SU Fuel Pump Problems on Classic Cars: Causes and Checks
Written by Petro Olekhnovych, mobile mechanic in Aylesham, Kent
Most SU fuel pump problems come down to a few things: dirty or worn contact points, a tired or ethanol-damaged diaphragm, a blocked filter, a poor earth or an air leak. The way the pump ticks tells you a lot about which one it is. Because you are dealing with petrol and electrical contacts in the same place, every check needs to be done with fire safety in mind.
How an SU electric pump works
The SU electric pump is a diaphragm pump worked by an electromagnet. The magnet pulls the diaphragm one way to draw fuel in, and a spring pushes it back to send fuel to the carburettor.
A set of contact points switches the magnet on and off. Each stroke makes the familiar tick.
When the carburettor float chamber is full, back pressure stops the diaphragm completing its stroke. The pump then slows right down and only ticks now and again as fuel is used.
Some classics have a mechanical pump instead, driven from the engine. That type has no points and does not tick at all.
What the ticking is telling you
With the ignition on and the engine off, a healthy pump ticks quickly at first, then slows to the odd tick once the carburettor is full.
Fast ticking that never slows down. The pump is not building pressure, or fuel is escaping somewhere. Likely causes are a split diaphragm, a leaking valve, an air leak on the supply side or an empty tank. It can also be a carburettor needle valve that is not shutting off. Fuel dripping from the float chamber overflow pipes points to that. One MG Magnette owners’ guide suggests that ticking more often than about once every ten seconds, with the engine off, means a leak somewhere.
No ticking at all. Check there is power at the pump first. If there is, the points may be dirty or burnt, the coil may have failed, or the moving parts may be stuck. A tap on the pump body sometimes gets it going again. Treat that as a warning that the pump needs attention, not a fix.
Ticking, but the engine runs out of fuel at speed. The pump may be working but not delivering enough. Suspect a blocked inlet filter, a collapsed supply hose, a weak diaphragm or a partly failed valve. Poor acceleration or a loss of top speed can be the first sign.
If the car will not start at all, our guide to why a classic car won’t start helps you work out whether it is fuel, spark or something else.
Classic car needs some care?
Servicing, recommissioning and mechanical restoration for older cars across Kent.
WhatsApp for a free quote Call 07563 350335
Labour £30 to £50 per hour depending on the job. Send your reg number and we will reply with a price before any work starts.
Points, earths and wiring
The points are the most common cause of an unreliable pump. They corrode or pit, especially on a car that has stood for a while.
Poor wiring causes similar symptoms. With the ignition on, the voltage at the pump should be close to battery voltage. A drop of more than about one volt suggests resistance in the wiring or connectors.
A bad earth is common. A simple test is to run a temporary wire from the pump body to the battery earth terminal and see if the pump runs better.
Polarity matters too. Some older SU pumps are polarity sensitive, so they can misbehave if the car has been converted from positive to negative earth. Our post on positive earth classic cars explains what to check.
Filters and fuel hoses
SU pumps have a mesh filter on the inlet side. If it clogs with rust or dirt from the tank, the pump struggles to deliver enough fuel. It should be cleaned or renewed whenever the pump is opened.
Check the rest of the supply as well. An old rubber hose can collapse inside and starve the pump, even if it looks fine from outside. Old, cracked fuel pipes are also a fire risk.
The pump should be mounted the right way up. The SU design relies partly on gravity, so it is normally fitted upright with the inlet at the bottom. Check your workshop manual for your model.
E10 petrol and the diaphragm
Standard 95 octane petrol became E10 in Great Britain in September 2021. It contains up to 10% ethanol.
GOV.UK lists classic, cherished and older vehicles as possibly not compatible with E10. It advises owners of vehicles not on its checker to keep using E5 super grade (97+ octane) petrol.
Older pump diaphragms and valve discs were not designed for ethanol. The rubber can swell, soften and eventually crack, which leads to pump failure. Ethanol also absorbs moisture, so fuel left standing over winter can cause extra trouble.
If the pump is rebuilt, use a kit with ethanol-resistant rubber, such as Viton. Our guide to E10 petrol in a classic car covers the wider fuel system.
Testing the pump safely
Fire is the real risk here. The points spark every time they open. If petrol is leaking nearby, that spark can light it.
- Work outdoors or in a well ventilated space, away from flames, cigarettes and anything that sparks.
- Keep the ignition off whenever a fuel pipe is disconnected.
- Disconnect the battery before taking the pump off or opening it.
- Catch any fuel in a proper container and wipe up spills before switching back on.
- Do not switch on to test the pump until you are sure nothing can ignite.
- Keep a suitable fire extinguisher close by.
A flow test is useful. Disconnect the pipe at the carburettor, run it into a container and switch on briefly. The delivery should match the figure in your workshop manual, as it varies between pumps.
Stop and get help if you smell petrol inside the car or boot, or see a leak at the pump. A failed diaphragm can let fuel into the pump body. Do not run the car near a hot exhaust or keep driving with a fuel leak.
How we can help
Fuel supply faults are part of our classic car servicing and repair across Kent. We can come out, find whether the problem is the pump, the wiring, the filter or the carburettor, and tell you what it needs before any work starts. We also change fuel filters as part of a service.
We are mobile, so we come to you in Aylesham, Canterbury, Dover, Deal, Sandwich, Folkestone and the villages around them. Labour is £30-£50 per hour depending on the job, and we agree the price on WhatsApp first. Parts are extra.
Frequently asked questions
Why does my SU pump tick constantly?
It is not building pressure, or fuel is escaping. Common causes are a split diaphragm, a leaking valve, an air leak on the inlet side, a stuck carburettor needle valve or an empty tank.
Can I just tap the pump to get it working?
It may get you going, but it shows the points or moving parts are sticking. Have it checked before it stops for good.
Is E10 bad for an SU pump?
Older diaphragms and valves were not made for ethanol and can fail. GOV.UK advises classic owners to keep using E5 super grade.
Need help? Send a WhatsApp to 07563 350335 with your car’s make and model and we’ll come to you.
Classic car needs some care?
Message us on WhatsApp with your reg and postcode for a price before any work starts.
WhatsApp for a free quote Call 07563 350335
Labour £30 to £50 per hour depending on the job. Send your reg number and we will reply with a price before any work starts.
