The Essential Guide to Understanding, Diagnosing, and Repairing Your SU Fuel Pump

SU fuel pumps are generally robust and long-lasting components found on many classic British and European vehicles, but they do eventually fail and require diagnosis, rebuilding, or replacement; understanding how they work, recognizing the signs of trouble, and knowing your repair options is crucial for keeping your vintage car running reliably.

For generations of classic car enthusiasts, particularly those devoted to British marques like MG, Triumph, Jaguar, Austin-Healey, and many others, the distinctive "tick-tick-tick" sound of a functioning SU fuel pump is a reassuring heartbeat. Named after its inventors, Skinners Union, SU developed a reputation for engineering simple, durable, and effective mechanical fuel pumps. Unlike modern electric pumps submerged in tanks, SU pumps are typically mounted on the engine bay firewall or frame rail, driven either mechanically by the engine's camshaft or via an electric motor in later AUF (A.U.F. is the model designation) versions. While known for their longevity and serviceability, even the best SU pumps will eventually succumb to wear, environmental factors, or incompatible modern fuels. Addressing SU fuel pump issues promptly is essential, as fuel delivery problems lead directly to poor performance, stalling, and failure to start, potentially leaving you stranded. Ignoring symptoms risks not only inconvenience but also potential engine damage from lean running conditions.

Understanding How an SU Fuel Pump Functions

Grasping the fundamental operation of an SU fuel pump is key to diagnosing problems. It's a positive displacement diaphragm pump, meaning it creates suction and pressure by the movement of a flexible diaphragm within a chamber.

1. The Core Mechanism: The diaphragm forms a seal within the pump body. Its movement is actuated either by a lever arm resting on an eccentric lobe of the engine's camshaft (mechanical pumps) or by an electromagnetic vibrator assembly (AUF electric pumps).
2. The Suction Stroke: As the actuating mechanism pulls the diaphragm downwards (away from the top chamber), it increases the volume within the pump chamber above the diaphragm. This creates low pressure (suction).
3. Fuel Entry: This suction opens the inlet valve (a simple one-way flap or ball check valve) and draws fuel from the tank into the chamber.
4. The Pressure Stroke: The actuating mechanism then pushes the diaphragm upwards, reducing the chamber volume and creating pressure.
5. Fuel Exit: This pressure forces the inlet valve closed and pushes the outlet valve open, sending fuel towards the carburetors. A pulsating flow of fuel is characteristic of this diaphragm action.
6. Regulation: SU pumps incorporate a clever pressure regulation system. A spring beneath the diaphragm pushes against the fuel pressure in the chamber. If the pressure becomes too high (for example, if the carburetor float valves are closed), the diaphragm cannot complete its full upward stroke against the spring tension. This limits the maximum pressure the pump can produce, preventing damage to the diaphragm or carburetors. Most SU pumps operate in the 1.5 to 4 psi range – much lower than modern fuel injection systems.
7. Venting: The chamber beneath the diaphragm (on mechanical pumps) or surrounding the vibrator assembly (on electric AUF pumps) is vented to atmosphere. This allows the diaphragm or vibrator mechanism to move freely. Blocked vents can cause pump failure. Mechanical pumps typically have a small vent hole on the main body, while electric AUF pumps have a small port or slot.

Common Symptoms of a Failing SU Fuel Pump

A weakening or failed SU fuel pump manifests through several tell-tale signs related to inadequate fuel delivery. Recognizing these symptoms early is crucial:

