Radiator Flush

The friendliest place on the web for anyone who enjoys boating.
If you have answers, please help by responding to the unanswered posts.


Senior Member
Feb 20, 2018
United States
Vessel Make
2007 Henriques 35 Express
Doing full cooler maintenance on my boat this winter.

I figure ill flush the freshwater side of the aftercooler while im at it.

Anyone have a system they like for a recommendation?
If you have kept up the FW side of the cooling system, changed antifreeze with a flush every 2 or 3vyears , the auto store can of cleanser will usually work.

If its an unknown time the 2 step, cleanser then neutralizer , as found in truck parts houses is required.

Either way flush multiple times with FW , then use antifreeze and Distilled water.

50/50 if you boat in freezing conditions , 35% antifreeze is better if you do not as antifreeze only has 5/8 the heat carrying ability as water.

If the engine is modern and requires antifreeze with SCA , the 2 part flush works best.
Last edited:
If the engine is a Yanmar be carefull and ask Yanmar what to use.
Some of their engines have somewhat sensitive, more so than many other engines, cooling systems due to the metals used in the heat exchanger and manifolds.

Ask about the antifreeze also for the same reason.

Failure to ask and use what is recommended could cost you a lot of $ and headaches.

You can go elsewhere to find competitive products but you need to know what the requirements are.
It is a Yanmar

I think the fresh water side is prob ok. Radiator fluid is always clean.
But would like to rinse out any buildup that may be on the inside of the aftercooler bundle.
The problem with car radiator cleaners is disposal of the flush. It's toxic to fish. I use Rydlyme on both fresh and salt sides. It's biodegradable and legal to dump over the side in most places.


  • Marine_Specifications_2015.pdf
    268.6 KB · Views: 42
  • RydlymeMarine_Recreational2.pdf
    1.2 MB · Views: 37
I can vouch for Barnacle Buster in my Yanmar salt water side. I am sure rydlyme will work also. If you want to flush the coolant side use a radiator flush for aluminum radiators. Prestone or the like. If you are changing the coolant every year or two and using Dexcool or the like you are probably fine. No green stuff!
Doing full cooler maintenance on my boat this winter.

I figure ill flush the freshwater side of the aftercooler while im at it.

Anyone have a system they like for a recommendation?

Do you mean the raw water side? If so, Barnacle Buster or Rydlyme. Note that won't preclude complete periodic aftercooler service, which will also address the air side of the aftercoolers.

Our engines are Cummins, and they recommend their own product (Restore) for the fresh water (coolant) side. Probably Yanmar has it's own recommendation.

Broken record - remove the after cooler every few years for cleaning and pressure testing. Much of the AC crud is not on the seawater side.

Not all ACs are compatible with suggested elixirs on the seawater side. As CE states, check with your manufacturer. For us slowpokes, a fouled AC is not normally a big deal. But once into the 60% and above fuel burn and rating curves, getting the AC clean, on both sides, is essential.
Here is the Yanmar service bulletin that addresses approved coolants
I use the prestone ELC just because its easily available, change every year per Yanmars' guidelines. We have twin 6lya-ste engines. Do the generator every year as well.
Wow, changing coolant every year seems overkill to me. However, if that is what Yanmar recommends, who am I to say different! Do you flush the coolant side annually as well?
I certainly second what Sunchaser has to say about servicing the raw water aftercoolers. There are many reasons to take them apart for a complete service every 2-3 years. First, the raw water tubes can foul up and require rodding out or cleaned with a mild acid (tubes only). Second, the air side clogs up with oily contaminants and can ultimately reduce air supply to the engine affecting performance. And third, most brands of these aftercoolers (Cummins for sure) are a mix of dissimilar metals put into an electolyte (salt water) that can cause large corrosion issues (one metal sacrifices itself (anode) for the other metal). Check out sbmar.com (a commercial site that also provides tons of free information for DIYers) for more detailed info. As can be seen in Tony's (site owner) aftercooler disasters, leaving these things alone can get very expensive.
By the way, I have no affiliation with that site or with Tony, just happy I found his site and info.
Flush every year?

If you drain and refill the coolant every year......or two there is no need to flush. Run lots of fresh water through the salt water side at least yearly / end of year before putting to bed. That will go a long way in keeping the coolers clean on the salt water side. This is based on my experience with 16 years owning my Yanmar 4lh. Great engine. Extended Life Coolant is a must. No silicates allowed :)
Last edited:
Easting. I agree that there is no need to flush if you regularly change the coolant, and I also am a big believer in the benefits of freshwater flushing of the salt water cooling system. I do it as often as I can (takes less than 10 minutes) and leave the boat with a freshwater with Salt Away mix in it all winter.

However, even with that preventative regime, I still recommend regular (although regular freshwater flushing coupled with the proper aftercooler reassembly including pressure testing will reduce the required full maintenance frequency) aftercooler maintenance at least every 3-4 years. Freshwater flushing will not clean the air side of the aftercooler and dissimilar metal corrosion can still become an issue.
I agree totally with you, but I was also wondering whether Helm was also flushing the coolant side annually as well as replacing coolant. Just curious :)
I am definitely no expert, but, I simply drain all the old coolant and add new, yearly as recommended by Yanmar in their service bulletin. Couldn't go wrong flushing it as well but just don't see the need.

We run our boat in the California delta, so the raw water side is generally fresh water, except for the occasional trip to the bay or offshore. Still we are due for an aftercooler service, thinking this winter.
Hi Eric,
Thanks for the reply. I agree that flushing the coolant side is not needed if you change as often as you do. You can't go wrong following what the manufacturer suggests, so I totally understand why you are following this routine. Running in freshwater most of the time should GREATLY reduce the build up of crud in the aftercooler tubes as well as the other corrosion issue I was describing. Good idea to service them anyway, as the air side cannot be cleaned any other way. Check out the site I mentioned prior to servicing, even if a "pro" is doing the work. Tony will explain why they should be well greased prior to reassembly.
Good luck, and Merry Christmas!
PS. I am no "expert" either, but I am repeating what I truly believe (after much research) to be good information.

Latest posts

Top Bottom