The SU electric fuel pump is one of the most dependable pieces of engineering on a classic car — but like any component that has been in service for decades, it has its weak points. Understanding how the pump works makes it much easier to diagnose problems before they leave you stranded.
How the SU Electric Fuel Pump Works
The SU pump is an electromagnetic diaphragm pump. At its heart is a solenoid coil surrounding a soft iron armature. When power is applied, the solenoid pulls the armature and with it a rubber diaphragm, creating a partial vacuum in the pump chamber. This draws fuel in through a one-way inlet valve. When the armature reaches the end of its travel, a set of contact points opens, cutting power to the solenoid. A return spring then pushes the diaphragm back, pressurising the chamber and forcing fuel out through a one-way outlet valve. The points close again, the cycle restarts, and you get the familiar rhythmic clicking of a healthy SU pump.
The genius of the design is its self-regulating nature. As the carburettor float chambers fill and the fuel system reaches equilibrium, the pump slows its cycling — eventually sitting almost stationary with the diaphragm held back against fuel pressure. When demand increases, it picks up the pace again automatically.
Common Failure Modes
Worn or Dirty Contact Points
The tungsten contact points are the most common source of trouble. Over time they pit, corrode and accumulate deposits from arcing. Dirty points cause erratic operation, slow cycling or complete failure. On older pumps this was addressed by periodic cleaning with fine emery paper — modern electronic conversions eliminate the problem entirely.
Perished Diaphragm
The rubber diaphragm hardens and cracks with age, particularly on pumps that have sat unused for long periods or been exposed to modern ethanol-blend fuels. A failed diaphragm means loss of suction and delivery pressure — the pump may click but deliver little or no fuel.
Stuck or Worn Valves
The inlet and outlet valves are small spring-loaded discs. If they stick open, the pump loses its ability to build pressure. If debris gets trapped under a valve, the same result follows. Valve failure often causes the pump to cycle continuously without building pressure.
Worn Rocker Mechanism
The rocker and pivot that operate the contact points wear over time, causing the points to float rather than make clean contact. The result is sluggish or intermittent operation.
Seized Armature
Corrosion or varnish deposits from old fuel can cause the armature to stick in its housing. The pump will typically be completely silent — no clicking at all — with power applied.
Rebuild or Replace?
Original SU pumps are worth rebuilding. The body castings are robust and the design has been proven over millions of miles. A properly rebuilt pump with new diaphragm, valves, points and seals will typically outlast a cheap pattern replacement and will perform to the original specification. Where the body is cracked or the armature housing is badly corroded, replacement may be the better option — but these cases are the exception rather than the rule.
If your SU fuel pump is showing signs of wear, our professional pump rebuild service will restore it to full working order using genuine specification parts, complete with a 2-year guarantee on all fitted components.