How to Clean the Raw-Water Strainer and Replace the Impeller on a Marine Diesel

Turn the engine off, remove the key, and let it cool down. Close the raw-water seacock (the handle is usually across the pipe when closed). Put a note on the ignition that says "Seacock closed" so nobody starts the engine and burns up the new impeller.

Put a rag under the strainer. Unscrew the lid, lift out the basket, and dump weeds, shells and grass into the bucket (not the bilge). Rinse the basket and scrub hard deposits off the screen with a brush. Wipe the sealing surface and check the lid gasket or O-ring: if it is flat, cracked or twisted, replace it. A leaky strainer lid can suck air and make the pump lose its prime, which looks like an impeller problem.

Also look at the bowl itself. If it is cracked, crazed or the threads are damaged, replace the strainer. Look especially hard in spring: a strainer that wasn't drained before winter can be split by ice, which is a common insurance claim.

Follow the hose from the strainer to the pump. Take photos of the hoses, the cover plate and the belt (if any) from a couple of angles. Check whether you can reach all the cover screws. If you cannot, plan to remove the pump: undo the hose clamps, then the bolts that hold it to the engine (and loosen the belt on belt-driven pumps). Your manual shows how.

Put a rag under the pump to catch water. Remove the cover screws and drop them straight into your cup; they are small and love to disappear into the bilge. If the plate is stuck, slide a thin blade into the joint or tap the edge gently with a plastic or rubber mallet. Pry carefully so you do not cut an O-ring if your pump uses one.

Before you pull the impeller, note which way the vanes are bent. Take a photo. The new one goes in the same way.

Grip the center hub (not the vanes) with an impeller puller or slip-joint pliers and wiggle it out. If it is stuck, use two flat screwdrivers on opposite sides of the hub and lever gently and evenly. Do not scratch the pump body. Some impellers are held by a screw or a snap ring in the center; remove that first.

Count the vanes and look closely:

• All vanes present and flexible, no cracks: it was still working, but replace it anyway since you are in there.
• Cracks at the base of the vanes or vanes stuck in a curved shape: it was worn out.
• Vanes torn off or missing: go to step 7 before you put anything back together.
• Burned, melted or glazed: the pump ran dry. Find out why (closed seacock, clogged strainer, air leak) before you run the engine again.

Broken pieces travel downstream and block the cooling water. Check the pump outlet and the hose leading away from it. On freshwater-cooled engines, the usual trap is the inlet end of the heat exchanger (or an oil or gear cooler on some engines). Many have an end cap you can remove to look inside. Account for every missing piece. If you cannot find them or cannot reach the heat exchanger, have a mechanic do it before you rely on the engine.

Wipe out the inside of the pump. Light scratches are normal. Deep grooves, heavy corrosion or a worn cam (the raised part inside that bends the vanes) mean the pump needs parts or replacement. Look at the inside face of the cover plate: if it is worn in a ring, many plates can be flipped over or replaced. Scrape off every bit of old gasket.

Look for a drip from the small weep hole under the pump. Water dripping there means the shaft seal is leaking. On some pumps, a leaking seal can let water reach the engine oil, so have it rebuilt soon.

Coat the impeller and the inside of the pump with the lubricant the pump or impeller maker recommends. Avoid petroleum grease or oil unless your pump maker approves it, since it can soften some rubber.

Bend the vanes the same way as the old ones (your photo), line up the drive slot, splines or pin with the shaft, and push it in with a twisting motion in the direction of rotation. A cable tie wrapped around the vanes can help squeeze them on stiff impellers; cut it off as the impeller slides in.

Always use a new gasket or O-ring. An old one will probably leak, and an air leak at the cover can stop the pump from priming. Line up the screw holes, then tighten the screws in a criss-cross pattern a little at a time so the cover sits flat. Snug is enough; do not overtighten small screws in a soft metal or plastic pump.

Bolt the pump back on, reconnect the hoses, and tighten the clamps. On belt-driven pumps, set the belt tension as your manual describes. Check that nothing touches the belt.

Open the seacock and remove your note. Start the engine and look at the exhaust right away: cooling water should spit out with the exhaust within several seconds. If it does not, shut down and check that the seacock is open, the strainer lid is tight, and the impeller is turning (a broken drive pin or loose belt can spin without pumping).

Let it idle for several minutes and check for drips at the cover, hoses and strainer lid. Watch the temperature gauge.

If the temperature still creeps up at idle and gets worse as you add RPM, something is restricting the raw-water flow. Westerbeke warns that judging flow by eye at the exhaust is unreliable (a weak flow can look fine); a mechanic can measure it. Its usual suspects, after the strainer and impeller: a dirty heat exchanger, a clogged water-injection fitting on the exhaust elbow, and an engine overloaded by a fouled bottom or prop.

Record the date and engine hours in BoatMend. Keep a spare impeller, gasket and the cover screws aboard. The old impeller, if it was still intact, can be cleaned and carried as an emergency spare.