1. Difficulty Starting: This is often the first sign. As the pump weakens, it struggles to fill the carburetor float chambers when the engine is cold. You might notice prolonged cranking before the engine fires, especially if it's been sitting for a while.
2. Engine Stalling at Low Speeds / Idle: Insufficient fuel pressure causes the engine to stumble and stall during low-demand situations like idling at a stoplight or driving slowly in traffic. The low pressure can't consistently maintain the fuel level in the carburetor float bowls.
3. Engine Sputtering or Hesitation Under Load: As you accelerate or climb a hill, the engine demands more fuel. A failing pump cannot keep up, leading to sputtering, hesitation, lack of power, or even backfiring as the mixture becomes dangerously lean.
4. Complete Engine Failure: The most obvious symptom – the engine simply stops running, either while driving or refuses to start at all. This occurs when the pump delivers no fuel.
5. Audible Pump Issues:
   • Mechanical Pumps: A healthy mechanical SU pump produces a distinct rhythmic ticking sound, often synchronized with engine RPM. Silence usually indicates a broken lever arm or disconnected linkage. Excessively loud ticking or clattering often points to excessive internal wear or lack of lubrication on the lever arm pivot or cam follower.
   • Electric AUF Pumps: A functioning AUF pump makes a clear, rapid buzz-click sound. An unusually loud buzz with no click could indicate a jammed mechanism or seized diaphragm. Silence means no electrical power or a completely failed internal component (like the points in the vibrator assembly).
6. Fuel Leaks: Visible fuel leaking from the pump body (around the seam where top and bottom halves meet, around the diaphragm center bolt, or from valves) is a serious failure sign. It indicates a ruptured diaphragm or deteriorated gaskets/seals – a major fire hazard that requires immediate pump removal.
7. Reduced Fuel Efficiency: While harder to pinpoint solely to the pump, extreme difficulty maintaining correct mixture due to low fuel pressure can sometimes lead to increased fuel consumption.

Diagnosing Your SU Fuel Pump Problem

Before rushing to rebuild or replace, thorough diagnosis pinpoints the exact issue:

1. Visual Inspection:
   • Look for obvious fuel leaks from the pump body or fuel lines connected to it.
   • Check the fuel lines (inlet and outlet) for cracks, hardening, or deterioration. Old rubber lines are prone to failure and can draw air.
   • Ensure all electrical connections on AUF pumps are clean, tight, and free of corrosion. Verify the ground connection is secure. For mechanical pumps, ensure the lever arm is correctly engaging the camshaft eccentric.
   • Check the pump vent hole(s) for blockage (dirt, paint, oil residue). A blocked vent prevents diaphragm movement.
2. Listen:
   • (Mechanical) With engine running (or cranking), listen for the characteristic ticking sound near the pump.
   • (AUF Electric) Turn the ignition on (without starting the engine). Listen for the distinct buzz-click sound. Try tapping the pump body gently – a stuck vibrator might free itself temporarily.
3. Electrical Checks (AUF Pumps Only):
   • Verify power to the pump. Use a multimeter or test light at the pump's power terminal (ignition ON). No power indicates an issue in the wiring, fuse (if equipped), or ignition switch.
   • Verify ground integrity.
4. Fuel Delivery Test: This is the most critical test. Caution: Perform this outdoors, away from sparks or flames. Have a fire extinguisher nearby. Disconnect the spark plugs or coil wire to prevent accidental starting. Disconnect the fuel line delivery hose at the carburetor(s). Hold the end of the hose over a suitable container (clear glass bottle/jar helps for observing flow). For mechanical pumps, crank the engine using the starter. For electric AUF pumps, turn the ignition ON (without cranking). Observe the fuel flow:
   • Good Flow: Strong, pulsing streams of fuel, consistent with the pump's action. Volume should be significant (often specs are in pints per minute or ml per stroke – but strong pulsing flow is a good indicator).
   • Weak/Intermittent Flow: Sputtering, dribbling fuel or spurts of fuel separated by air bubbles indicates a failing diaphragm, clogged valves, air leaks in the inlet line, or blockage.
   • No Flow: Confirms a serious pump failure, blockage in the inlet line or tank pickup, or (for AUF) an electrical issue despite voltage being present.
5. Pressure Test: While a good flow test is often sufficient, a pressure test provides precise data. Disconnect the outlet hose and install a low-pressure fuel pressure gauge (0-15 psi range is ideal) between the pump outlet and a return line to a container, or cap the gauge and run the pump very briefly. Good SU pump pressure should fall within the 1.5 psi (low) to 4 psi (peak) range. Consult your workshop manual for exact specifications. Pressure significantly below spec or failing to hold indicates pump problems.

