How do I troubleshoot an Onan 8 kW generator that isn't pumping cooling water?

Stop the generator if seawater flow has disappeared, then check the correct discharge outlet, inlet seacock and strainer before opening the seawater pump. Start by looking for a blocked intake, suction-side air leak or damaged impeller; repeated running without water can damage the pump and wet exhaust.

Confirm the discharge outlet

Establish whether water normally leaves with the exhaust or through a separate outlet. Some installations have an exhaust-water separator, so a relatively dry exhaust outlet can be normal: cooling water discharges elsewhere, sometimes below the waterline.

Read the generator identification plate and record its full model, specification and serial number. Onan's 8 kW rating alone does not identify the pump, impeller or drive arrangement. These vary between models and production specifications.

The generator has two cooling circuits. The seawater pump draws outside water through the cooling system and towards the exhaust; the engine's circulating pump moves coolant around the closed circuit. Low coolant can cause overheating, but topping it up will not restore missing seawater discharge.

Before dismantling, disable automatic starting and isolate the starting supply. Close the seacock before opening seawater plumbing, and let hot components cool.

Check the seawater supply

Work from the hull inlet towards the pump:

1. Check the seacock. Confirm it is fully open and that its handle operates the valve. Weed, plastic bags and marine growth can obstruct the outside intake.
2. Clean the strainer. Close the seacock, remove the basket and clear weed, shells and debris. Inspect the housing and lid seal, clean the sealing faces and refit the lid evenly.
3. Inspect the suction hose. Look for kinks, soft sections, loose clamps and internal delamination. A hose can collapse under suction even when its outside looks sound.
4. Check for air entry. A cracked strainer lid, flattened O-ring or loose hose connection can admit air without leaking water into the bilge.

If the fault began immediately after strainer cleaning or winter servicing, recheck the lid seal and valve positions first. An empty strainer or persistent bubbles during a brief test points towards lost prime, restricted supply or air entry, though the strainer's position affects what you can see.

If you need to check inlet flow with a hose disconnected below the waterline, have the correct tapered bung ready and control the opening with the seacock. Ask a marine engineer if access is poor.

Inspect the impeller and drive

With the seacock closed and starting disabled, remove the seawater-pump cover. Photograph the arrangement, then inspect the rubber impeller for missing vanes, cracks, permanently folded blades or a hub that has separated from the rubber.