Mercedes Mullins
Fuel lines and hoses carry fuel from the tank to the engine. The main fuel line allows the fuel pump to draw fuel out of the tank. The fuel is then pulled through this line to the pump and then to the carburettor or metering section of the injection system. Fuel lines are typically made of double wall steel tubing, while fuel hoses, made of synthetic rubber, are used where there is severe movement between parts. When checking for restrictions in the fuel lines, it is important to disconnect the battery to prevent stray sparks from igniting petrol or petrol vapour. It is also crucial to look at the joint between the fuel line and the carburettor and check for splits, cracks, or loose connections.
Characteristics and Values for Checking Restrictions in Fuel Lines
| Characteristics | Values |
|---|---|
| Fuel line size | The diameter and length of the line must be considered to deliver fuel at the correct pressure and rate |
| Pressure loss | Fuel line size can cause pressure loss if not taken into account; friction is a source of pressure loss and increases as the flow rate through the line increases |
| Fuel supply restriction | Fuel supply restriction or "pressure drop" is always measured as a negative number because it is on the suction side of the fuel lift or supply pump |
| Fuel system design | The design of the fuel system dictates the total fuel flow |
| Maximum pressure drops | Know the maximum pressure drops the engine can handle and still work correctly for both the supply and return sides |
| Fuel line length | Longer lines will have more valves, manifolds, and fittings, so a larger fuel line will result in less fuel supply restriction; fuel lines are longer on the supply side |
| Fuel return system | The maximum pressure drop is usually two to four times more than the supply side, and the return flow is less than the total fuel flow, so fuel lines can be smaller |
| Fuel pump pressure | Connect a pressure gauge to the output line of the fuel pump and compare the reading to the manufacturer's specifications |
| Fuel pump volume | The amount of fuel the pump can deliver in a specific amount of time; measure by allowing fuel to pour into a graduated container for a certain amount of time |
| Fuel pump vacuum | If the fuel pump fails both pressure and volume tests, check the fuel pump vacuum to eliminate possible problems in the fuel lines, hoses, filters, and pickup screen in the tank |
| Fuel tank | Check for leaks and clogs; certain petrol blends can cause plastic fuel lines to expand or swell, and sagging fuel pipes are a sign of this |
| Fuel line material | Metal fuel lines rarely cause problems, but replace if they become smashed, kinked, rusted, or leaking; fuel lines are normally made of double wall steel tubing |
| Fuel hose | Fuel hoses can become hard and brittle from exposure to engine heat and the elements, and engine oil can soften and swell them; replace any in poor condition |
| Fuel filter | A blocked fuel filter can cause fuel starvation at high speed, which is hard to diagnose; if suspected, blow through the pipe to check |
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What You'll Learn
Check for fuel leaks
Checking for fuel leaks in a car is important, as even a small leak can cause a fire. If you suspect a fuel leak, do not drive the car until it has been cured. To check for fuel leaks, follow these steps:
- Disconnect the battery: Before inspecting the fuel system, disconnect the battery to prevent stray sparks from igniting petrol or petrol vapour. Ensure that any naked lights or cigarettes are extinguished, and use a torch instead of an inspection lamp to avoid the risk of fire.
- Inspect the fuel pipes: Raise the car on ramps or axle stands to access the fuel pipes under the floor. Check the entire fuel line for any signs of damage, such as splits, cracks, loose connections, or wet joints. Pay close attention to the joint between the fuel line and the carburettor, as this is a common area for leaks.
- Check for sagging or swollen pipes: If you have a plastic fuel line, look for sagging pipes, which could indicate that the line has expanded or swollen due to certain petrol blends. Hardened or inflexible plastic pipes should also be replaced, as they may have deteriorated with age.
- Perform a pressure test: If you are unable to find any visible signs of a leak, you can perform a pressure test. Remove the return line from the fuel pressure regulator and use a bike pump to pressurise the fuel system with air. Listen for any unusual sounds and check under the car for any signs of fuel leaking.
- Check for blockages: If you suspect a blockage in the fuel line, free both ends of the pipe and use your mouth to blow through it. Do not blow into the fuel filler hole, as this can be dangerous if the tank is full. Blockages may also be caused by dirt or rust in the fuel tank, so the tank may need to be cleaned or replaced.