Rebuilding vs. Replacing Your SU Fuel Pump: Making the Right Choice

Finding your SU fuel pump is faulty presents two main paths: rebuilding your existing unit or purchasing a replacement. Each has pros and cons:

The Case for Rebuilding:

• Cost-Effective: Rebuild kits are significantly cheaper than a new or reconditioned pump. Kits typically include all consumable parts: diaphragm, valves (seats, pins, and balls/seals as needed), gaskets, O-rings, and sometimes the lever arm pivot pin/bushing (for mechanical) or points/condenser (for AUF). You pay mostly for parts.
• Preserve Authenticity: For concours restorations or purists, rebuilding the original, date-stamped pump maintains the car's originality.
• Educational/Satisfying: Doing it yourself provides valuable mechanical insight and the satisfaction of restoring a core component. The design is inherently rebuild-friendly.
• Common Issues Solved: A rebuild directly addresses the most frequent failure points: perished diaphragm, dirty/worn valves, leaking gaskets.

Rebuilding Limitations/Challenges:

• Mechanical Wear: If the pump body is cracked, the lever arm pivot/bushing excessively worn, the cam follower badly damaged (mechanical), the drive dog worn (AUF), or the magnet coil burnt out (AUF), a simple kit won't fix it. These require component replacement or sourcing a different pump core.
• Skill and Time Required: Rebuilding requires mechanical aptitude, careful disassembly/assembly (sometimes with small, intricate parts), cleanliness, and specific torque settings. Mistakes can lead to immediate or premature failure. Lack of a suitable workspace is a barrier.
• Finding Kits: While kits are widely available online and from specialists, ensuring you get the correct kit for your exact pump model (SU coded their pumps differently) is essential. AUF pump kits often vary internally.
• Material Quality: Kits vary significantly in quality. Inferior diaphragms (not compatible with modern ethanol-blended fuel) or valve materials will fail quickly. Seek kits specifically made from ethanol-resistant materials (like Viton). Reputable suppliers like Burlen Fuel Systems manufacture high-quality kits.

The Case for Replacement (New or Reconditioned):

• Convenience and Reliability: Installing a guaranteed new or professionally reconditioned pump saves time and effort. It offers near-zero risk if sourced from a reputable vendor.
• Addresses Core Issues: New or properly reconditioned units incorporate a new or re-machined body casting where necessary, plus a new diaphragm, valves, and seals. They effectively address wear beyond just consumables.
• Warranty: Most new or professionally reconditioned pumps come with a warranty (e.g., 12 months), offering peace of mind a rebuild kit doesn't provide.
• Good for Non-Purists: If originality isn't critical, a reliable replacement is often the quicker, simpler solution.

Replacement Limitations/Challenges:

• Cost: New pumps, especially high-quality ones, are considerably more expensive than a rebuild kit. Reconditioned units fall in the middle but still cost more than DIY.
• Potential Fitment Issues: While most replacements are exact replicas, slight variations in mounting bracket holes, lever arm length/angle (mechanical), or inlet/outlet ports can occur. Double-check compatibility before purchase.
• Quality Variance: As with rebuild kits, the quality of replacement pumps varies widely. Avoid cheap reproductions from unknown sources – failures are common. Stick with established brands like Burlen Fuel Systems or known specialists.
• Loss of Originality: Replacing the original pump may detract from the vehicle's originality for judging purposes.

Decision Guide:

• Rebuild If: The pump body is undamaged, minimal core wear exists, you are comfortable with the task, cost is a major factor, and/or maintaining strict originality is paramount. Especially true for less common SU pump variants where direct replacements might be hard to find.
• Replace If: The pump shows significant wear to non-kit parts (body, lever pivot, cam follower, AUF coil), you lack the tools/confidence to rebuild, time is limited, or you prefer guaranteed reliability with warranty. Generally the best choice for primary daily-driven classics or when uncertain about the core condition.

Step-by-Step Guide to Rebuilding Your SU Fuel Pump

Rebuilding an SU pump requires patience, cleanliness, and care. Always work in a well-ventilated area away from ignition sources. Handle fuel carefully. This guide covers the general process – always consult a workshop manual or specific rebuild guide for your pump model number.

