What You're Replacing and Why It Matters

The impeller on your Volvo Penta 5.0Gi is a rubber-bladed pump inside the raw water cooling system. It pulls seawater or lake water through the engine block to keep it from overheating. The impeller wears out — usually every two to three years depending on use — and when the blades crack, fray, or separate, your engine loses cooling flow and can overheat in minutes.

Replacing it is a maintenance task you can do yourself with basic tools. The job takes one to two hours and costs between $40 and $80 for the impeller itself, plus a new gasket. You do not need to haul the boat or remove the engine.

Key Takeaways

  • The impeller sits inside the raw water pump housing on the starboard side of the engine block, behind a single cover plate held by four bolts.
  • You must drain the raw water system or accept that water will spill when you open the housing — have towels and a bucket ready.
  • The impeller key is a small metal pin that can fall into the bilge; work over a tray and keep your hand under the housing when you remove it.
  • Volvo Penta part number 21951352 is the correct impeller for the 5.0Gi; order a new gasket (part 3842652) at the same time.
  • The new impeller must be installed with the curved blades facing the direction of rotation, and the housing must be hand-tight only — over-torquing cracks the cover plate.

Locating the Raw Water Pump Housing

On the Volvo Penta 5.0Gi, the raw water pump is mounted on the starboard (right) side of the engine block, roughly midway along its length. Look for a cylindrical aluminum housing about the size of a soda can, with a flat cover plate facing outward. A hose connection labeled "raw water in" enters the bottom of the housing; another labeled "raw water out" exits higher up.

The cover plate is held by four bolts arranged in a square pattern. These bolts are typically 3/8-inch or 10mm and should be snug but not tight — Volvo specs them at around 10 foot-pounds, which is hand-tight with a wrench, not a power tool.

Draining and Opening the Housing

Before you crack the cover plate, decide whether to drain the raw water system. If you have a through-hull seacock (a valve in the hull where water enters), close it. This stops new water from flowing in, but water already in the lines will still spill when you open the housing. Lay towels around the pump and position a bucket underneath.

If you do not have a seacock or prefer to drain completely, locate the raw water intake hose at the through-hull fitting and disconnect it, or open any drain plug on the raw water strainer if your boat has one. Let the system drain into a bucket for a few minutes, then proceed.

Using a wrench or socket, remove the four bolts from the cover plate slowly and evenly — do not unscrew one completely and then move to the next, as this can warp the plate. Once all four are loose, lift the cover plate straight out. Water will drip; this is normal.

Removing the Old Impeller and Key

Inside the housing you will see the impeller: a rubber wheel with curved blades arranged around a central hub. The blades may look cracked, frayed, or partially missing — that is why you are replacing it. Directly behind the impeller hub is a small metal pin called the impeller key. This key is straightforward to lose.

Grip the impeller hub with your fingers and pull straight out toward you. The impeller should come free with gentle pressure. If it is stuck, do not force it — spray a small amount of penetrating oil around the hub and wait five minutes, then try again.

Once the impeller is out, look into the housing for the impeller key. It is a flat metal pin, usually about 1/4 inch wide and 1 inch long, sitting in a slot on the shaft. Reach in carefully and remove it. If you cannot see it, it may have fallen into the bilge — shine a flashlight and feel around the bottom of the housing with a small magnet or your fingers.

Installing the New Impeller

Before you install the new impeller, clean the inside of the housing with a dry cloth to remove any debris or old rubber fragments. Inspect the shaft for corrosion or damage; if it looks pitted or rough, you may need to replace the entire pump assembly, but this is rare.

Insert the impeller key back into its slot on the shaft. The key should sit flush and not protrude. Now take the new impeller and look at the blades — they curve in one direction. The impeller must be installed so the blades curve in the direction the shaft rotates. On the Volvo Penta 5.0Gi, the shaft rotates clockwise when viewed from the front of the engine, so the blade curve should point clockwise as you look at the impeller face-on.

Slide the new impeller onto the shaft, aligning the hub slot with the impeller key. Push it in until it seats fully against the internal shoulder of the housing. It should go in smoothly; if you feel resistance, stop and check that the key is seated correctly.

Sealing and Testing

Remove the old gasket from the cover plate and discard it. Wipe the gasket surface of the cover plate and the housing face with a clean, dry cloth. Position the new gasket on the housing face, aligning the bolt holes.

Place the cover plate over the gasket and insert the four bolts. Tighten them by hand first, then use a wrench to snug them evenly in a crisscross pattern (top-left, bottom-right, top-right, bottom-left). Stop when you feel firm resistance — this is hand-tight. Do not use a power drill or impact wrench; over-torquing will crack the aluminum cover plate.

Reconnect the raw water intake hose if you disconnected it, or open the through-hull seacock if you closed it. Start the engine and let it idle for two minutes while you watch for leaks around the cover plate. A few drops are normal as the gasket seats; steady dripping means the bolts need another quarter-turn. Once the engine reaches operating temperature, shut it down and check again after it cools.

Frequently Asked Questions

What happens if I install the impeller backward?

The engine will still run, but the impeller will not pump water efficiently and the engine will overheat within minutes. You will see the temperature gauge climb and may hear an alarm. Stop when ready, let the engine cool, and remove the cover plate to reinstall the impeller with the blades facing the correct direction.

Can I reuse the old gasket?

No. Gaskets compress when tightened and lose their seal when removed. Reusing an old gasket almost always results in a leak. A new gasket costs $8 to $15 and is worth the money to avoid water in your bilge.

My impeller key is missing and I cannot find it — what do I do?

Order a replacement key (Volvo Penta part 3842652 includes a new gasket and key) or call a marine dealer for the key alone. Without the key, the impeller will spin on the shaft and not pump water. Do not run the engine without it.

How often should I replace the impeller?

Volvo Penta recommends inspection every season and replacement every two to three years, or sooner if you run in sandy or silty water. If you trailer your boat and store it in winter, inspect the impeller each spring before launching.

Do I need to bleed air from the cooling system after I close the housing?

The Volvo Penta 5.0Gi raw water system does not require bleeding. Once you reconnect the intake hose and open the seacock, water will flow through naturally as the engine runs. If you see steam or hear gurgling from the exhaust, stop the engine and check that the intake hose is fully connected and the seacock is fully open.