If you are unable to identify the source of the leak or are unsure about performing these checks yourself, it is recommended to consult a mechanic or a specialist, such as Fisher Auto, who can inspect and replace the fuel lines if necessary.
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Inspect rubber connectors for damage
To inspect rubber connectors for damage, you must first locate the fuel lines. Typically, a single fuel pipe runs from the tank, under the floor of the car, to a mechanical pump on the engine. The pump delivers fuel to the carburettor. The section of the fuel line in the engine bay is less exposed to damage than the piping under the car. However, heat and oil can damage the rubber connectors in the engine bay.
Before inspecting the rubber connectors, it is crucial to disconnect the battery to prevent stray sparks from igniting petrol or petrol vapour. It is also important to ensure that all naked lights and cigarettes are extinguished, as fuel leaks can cause fires. Use a torch instead of an inspection lamp, as the bulb of the lamp can break accidentally and start a fire.
Once you have taken the necessary safety precautions, you can begin inspecting the rubber connectors. Carefully examine the rubber joints for any signs of fraying or loosening. Metal joints tend to leak when they have been pulled or pushed, distorting the seal with the connector. Feel around the joints to check if they are wet, as petrol may run along the pipe before dripping. Additionally, bend the flexible pipes between your fingers to reveal any splits or cracks.
If you suspect a blockage, do not blow into the fuel filler hole as it can be dangerous if the tank is full. Instead, free both ends of the pipe and blow through it gently to check for blockages. If you find a blockage, it may be at a filter, as some filters mounted in the fuel line resemble rubber connectors. Repeated blockages may be caused by dirt or rust in the fuel tank, which will require the tank to be cleaned or replaced.
It is important to note that certain petrol blends can cause plastic fuel lines to expand or swell, leading to sagging fuel pipes. In such cases, the entire line must be replaced, as improvised joints will not remain leakproof.
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Check for blockages
If your engine won’t turn over, or your vehicle’s engine is sputtering or dying, you may be having issues with a clogged fuel line. A blocked fuel line will cause the engine not to get enough fuel, or it will receive fuel in inconsistent spurts. This will cause the engine to sputter or cut out.
To check for blockages, you can start by looking at the joint between the fuel line and the carburettor, and then checking other joints, working your way back to the tank. Place the car on axle stands to inspect the section under the floor. Bend flexible pipes between your fingers to show up any splits or cracks. Push clips and metal joints to see if they are loose. Feel around the joints to see if they are wet, as petrol may run along a pipe before it begins to drip.
If you suspect a blockage, the quickest way to check is to free both ends of the pipe and blow through it. Use your mouth, not a high-pressure air line, which may cause damage. An air line may also loosen dirt, which could then fall back and block the pipe again. Do not blow into the fuel filler hole, as this will not reveal a blockage and could be dangerous if the tank is full. You might inhale petrol or fumes, and get it in your eyes. If you find a blockage, it may be at a filter. Some filters mounted in the fuel line look just like rubber connectors.
Kinks and dents in the line itself can cause a blockage. Old rubber hoses, especially the braided type, can collapse internally without this being evident on the outside. Check by blowing, not by poking anything through the hose, which may damage it. Repeated blockages may be caused by dirt or rust in the fuel tank. If so, the tank must be removed, cleaned, and flushed out, or replaced. When working on the fuel system, do not wipe the ends of pipes with soft rags. Small threads or bits of fluff can come off and block a pipe.
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Measure fuel pump pressure
To measure fuel pump pressure, you will need to perform a fuel pump test. This will show the performance condition of the pump. Before you begin, ensure that your vehicle is on flat ground, in park, with the emergency brake set and the engine off. It is important to take the necessary safety precautions when working with fuel systems, so keep a fire extinguisher nearby.
The first step is to locate the fuel pump test point, which is usually near the fuel injectors. You will then need to find the point at which the pump hooks up with the filter injector rail. There should be a separation joint or a test port where you can attach the pressure gauge. Different vehicles have different locations for the fuel pump and test port, so consult your owner's manual for specific instructions.