1. Prepare:
   • Safety: Disconnect the negative battery terminal. Relieve fuel pressure by running the engine until it stalls (unless pump is dead). Allow the engine to cool. Place absorbent rags under the pump.
   • Remove Pump: Clean the pump body externally with a degreaser. Disconnect the inlet and outlet fuel lines (plug ends to contain spillage). Disconnect the earth wire (AUF). Undo the mounting bolts/nuts. Carefully remove the pump. For mechanical pumps, note how the lever arm engages the eccentric lobe. Remove gasket material from the mounting surface.
   • Gather Tools & Kit: Obtain the correct rebuild kit (double-check model number!). Needed tools typically include screwdrivers (flathead, possibly Phillips), wrenches/sockets (appropriate sizes), pliers (needle nose), a soft-jaw vice or clamp is helpful, a small pick or scribe, new gasket for block surface (mechanical), solvent tank or parts cleaner brushes, lint-free rags, compressed air (optional). Use ethanol-resistant thread sealant on valve plug threads.
2. Disassemble: (General steps – model specifics vary)
   • (AUF Electric): Remove the small screws securing the plastic end cap. Note the orientation of wires. Unbolt/unscrew the large central stud/nut holding the diaphragm and internal assembly to the main body. Carefully separate the assembly. Note the order of washers/spacers. The vibrator assembly usually resides in the main body – points might need cleaning/adjusting later.
   • (Mechanical): Unbolt/unscrew the large central stud/nut holding the top cover (which includes the diaphragm and valves) to the main body. Carefully separate. Note the orientation/order of any shims or spacers under the diaphragm head.
   • Remove Diaphragm: Unbolt the diaphragm from its center attachment point. Set the metal parts aside.
   • Remove Valves: Carefully remove the inlet and outlet valve assemblies. Pay close attention to the orientation and order of components (balls, pins, springs, valve bodies). Note any wear or damage. Inspect valve seats in the body/cover. Clean all parts meticulously in solvent, blowing out passages with air. Replace all components from the kit – valve parts, O-rings, gaskets, diaphragm.
3. Inspect & Clean:
   • Body: Inspect the main body for cracks, warping, or significant corrosion. Check the lever pivot hole/bushing for excessive oval wear (mechanical). Check the AUF drive mechanism/armature pin for wear.
   • Diaphragm Mounting Areas: Ensure the sealing surfaces for the diaphragm are smooth and undamaged.
   • Vents: Thoroughly clean all vent holes/passages.
   • Valve Seats: Ensure they are smooth and undamaged. Light polishing with a Q-tip and fine metal polish can be done if necessary.
   • Clean Everything: Scrub all metal components (excluding electrical AUF parts) in solvent. Blow dry with compressed air. Ensure all passages are clear. Let components air dry completely. Use lint-free rags.
4. Rebuild Assembly: (Refer to kit instructions/guide for your specific model)
   • Lubricate: Lightly lubricate moving parts (diaphragm sealing edge, lever pivot on mechanical pumps, valve pins) with engine oil or assembly lube compatible with fuel. Never use grease!
   • Install Valves: Carefully install new inlet and outlet valve components exactly as the old ones were removed. Apply a small amount of ethanol-resistant sealant to valve plug threads (if applicable) before installing/screwing them in. Torque valves gently.
   • Install Diaphragm: Secure the new diaphragm to the center stud/bolt using the provided nut/washers. Ensure it's centered. For mechanical pumps, the lever arm must be correctly positioned under the diaphragm center pin (manual specifics vary here – crucial step!).
   • Assemble Main Body:
     • (AUF): Carefully place the diaphragm assembly onto the main body, ensuring the lever mechanism engages correctly with the AUF drive mechanism inside the body. Insert and tighten the central stud/nut evenly and securely to the specified torque.
     • (Mechanical): Place the diaphragm assembly onto the main body. Ensure the lever arm sits correctly on its pivot. Insert and tighten the central stud/nut evenly and securely to the specified torque. Any shims noted during disassembly must be reinstalled.
   • AUF Specific: Reassemble the end cap, ensuring wires aren't pinched. Install any provided points/condenser if part of the kit.
   • Final Torque: Ensure all fasteners are tightened to specifications. Avoid overtightening, especially plastic AUF parts.
5. Bench Test (Before Installing):
   • (Mechanical): Manually operate the lever arm through its full stroke. You should feel suction and pressure. Cover the inlet port with your finger – strong suction should be felt on the intake stroke. Cover the outlet port – resistance and slight pressure build-up should be felt on the exhaust stroke. Fuel or oil can be used minimally for this test. Listen for valve clicks.
   • (AUF): Connect the pump directly to a 12v source (e.g., a jumper from the battery). You should hear a consistent, crisp buzz-click and observe the diaphragm visibly pulsing. Cover the ports to test suction/pressure as above. Pump must be held firmly – vibration is strong.
   • Check for leaks around the body seam, center stud, and valves by applying light oil and looking for bubbles (do this dry first!). Fix any leaks before engine installation.
6. Reinstall Pump:
   • Install a new mounting gasket for mechanical pumps (unless sealing compound is recommended).
   • Position the pump on the engine. For mechanical pumps, ensure the lever arm correctly engages the camshaft eccentric lobe. Refer to your manual; improper engagement can break the lever arm. This often requires rotating the engine to position the cam lobe appropriately. Hand-press the pump body against the gasket and finger-tighten bolts to hold it. Move the lever arm to feel smooth engagement – it should rest against the eccentric. Tighten mounting bolts securely.
   • Reconnect the inlet and outlet fuel lines securely using new hose clips if necessary. Ensure lines aren't kinked or near exhaust heat. Reconnect the earth wire (AUF).
7. Final System Checks & Start-Up:
   • Prime the carburetor(s): Crank the engine briefly or operate the pump manually/AUF electrically until resistance is felt or fuel is seen at the carburetor connections if disconnected.
   • Recheck all fuel connections for leaks before reconnecting the battery or starting. Double-check!
   • Reconnect battery (negative terminal last).
   • Start the engine. The pump should now produce its characteristic sound. Monitor fuel line connections again carefully for any drips. If equipped with a clear fuel filter, check for strong, bubble-free flow. Test drive cautiously initially, monitoring for the previously experienced symptoms.

