The in-tank pump normally makes a short, quiet buzz at key-on and a faint hum while running. A loud whine, a rising or rattling note, or a pump that keeps running after the priming seconds with the engine off, means something is wrong. A pump that works hard is often starving: the tank is low or its strainer is blocked. A pump that never stops is being kept on by a stuck relay or a wiring or ECU fault, and it will flatten the battery.
Is it safe to ride?
With care, for short distances, and keep the tank well filled. A starving pump can fail and stop the engine without warning.
Quick checks first
- How much fuel is in the tank? Pumps get louder and hotter on a low tank, because fuel cools them.
- Is the noise only at key-on, or all the time the engine runs?
- With the key on and the engine off, does the pump stop after a few seconds?
- Has the bike had dirty or rusty fuel?
Diagnose it step by step
1Fill the tank to at least half. Is the noise now normal?
- Yes: the pump was running near-dry. Do not run the tank low. The K-Light 125 manual warns that running the pump with little or no fuel can burn it out.
- No: go to step 2.
2Key ON, engine off. Does the pump stop after its priming seconds (K-Light 125: about 3 s)?
- No, it keeps running: the pump is being held on. A stuck relay or a wiring or ECU fault. Turn the key off. Check fuses and relays (swap the relay with an identical one), then read fault codes (K-Light 125: P0629 pump circuit shorted). See Find a wiring fault. Check the battery has not been drained: Test and charge a 12 V battery.
- Yes, it stops normally: go to step 3.
3Has the bike had dirty, rusty or watery fuel, or a long time since the fuel filter was changed?
- Yes: the pump strainer or filter is probably blocked and the pump is straining. Drain the fuel and ask a workshop to clean or replace the in-tank strainer and filter. See Rust or flakes inside the fuel tank.
- No: go to step 4.
4Does the engine also lose power, hesitate, or cut out?
- Yes: the pump may be wearing out, or a pressure fault. A workshop measures fuel pressure (K-Light 125: about 300 kPa; below 270 kPa is a fault). The K-Light 125 manual also notes that a reversed pump connector makes the pump run backwards with no pressure. Check the fuel pump.
- No, it runs well but is noisy: monitor it. Note whether the noise grows. Replace the pump assembly when it starts to affect running.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Low fuel level | Quieter after filling | Keep tank above a quarter |
| Blocked strainer or filter | Dirty fuel history, low pressure | Fuel pump check |
| Stuck relay or wiring fault | Pump runs with key on, engine off | Fuses and relays |
| Worn pump | Loud and low pressure | Workshop replacement |
| Reversed connector after repair | Runs, no pressure | Workshop |