How To Service Your
Diesel Marine Engine!
Part II

Let's continue our diesel marine engine maintenance!

Raw Water Circuit

Most modern diesel marine engines are cooled through a raw water or salt water circuit. The salt water takes the heat away from the coolant that is circulating in a closed circuit. The heat transfer happens in the heat exchanger.

Now, the sea water is pumped up by the sea water pump, which contains the impeller. This is a special kind of paddlewheel that draws het seawater in and pumps it further.

This impeller needs to be checked from time to time. It's function is not a small one. It ensures that the engine is cooled, without (a proper) cooling, we will have damage to our engine.

How do you check? Easy, just look over the side to see if the water is gushing out with strong pulses. This means the seawater pump is working properly. Also look at the temperature gauge in a while.

This is also the reason why we check the waterflow out of the exit when we start the engine. This is basic seamanship!

The danger for the impeller is that something, for example a plastic bag, is blocking the seawater inlet, which causes the impeller to break down. Impellers don't like to spin around without their fluid!

So, when everything goes fine during our cruising season, I replace the impeller just before the new cruising season. That way, you start with a fresh impeller.

impeller housing
impeller removal tool

When you take out the old one, with a special removal tool or just by fiddling it out with 2 screwdrivers, check it. If the blades are bent, then it's time for a new one. If blades are missing, then you have to go looking for the missing pieces. Follow the pipe and open up the heat exchanger. Those pieces could block the small tubes in the heat exchanger wich reduce the cooling of the engine!

Of course, we also clean the sea water filter, which is situated between the seawater inlet and the seawater pump. Close the seacock first!

Cleaning the seawater or raw water filter is necessary every time you see rubbish. Don't delay this task, because it will get back to you!

seawater filter

Coolant Circuit

Like I mentioned before, this circuit is a closed one, so the same coolant fluid is pumped around and around.

The coolant takes the heat from the engine and gives it through to the seawater. That way it is cooled and it can do its function again.

The maintenance we do here is to check if there is enough coolant in the system.

coolant expansion tank

On the outside of our diesel marine engine we find a coolant expansion / recovery tank with min and max marks on it.

When the engine is cold, the level of the coolant should be at or just above the minimum mark.

Add coolant to the recovery tank if necessary.

When the engine is hot, after running at its operation temperature, the coolant level should be at or near the maximum mark.

Now, from time to time, you also need to check the level of the coolant at the pressure cap. Do this only when the engine is cold. 

Opening the pressure cap while the engine is still hot is extremely dangerous as the hot coolant will squirt out of the system.

So, when you open it slowly, you will hear the pressure escaping, you have to see the coolant reaching just under the opening of the overflow tube. If not, fill it up until it does.

Again, only add cold coolant to a cold engine!

coolant pressure cap
inspecting coolant pressure cap

The function of the pressure cap is to put pressure on the coolant. The effect of this is that the coolant will boil at a higher temperature.

If you see that the pressure cap is leaking or not functioning well, replace it by another cap with the same pressure rating. This last one is stamped on the cap.

Continue or go back to diesel marine engine part I

Return from Diesel Marine Engine part II to Boat Maintenance

Return to's Home Page

Like this Page?

New! Comments

Have your say about what you just read! Leave me a comment in the box below.
Share this page:
Enjoy this page? Please pay it forward. Here's how...

Would you prefer to share this page with others by linking to it?

  1. Click on the HTML link code below.
  2. Copy and paste it, adding a note of your own, into your blog, a Web page, forums, a blog comment, your Facebook account, or anywhere that someone would find this page valuable.


Support This Site

Do you enjoy reading and want to help support this site?

You can, by shopping at!

This doesn't cost you anything, but it surely helps supporting this site!

Thank You!

Get the Free

'I Love Yachting'


Enter Your E-mail Address
Enter Your First Name (optional)

Don't worry — your e-mail address is totally secure.
I promise to use it only to send you I Love Yachting!.