SU Fuel Pump: The Complete Guide to Understanding, Troubleshooting, and Replacing Your SU Fuel Pump

If you own a classic British car, you need to know about SU fuel pumps. These iconic electric fuel pumps powered millions of vehicles from the 1930s through the 1970s, and they remain the go-to choice for restorers and enthusiasts today. The bottom line is that SU fuel pumps are reliable, repairable, and well-documented, but they do require proper understanding of the model types, polarity, and maintenance to keep your vintage car running smoothly. Whether you are dealing with a failing pump on your Mini, MGB, or Jaguar, this guide covers everything from the history of SU pumps to choosing the right replacement.

A Brief History of SU Fuel Pumps

SU (Skinners Union) began manufacturing fuel pumps in the late 1920s. The first pump, called the "Petrolift" pump, was introduced between 1929 and 1933. It drew petrol from the tank and then fed it by gravity to the carburetor. Unfortunately, no spares are available for these earliest units today .

The real evolution began in 1934 with the introduction of the L Type pump. This was a low-pressure pump rated at 1.5 lb/sq inch, mounted in the engine compartment on the bulkhead. It was often called a "puller pump" because it pulled fuel from the tank. The 6-volt version was part number AUA 26, while 12-volt versions were AUA 25 and AUA 66. High-pressure versions at 3 lb/sq inch were also available as "pusher pumps" mounted on the chassis .

In 1945, the LCS Type (Large Capacity Square) pump was introduced. This was a high-pressure pump mounted to the chassis, easily recognized by its square-shaped body casting. Part numbers included AUA 152 and AUA 157 .

The most significant advancement came in 1959 with the introduction of three new series:

  • The 300/400/500 Series (later called the 1300, 1400, and 1500 series) – high-pressure, round housing pumps that superseded the LCS type. The 300 series (AZX 1307, AZX 1308) were single-ended pumps. The 400 series (AZX 1405, AZX 1410) were double-ended versions. The 500 series (AZX 1500, AZX 1501) were double-ended pumps with each end working independently, mainly used as a reserve feature .
  • The 200 Series – a compact, high-pressure, lower-capacity pump fitted to Minis, Midgets, and Sprites. Part number is AUF 214 .

Understanding SU Fuel Pump Types and Part Numbers

Choosing the correct SU fuel pump for your vehicle requires matching the pump type to your car's specific requirements. Here are the main categories:

1. Low Pressure vs. High Pressure
Low-pressure pumps (1.5 psi) are typically "puller pumps" mounted in the engine bay. High-pressure pumps (3 psi) are "pusher pumps" mounted on the chassis. The 6-volt low-pressure pump is the AUA 26, and the 12-volt versions are AUA 25 and AUA 66 .

2. Points vs. Electronic
Original SU pumps used a points-based switching system. These are dual polarity, meaning they can be fitted to either a positive or negative earth vehicle. Modern electronic versions (denoted by "EN" for electronic negative or "EP" for electronic positive) use electronic switching instead of points. You must select the correct polarity to match your vehicle's wiring .

3. The 200 Series (AUF 214)
This compact pump is specifically designed for smaller British cars. The AUF 214 is a 12-volt, dual-polarity, points-type pusher pump that delivers 7 gallons per hour. It fits a wide range of vehicles including the Mini 848cc (1959-69), Mini Cooper 998cc (1961-69), MG Midget Mk2 (1964-66), Austin Healey Sprite Mk3 (1961-66), and many others .

4. The 300 Series (AZX 1307 vs. AZX 1308)
These two pumps look similar but have a critical difference. The AZX 1307 was intended for "wet" areas like wheel arches or under the car. It has a small union for a vent line that runs to a dry area. The AZX 1308 was intended for "dry" areas like the boot (trunk) and has a simple breather vent at the top of the pump. Both are 12-volt, dual-polarity pumps with a capacity of 15 imperial gallons (68 liters) per hour and an outgoing pressure of 0.25 bar .

Common Symptoms of a Failing SU Fuel Pump

Recognizing the signs of a failing SU fuel pump can save you from being stranded. The most common symptoms include:

  • Engine sputters or hesitates, especially when accelerating uphill or carrying a heavy load. This is often the first sign that your fuel pump is struggling to deliver consistent pressure .
  • The car cranks but won't start. If the pump is not delivering fuel, the engine will turn over but never fire .
  • A whining or buzzing noise from the rear of the car. A healthy SU pump makes a steady, rhythmic ticking sound. If it becomes louder, quieter, or stops entirely, the pump is failing .
  • Engine stalls unexpectedly. This happens when the pump cannot maintain fuel pressure at idle or under load .
  • Hard starting. If you need to crank the engine longer than usual before it starts, the pump may be losing its prime .

