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When it comes to diesel engine maintenance, I am a definite newbie. The small amount of info that I have learned has basically been acquired through trial and error. With my time on the Leopard, I had become somewhat familiar with the Yanmar engines that powered it, at least the user-serviceable bits. Frost has an older Perkins 4.154 engine, and I am working hard to develop some familiarity with it too.

In my layman’s words, the cooling system on a marine diesel engine basically works like this: raw (sea) water is brought into the engine room through a below-the-waterline seacock, and after flowing through a strainer, it is pumped into a heat exchanger where it absorbs the engine’s heat from the coolant, later to be discharged overboard.

The thing that moves the water in the pump is a small multi-vaned rubber impeller, a part which can, over time, degrade and break into pieces. If you allow the vanes to break off, they can flow from the pump into the heat exchanger’s core, clogging the small holes and degrading its performance. This is something to be avoided which is why, at least once a year, the impeller should be checked/changed.

Unfortunately, accessing the impeller is not always easy. On the Yanmar engines, the water pump was reasonably easy to get to. On our Perkins, it is decidedly less so. In fact, the only way to access the impeller is to completely remove the water pump from the engine, not something that I was looking forward to doing. Not knowing if/when the impeller had ever been checked though, it was a job that needed to be done, and as we had moved onto the dock yesterday to have some other work done, I figured there was no better opportunity to get on with it.

The raw water pump removed.

I’m not going to say that the job of removing the water pump was all that complicated. I did need to figure out the process as I was going along, and I was fortunate to be able to complete it all without dropping any nuts or washers into the black hole that is our engine bilge.

One unpleasant surprise came when I actually got the old impeller removed. The impeller that was in the pump was actually in pretty good shape but when I went to find a replacement from the 6 spares that I had on the boat, only one of them was the appropriate model (I found 6 spare impellers on the boat, 4 of the same type and 2 other misc. ones. I had assumed the 4 that were the same would fit the engine. I was wrong. Lesson: don’t assume!).

Old (left) and new (right) impellers.

When I finally got the new impeller put in and the water pump reinstalled, all without dropping any pieces, I tested the engine and found that we had great water flow. Unfortunately, the pump was also leaking, forcing me to go through the entire process of removing it again. While something like this normally would have prompted a fair amount of expletives to leave my mouth, I calmly convinced myself that this was all good practice.

I’m pretty convinced that the leak was due to having the wrong gasket. Without anything better, I used some Permatex liquid gasket material to seal it. After installing it once again, that seemed to do the trick in preventing any leaks.

What did I learn from all this? I learned that I definitely do not want to have to do that job while out at sea in some less-than-calm weather! As with most bits of engine maintenance, taking preventative action is always the best.

Note: After finding out that I no longer had any spares, I went to Island Water World yesterday and bought four spare impellers of the appropriate type!


  1. On the job training, I always find, is the best way for me to learn. Went through this exercise last year for the first time, good learning experience. I was also lucky enough to get my hands on the service manual for my engine (25 year old Yanmar 2QM15) which is extremely detailed and comprehensive. I’ve printed out a copy to leave on the boat as a backup to the electronic version. Like you I was a lot more familiar with gas, however pulling up schematics and working diagrams of general diesel engine mechanics and studying the service manual, I feel a lot more confident in my ability to handle issues that might arise engine wise, as well as have a pretty good idea of troubleshooting problems.

    Just a quick question, as getting gaskets for the old engine is difficult and typically need to be ordered from overseas, how is the liquid gasket working out. I have a tube but haven’t used it yet, it is coming up time for the maintenance time for my own engine and was going to give it a go.

    Cheers mate,


  2. 2 suggestions, budget aside:

    1. Purchase a spare water pump with new impeller in place. Then in an emergency you would only need to switch out pumps (gaskets on hand, of course).

    2. Not knowing your specific engine, if there is a hose between the water pump and the heat exchanger, install a good strainer in line between the pump and exchanger in a location that is accessible and easy to view. With routine inspection, you will catch broken vanes before they reach the exchanger and you will get a heads up on when an impeller change will be necessary.
    I set this system up on our genset and it works flawlessly.

  3. Good idea on the extra strainer

  4. … and 4 new gaskets? Looking at the trip you are planning I would purchase an engine overhaul gasket set, if this even exists for your engine. My background is with small block GM V8 gasoline engines.

  5. One thing to keep in mind is that ‘time’ is what kills the impellers typically, so get as many as you will need in a year or two, but don’t overstock. When buying them, there should be a date code on the package and don’t purchase them already sitting on a shelf for 5-10 years. The other thing that really tears them up is running them dry. On old outboard motors, people tend to ‘test run’ them dry, which kills the impeller, without the water for lubrication they get hot fast and dry up. When putting the pump back together, use a little petroleum jelly or oil inside the pump to lubricate the surfaces until it starts drawing water.

  6. As already commented, purchase a spare sea water pump and a rebuild kit. They do leak. When reinserting the impeller, it makes it easier if you wrap the blades with a plastic tie wrap, insert and then cut the tie wrap off.

  7. Old paper charts are nice thick paper (especially B.A. ones). They make great gaskets!

  8. Mike, look into a Speedseal cover for the pump ( It replaces the pump cover with one that has an “O” ring seal and is attached with four knurled thumbscrews. Only two of them need not be completely removed to remove the cover; the other two only need to be loosened.

    On my last engine, a Yanmar 3HMF, the pump cover attached with six tiny screws. One one occasion I had to check the impeller at sea. It was not fun. The Speedseal cover changed everything. My new engine, a Yanmar 3JH4E, has one as well. I wouldn’t be without it.

    Tim Metcalf
    C&C 41 “Insatiable”

  9. I wonder if the other 2 impellers are for the outboard.

  10. Please forgive my comments off topic:

    I just finished “catching up” on reading your blog from the beginning to today’s post (a couple months to do).

    Thank you for sharing your gained knowledge, and the wonderful experiences/ adventures you guys are having. Your posts have re-energized our plans to cut the dock lines and follow in your wake (just not to the colder spots lol). I continue to look forward to your future endeavors and experiences.

    Again, thanks and continued good luck!

  11. On our first boat we had a Volvo with the paper gasket on the water pump. It was very susceptible to any dirt or roughness left on either surface before attaching it (although it did seal most times).

    I would second the recommendation for the speedseal especially since yours is hard to access. We also had a spare pump which we could rebuild at our leisure after swapping it out. Thats also useful when you need to change seals or bearings in the pump 🙂 (hint – get a rebuild kit before heading to Patagonia!)

  12. Regarding dual functionality, I can’t remember where I read this, but is something that I”m going to implement just to be on the safe side…..rigging the intake hose to the water pump with a T connector and a valve. Have extra hose leading into the boat with screen mesh on the end so the water pump for the engine can act as an auxiliary bilge pump should you spring a leak that might overwhelm your primary bilge pump(s). Easy to setup…..

  13. Once, when a pump failed just days before a scheduled cruise, I had to remove the lower unit and replace the impeller, with no time for hauling. I didn’t really have time to haul up the engine. So I did it swimming, at the dock.

    No, not my first choice. PM is better.

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