Maintaining Your SU Fuel Pump for Longevity

Regular maintenance dramatically extends the life of your SU fuel pump and prevents unexpected failures:

1. Use Quality Fuel Stabilizer (Especially For Storage): If your classic car sits unused for extended periods, add an ethanol-rated fuel stabilizer to a full tank before storage. This helps prevent the formation of gum and varnish inside the pump and fuel system which can clog valves and seize diaphragms.
2. Inspect Regularly: Make visual checks of the pump during routine oil changes or servicing:
   • Look for any signs of fuel weeping or leaks around the body seam, valves, or center stud.
   • Check fuel lines (inlet and outlet) for cracking, hardening, or brittleness. Replace every 5 years regardless of appearance; ethanol degrades rubber faster.
   • Ensure vent holes are clean and unobstructed. Remove any accumulated dirt or oil residue.
   • Listen to the pump sound during operation for any unusual noises.
3. Replace Fuel Lines Periodically: Rubber fuel lines deteriorate over time, especially with modern ethanol fuels. Replace the flexible hose sections every 3-5 years. Use only ethanol-resistant fuel hose (SAE J30R7 or R9 specification). Ensure clamps are tight and replace deteriorated ones. Old hoses can collapse internally, restrict flow, or suck air.
4. Install an Inline Fuel Filter (Crucial): Protect your SU pump and carburetors by installing a fine mesh inline fuel filter (rated for low-pressure systems) between the tank and the pump inlet. This traps debris from the tank before it reaches the pump valves and carburetors. Check and replace this filter annually or as needed. Avoid using a filter after the pump as debris could clog valves before it even gets filtered.
5. Consider a Tank Sedimentor/Pre-Filter: For older tanks with known sediment issues, an additional in-tank pickup filter or coarse sedimentor bowl mounted before the main inline filter adds valuable protection. Clean these periodically.
6. Address Ethanol Concerns (E10/E15 Fuel): Most modern petrol contains ethanol. While high-quality Viton or other ethanol-resistant diaphragms and seals (as found in good rebuild kits or new pumps) handle E10 reasonably well, prolonged storage or exposure can still cause issues:
   • Minimize Storage Time: Try to use the car regularly. If storing, add stabilizer and top the tank to minimize air space where water absorption can occur.
   • Seal Integrity: Ethanol is more aggressive on older materials. If your pump hasn't been serviced recently, its seals are likely compromised. Pre-emptively rebuild or replace with ethanol-proof components.
   • Water Absorption: Ethanol attracts water, which can cause corrosion inside the pump and fuel system. Keeping the tank full helps, but periodic water-removing fuel additives are advisable in humid climates, especially before storage.
   • Non-Ethanol Fuel: If readily available and practical, using ethanol-free fuel eliminates these specific issues. However, it is often more expensive and less widely available.
7. Operate the Vehicle Regularly: Regular use keeps components lubricated internally with fuel and prevents seals from drying out or mechanisms from seizing. Short trips can allow condensation build-up; occasional longer drives are beneficial.