Once you have located the test port, attach the pressure gauge to it. If your vehicle does not have a built-in pressure port, you may need to use an adapter from a test kit. Attach the adapter at the fuel filter or fuel pressure feed line headed to the engine's fuel rail. When attaching the pressure gauge, a small amount of fuel may be present, so have a shop towel ready.
With the pressure gauge attached, turn the ignition key to the "on" position without starting the engine. The gauge should read between 45 psi and 58 psi. If there is no pressure, recheck the hose or adapter connection and recycle the ignition switch to recheck. If there is still no pressure, you may have a bad fuel pump, fuel pump relay, or fuse.
If the initial pressure test is successful, start the engine and test the performance of the pump. The fuel pressure should drop by about 5 psi from the static prime pressure, and then snap the throttle, causing the fuel system pressure to jump up by about 5 psi. This indicates that the fuel pump and pressure regulator are working correctly. If the system fuel pressure does not respond, inspect the regulator vacuum feed line, as the fuel pressure regulator may have failed.
The final check is the load test, which will determine how the pump performs under load. This can be done by loading the engine and observing the fuel pressure or by driving the car while the gauge is visible to the driver. If the fuel pressure is low under load or while idling, the fuel filter may be clogged, or the fuel pump may need to be replaced.
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Check for restrictions in the fuel return system
The fuel return line is a crucial component of a vehicle's fuel system, responsible for returning excess fuel from the engine to the fuel tank. This line is typically found in mechanical diesel fuel systems with injector pumps and plays a significant role in engine operation and performance. A faulty fuel return line can cause fuel leaks, poor engine performance, and even safety hazards. Here are some detailed instructions to check for restrictions in the fuel return system:
First, it is important to understand the function of the fuel return system. The return line ensures that excess fuel is directed back to the fuel tank, preventing fuel accumulation in the engine, which could lead to potential issues.
To check for restrictions, begin by inspecting the fuel return line for any signs of damage, leaks, or blockages. Look for any punctures, kinks, or worn-out sections on the hose. Feel around the joints and connections for any wetness, as fuel may run along the pipe before dripping. If you suspect a blockage, do not blow into the fuel filler hole, as this could be dangerous. Instead, free both ends of the pipe and blow through it to check for any obstructions.
Next, consider the pressure drop in the fuel return system. The maximum pressure drop is usually two to four times higher than the supply side, and the return flow is less than the total fuel flow. This means that fuel lines can be smaller on the return side. However, using a larger line on the return side is also an option, although it may be more manageable and cost-effective to use smaller lines.
Finally, if you suspect any issues with the fuel return system, it is always best to consult a competent mechanic. They will have the expertise to diagnose and address any problems with the fuel return line, ensuring the safe and efficient operation of your vehicle's fuel system.
By following these steps and staying vigilant about the condition of your fuel return system, you can help maintain the performance and safety of your vehicle.
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Frequently asked questions
To check for restrictions in the fuel lines of a car, follow these steps:
- Disconnect the battery to prevent sparks from igniting fuel or vapours.
- Inspect the fuel line under the car, which is more susceptible to damage.
- Check the rubber connectors in the engine bay for damage from heat and oil.
- Look for leaks or blockages in the fuel line.
- If you suspect a blockage, free both ends of the pipe and blow through it.
- Check the fuel pump pressure by connecting a pressure gauge to its output line and comparing the reading to the manufacturer's specifications.
- If the pump fails the pressure test, check the pump volume, lines, and filters.
- Refer to the service manual for specific pressure and volume specifications.
There are several signs that may indicate restrictions or issues with your car's fuel lines:
- Fuel leaks, even small ones, can be a fire hazard.
- Sagging fuel pipes may indicate that certain petrol blends are causing plastic fuel lines to expand.
- A blocked fuel line will stop the car, while a partial blockage may cause fuel starvation at high speeds.
- Repeated blockages may be caused by dirt or rust in the fuel tank.
If you discover a restriction or blockage in the fuel line, you should:
- Replace any damaged or restricted sections of the fuel line.
- Use the correct grade of fuel line recommended by the manufacturer.
- Avoid using copper or plastic tubing for fuel lines.
- Ensure that the fuel line is securely attached to the frame, engine, and other units to minimise the effects of vibration.
- Refer to the service manual or a specialist for specific instructions and safety precautions.