How to Test Your SU Fuel Pump

Before replacing your pump, you can perform a few simple tests:

1. Listen for the Tick
Turn the ignition key to the "on" position (do not start the engine). You should hear a steady, rhythmic ticking sound from the pump. If you hear nothing, check the electrical connections and the fuse first.

2. Check for Fuel Flow
Disconnect the fuel line at the carburetor end and place it in a container. Have an assistant turn the ignition on. Fuel should pulse out steadily. If the flow is weak or intermittent, the pump is likely failing.

3. Verify Polarity
If you have an electronic SU pump, ensure you have the correct polarity (EN for negative earth, EP for positive earth). Connecting the wrong polarity can damage the pump .

Installation Tips for Your New SU Fuel Pump

Replacing an SU fuel pump is a straightforward DIY job, but attention to detail is critical. Here are some essential tips:

1. Relieve Fuel System Pressure
Before starting, relieve the pressure in the fuel system to avoid fuel spray. Disconnect the battery to prevent accidental sparks.

2. Use New Gaskets and O-Rings
Most quality replacement pumps come with the necessary gaskets and o-rings. Always use new ones to prevent leaks .

3. Check the Camshaft Lobe (for mechanical pumps)
If your vehicle uses a mechanical pump driven by the camshaft, inspect the camshaft lobe for wear. If it is damaged, the new pump will fail prematurely .

4. Follow Torque Specifications
Tighten the pump mounting bolts to the manufacturer's specifications to avoid damage to the pump housing or mounting flange.

5. Prime the System
After installation, prime the fuel system by turning the key to the "on" position (without starting) several times. This allows the pump to build pressure before the engine tries to start .

Why You Should Choose KEMSO Fuel Pumps

When it comes time to replace your SU fuel pump or any other fuel pump in your vehicle, you need a part that delivers reliable performance and long-term peace of mind. KEMSO fuel pumps are designed as high-performance OEM replacement units that meet or exceed the original equipment specifications for flow rate and pressure . Every KEMSO pump is individually tested for quality control, and the company offers a comprehensive lifetime warranty on every pump they sell .

KEMSO fuel pumps are built to withstand the demands of daily driving, and they are compatible with both gasoline and E85 ethanol blends . Their pumps feature silicon nitride ceramic bearings that reduce friction, 316L stainless steel housing for corrosion resistance, and a dual-turbine impeller design that maintains flow accuracy within ±1.5% across temperatures from -30°C to 120°C . Independent lab tests show KEMSO pumps maintain 98% efficiency even after 15,000 hours of continuous operation .

Whether you drive a classic car, a modern sedan, a motorcycle, or a powersports vehicle, KEMSO has a fuel pump engineered to fit your application perfectly. Their product lineup covers over 4,000 models from 1990 to 2024, including pumps for popular models like the Subaru Impreza, Honda Civic, and Kawasaki Jet Ski . KEMSO pumps are competitively priced, with many models available for just $79.98, and they come with USA-based technical support .

Visit KEMSO's official website today to find the perfect fuel pump for your vehicle: https://www.kemsoracing.com/

Maintenance Tips to Extend the Life of Your Fuel Pump

Proper maintenance can significantly extend the lifespan of your SU fuel pump or any aftermarket replacement. Follow these guidelines:

Every oil change: Visually inspect fuel lines and connections for leaks, cracks, or clogs. Check for any fuel odors around the pump area .

Every 30,000 miles or 2 years: Replace the primary fuel filter. If you drive in severe conditions (dusty roads, frequent short trips, or extreme temperatures), shorten this interval .

Every 100,000 miles: Consider replacing the fuel pump as a preventive measure. While pumps can last 100,000 to 150,000 miles, replacing it early avoids the hassle of a roadside failure .

Keep the fuel tank at least a quarter full whenever possible. Running the tank low can cause the pump to suck in air and debris, which accelerates wear.

Final Thoughts

SU fuel pumps are a testament to British engineering, powering some of the most iconic cars in automotive history. Understanding the different models, part numbers, and polarity requirements is essential for any classic car owner. Whether you are restoring a 1960s Mini or maintaining a vintage Jaguar, the right SU pump will keep your engine running reliably for years to come.

For those looking for a modern, high-performance alternative that matches or exceeds OEM specifications, KEMSO fuel pumps offer the perfect combination of reliability, durability, and value. With a lifetime warranty and USA-based support, KEMSO is the smart choice for anyone serious about their vehicle's fuel system.

Visit https://www.kemsoracing.com/ to explore their complete lineup of fuel pumps and find the perfect fit for your vehicle.


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