Sourcing Parts and Professional Help

Finding the right parts and knowing when to seek expert help ensures success:

• Reputable Parts Suppliers: Stick with specialists known for quality and expertise in SU pumps:
  • Burlen Fuel Systems: The direct successor to SU Carburetters. They manufacture genuine SU parts, rebuild kits, and new/reconditioned pumps. Widely considered the gold standard.
  • Moss Motors, Victoria British, Rimmer Brothers, SNG Barratt: Major classic car parts suppliers who carry Burlen kits/pumps and sometimes other vetted brands.
  • SU Pump Specialists: Independent workshops like SU Carburetter Exchange or local classic car experts known for fuel system work. They often carry rare parts or can recondition worn core bodies/components.
• Avoid Cheap Kits/Pumps: Deep discount kits or pumps often use inferior materials (like non-ethanol resistant nitrile diaphragms) and poorly-machined valve components. They often fail quickly. Investing in quality pays off in reliability. Look for Viton/FPM or FKM diaphragms specifically stated.
• Professional Rebuilding Services: If your pump body is worn (lever pivot, cam follower on mechanical; internal components on AUF) or you lack the tools/confidence for a rebuild, professional shops specializing in SU pumps offer reconditioning. They replace consumables and fix core wear issues you cannot address with a standard kit. Costs more than DIY but less than a new pump and includes expertise. Insist on seeing warranty terms.
• Finding Service: Search specifically for "SU fuel pump rebuild service," "classic British car specialist," or "vintage fuel system repair" in your area. Classic car clubs and online forums are excellent resources for recommendations. Ensure the shop understands SU pumps specifically, not just modern fuel systems.

Conclusion: Ensuring Reliable Fuel Flow for Your Classic

The SU fuel pump, a marvel of simple and effective engineering, is a vital lifeline for carbureted classic vehicles. Its distinct sound is a signature of a bygone era. While known for durability, neglect, time, and modern fuel challenges inevitably take their toll. Recognizing the symptoms of failure – difficulty starting, stalling, hesitation, leaks, and unusual sounds – allows for timely intervention. Diagnosing the issue correctly involves visual inspection, listening, flow testing, and potentially pressure measurement.

The choice between rebuilding with a quality ethanol-resistant kit and replacing with a new or professionally reconditioned unit depends on the pump's condition, your skill level, budget, and the importance of originality. Rebuilding offers satisfaction and cost savings but requires diligence and the right kit. Replacement provides convenience and warranty-backed assurance.

Whichever path you choose, understanding the SU pump's operation gives you the power to maintain it effectively. Regular inspections, the essential installation of an inlet fuel filter, periodic fuel line replacement, careful attention to ethanol fuel concerns, and consistent vehicle operation are the cornerstones of longevity. With proper care and maintenance, your SU fuel pump will continue its faithful "tick-tick-tick" for thousands of miles more, ensuring your cherished classic car delivers the driving experience it was engineered for. Understanding this key component empowers you to keep the adventure alive.