starting work, and avoid skin contact with gasoline and inhalation of vapors.

Frequently asked questions