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Old 17-02-2024, 02:00   #1
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Coolant Flush - Brown rather than Clear

Greatly appreciate advise on my flush process and the colour to the water I am seeing coming out.

I have twin Yanmar 4JH57s with about 1700 hours on each. I am due a coolant change and am moving from the green Volvo Penta coolant to red Rotella ELC as this is recommended.

I have followed the process below and on the 3rd flush the water coming out is no longer green, but rather than being fairly clear as I expected, it is brownish in colour (see pictures). Any advice on what might cause this and what if anything should be done about this would be greatly appreciated. This is the first time the coolant has been fully flushed, as previous changes were just a drain and replace.

PROCEDURE (thanks to Mistroma on YBW)
1) Start with a cold engine:
2) Empty out header tank (it slides out of holder on my Jeanneau, so easy to tip it out and put back with lid on loosely
3) Start to drain via port side fitting, then open filler cap and leave to one side
4) When flow stops, transfer hose to starboard fitting and then drain from that point
5) Close both drains, fill with water, start engine and run for about 5 minutes whilst topping up with water
6) Replace cap and then run engine up to temperature
7) Stop engine and allow to cool completely (and I do mean completely)
8) Repeat steps 3-6 until water looks free of old coolant. I think that I repeated this at least 2 times.
9) Final coolant drain by repeating steps 3 & 4 (coolant should be clear) and close drain points
10) Fill with fresh coolant, then start engine and top up with coolant whilst engine runs for 5 minutes
11) Replace cap, tighten and run for 15-20 minutes checking drain points for leaks
12) Stop engine and top up expansion tank to bottom level
13) Leave engine to cool completely before lifting lid clear of expansion tank (stops contents syphoning out)
14) Release filler cap slowly and add more coolant if required. Replace cap and expansion tank lid.
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Old 17-02-2024, 05:07   #2
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Re: Coolant Flush - Brown rather than Clear

I believe what you are seeing is sludge being flushed out. I would repeat the flushing a couple more times to see if the water becomes clearer.

There are a few common causes of sludge.
1. Mixing of incompatible coolant types.
2. Oil contamination of coolant from a head gasket leak.
3. Corrosion from imbalanced coolant chemistry.

If it were me, I would do more flushing to clear out remaining sludge, complete the flush procedure, and then keep a close eye on the coolant over the next months, especially watching for oil contamination. Keep an eye on your oil, too. Hopefully, the sludge was just some corrosion, not an oil leak.
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Old 17-02-2024, 05:25   #3
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Re: Coolant Flush - Brown rather than Clear

Yes, keep flushing, get all that crud out

After draining, I leave the lowest drain open with a clean glass jar beneath and add some water at the coolant screw top and see if it comes out clean. If not, I keep doing that before filling with water and running.

But after two flushes you are already beyond this being helpful so just keep doing what you did.
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Old 17-02-2024, 06:56   #4
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Re: Coolant Flush - Brown rather than Clear

As Joe indicated, mixing of mismatched coolants can cause the additives to precipitate, out of the solution, and form sludge, or slime.
Sometimes, when ‘Dex-Cool’ was used, only a chemical flush will remove the gel, or sludge [a silicate precipitant].
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Old 17-02-2024, 07:28   #5
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Re: Coolant Flush - Brown rather than Clear

I would be good to check the pH. If it is less than 8, that indicates excessive rusting, which is caused by incompatibility. It is an oxidizing condition, generally...
  • Air (oxygen) getting in the coolant. Bad head gasket, no or incorrectly functioning expansion tank system, or other air leak on the suction side of the water pump.
  • Wiring error. Anything incorrectly wired in the engine room can be a problem. on cars, replacement fans and collision repair are a common culprits.
  • Salt. A seawater leak above about 200 ppm will cause that. This is why marine engine manufacturers recommend annual changes, even with "long life" coolants. The heat exchanger is a common culprit.
But if you have flushed the coolant and discarded the old it is too late to check.

There is very little silicate level in Dexcool. Originally there was none.


(35 years in the engine coolant formulations business, including troubleshooting. One long-life formulation patent.)
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Old 17-02-2024, 12:18   #6
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Re: Coolant Flush - Brown rather than Clear

Thank you all for the amazing replies, the brown colour is definitely caused by very small particles in the water, leave it for a while and they all settle. The more I flush the less there are so I keep flushing. I have got the old coolant so will see if I can find some way of checking the PH. Both engines are identical so I am erring away from a leak or wiring issue as it seems unlikely (but not implausable) that both would have the same issue.
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Old 17-02-2024, 12:49   #7
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Re: Coolant Flush - Brown rather than Clear

Quote:
Originally Posted by thinwater View Post
... There is very little silicate level in Dexcool. Originally there was none...
Thanks for sharing your expertise!
If not silicate precipitate, what causes the gel/goop, often found /w Dexcool?
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Old 17-02-2024, 13:21   #8
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Re: Coolant Flush - Brown rather than Clear

Quote:
Originally Posted by GordMay View Post
Thanks for sharing your expertise!
If not silicate precipitate, what causes the gel/goop, often found /w Dexcool?
I've seen it when a Dexcool type OAT coolant gets mixed with HOAT or IAT type coolants. I'm not sure what the gel is actually made of, but some reaction between the incompatible coolants causes it to form.
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Old 17-02-2024, 19:21   #9
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Re: Coolant Flush - Brown rather than Clear

Caterpillar offers 2 flavours of coolant passage cleaners, one that works in 2 hours, the other in about 200 hours.
Either way multiple rinses and a distilled water and coolant blend fill is the order of the day, once things are clean.

Perhaps Yanmar offers something similar.
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Old 22-02-2024, 08:07   #10
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Re: Coolant Flush - Brown rather than Clear

I have now finished the flushing and added the new coolant.
After 6 flushes I was still getting gunk out on one side so I ended up flushing through using a hose with both engine drains open. I also took both sides of the hot water loop off from the engine and run the hose in one side with the other to a big bucket. This was by far the most effective way to get the majority of the gunk out quickly. If I was to start from scratch I would definitely take this approach before doing the flush process in my first post above.

At various stages throughout the process I collected a sample of the flushed water, interestingly the colour of the gunk itself becomes lighter the more I flushed. This gunk is not at all gritty and is hard to feel anything when running fingers over it.

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Old 26-02-2024, 00:01   #11
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Re: Coolant Flush - Brown rather than Clear

Quote:
Originally Posted by thinwater View Post
I would be good to check the pH. If it is less than 8, that indicates excessive rusting, which is caused by incompatibility. It is an oxidizing condition, generally...
  • Air (oxygen) getting in the coolant. Bad head gasket, no or incorrectly functioning expansion tank system, or other air leak on the suction side of the water pump.
  • Wiring error. Anything incorrectly wired in the engine room can be a problem. on cars, replacement fans and collision repair are a common culprits.
  • Salt. A seawater leak above about 200 ppm will cause that. This is why marine engine manufacturers recommend annual changes, even with "long life" coolants. The heat exchanger is a common culprit.
But if you have flushed the coolant and discarded the old it is too late to check.

There is very little silicate level in Dexcool. Originally there was none.


(35 years in the engine coolant formulations business, including troubleshooting. One long-life formulation patent.)
Thanks for your help. Any thoughts on the different colours of the stuff coming out with the flushing in my post below? It doesn't feel gritty so it doesn't seem like rust, but I welcome your expert view.
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