Before you go any further – do you have a Japanese or a European Engine?

How the impeller works in a water pump

Let’s take a moment to think about how that little impeller drives cooling water around our engines. The impeller is the rotating component of a centrifugal pump. The rotational motion of the drive-shaft is used, often via a belt or a direct link, to rotate the impeller in the fixed housing. As the fins of the impeller rotate they are forced to change shape. Near the input pipe this change in shape creates a suction as the cavity between adjacent fins grows in size. This suction draws fluid in to the cavity. As the impeller continues to rotate, the fluid is carried around in the cavity until, close to the output, the cavity is squeezed and the fluid escapes through the output pipe.

Obviously, the constant pressurised environment, the salt water and repeated deforming of the rubber impeller makes this a part prone to failure. Add to that the nature of a failure in this cooling system; should the cooling water supply fail, the engine block will continue to run but will soon overheat. Often, the first warning that something is wrong is the over-heating alarm. This is why so many skippers hang over the stern just after starting the engine – they are performing a visual check that the cooling water is pumping.

Type 1: Fitted 1980 – 1996

Engine models:

• 2GM
• 2GM20
• 3GM
• 3GMD
• 3GM20
• 3GM30

All the above engines are raw water cooled, fully Japanese assembled and have no ‘E’ in the serial number

The Yanmar part number for the raw water pump assembly is: 728270-42002 (previous part number: 128270-42000 & 128270-42001) and it looks like this:

Note the 45 degree removable inlet and outlet elbows. It uses a gasket to seal the cover and has a replaceable wedge. It takes impeller part number: 104211-42071 (Previously 104211-42070) that looks like this (Note the cross pin):

Type 2: Fitted 1980 – 1996

Engine models:

• 2GMF
• 2GM20F
• 3GMF
• 3GM30F

All the above engines are heat exchanger cooled, fully Japanese assembled and have no ‘E’ in the serial number

The Yanmar part number for the raw water pump assembly is: 721575-42702 (previous part number: 721575-42701) and it looks like this:

Note the straight, removable inlet and outlet connections. It uses a gasket to seal the cover and has a replaceable wedge.
It takes impeller part number: 124223-42092 (Previously 124223-42091, 124223-42090). See the image below. Note the key way and the fact that it has no crosspin.

Type 3: Fitted 1996-2004

Engine models:

• 2GM20-YEU
• 3GM30-YEU

All the above engines are raw water cooled, partly European assembled and do have an ‘E’ in the serial number
The Yanmar part number for the raw water pump assembly is: 128377-42500 and it looks like this:

Note the non-removable inlet and outlet connections. An O-ring is used to seal the cover and it does not a have a replaceable wedge.
It takes impeller part number: 128990-42570 (Previously 128990-42200, X08810B) that looks like this. Note that the impeller has no key way for rotation but has a cross pin instead.

Type 4: Fitted 1996-2004

Engine models:

• 2GM20F-YEU
• 3GM30F-YEU

All the above engines are heat exchanger cooled, partly European assembled and do have an ‘E’ in the serial number

The Yanmar part number for the raw water pump assembly is: 128397-42500 and it looks like this:

Note the non-removable inlet and outlet connections. An O-ring is used to seal the cover and it does not a have a replaceable wedge.
It takes impeller part number: 128990-42570 (Previously 128990-42200, X08810B) that looks like this (Note the cross pin):

Note: The YEU models don’t actually have ‘YEU’ on the serial number plate but instead have an ‘E’ at the beginning of the serial number.

All 4 pumps have been carefully matched to their respective engines by Yanmar. Over-cooling or under-cooling can occur if an incorrect pump is used.

Top Tip

You can easily remove the impeller from a water pump with a pair of water pump pliers like shown below.