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Old 04-07-2020, 01:04   #1
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Rust in Coolant Reservoir - Mercruiser

I'm looking at buying a boat. It has an 8.1 L Mercruiser (two of them) on it showing 2100 hours. I have not gone to survey yet. One of the pictures of the boat shows the coolant reservoir on both engines is full of rust. I know on diesels this is bad news. Is it the same situation on an Inboard? Is this for certain a cracked water jacket or is there a less sinister potential cause that could be easily rectified? Should I slowly back away from this boat or run screaming from the building? Crappy picture attached. Thanks.
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Old 05-07-2020, 05:47   #2
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Re: Rust in Coolant Reservoir - Mercruiser

The best case is that the engines have not been properly maintained, including properly draining, flushing and replacing the antifreeze.

Assuming no cooling jacket leak, then one big effect from that corrosion is decreased cooling system efficiency. My experience is that those liquid cleaners have negligible effect on improving the passages.

The engine disassembly, including freeze plug removal, followed by detailed scraping of internal raw water passages is the best / only way to restore proper cooling efficiency.

The bigger point is if this maintenance aspect was obviously neglected, what does that portend for all the other maintenance on the boat?
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Old 05-07-2020, 05:58   #3
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Re: Rust in Coolant Reservoir - Mercruiser

It's quite possible that someone ran them with fresh water and no coolant in the system at some point. If everything else checks out, I'd do a combustion gas check on the cooling system, then flush thoroughly and refill with the proper coolant mix. If the only issue is the coolant looking bad, chances are it'll clean up fine with no issues.

Definitely worth confirming that it's not a symptom of another issue. But being both engines, there's a good chance it's just a bit of internal rust from running water without the rust inhibitors coolant has.
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Old 05-07-2020, 09:04   #4
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Re: Rust in Coolant Reservoir - Mercruiser

Thank you for the replies much appreciated. I'll look into a survey on the rest of the mechanicals. Appreciate it!
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Old 05-07-2020, 09:23   #5
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Re: Rust in Coolant Reservoir - Mercruiser

I’m pretty sure that those exhaust manifolds don’t last long either, expect to have replace them if you buy the boat, if not they often result in buying a new motor.
I’ve had good luck with flushing and flushing and flushing until you only get clear water, then the 50/50 mix, run a year and flush again until clear water and often that’s the end of it.
Install the Prestone flush kit in the heater hose and flush until clean, run the motor flushing still, then close of drains and run until hot, drain and flush, then add antifreeze.
Sounds like a lot of work but it’s not really.
When you flushing leave the radiator cap off and all drains open, and just sit back and let the bilge pump do it’s thing.
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Old 05-07-2020, 10:16   #6
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Re: Rust in Coolant Reservoir - Mercruiser

Assuming they didn't just use tap water for years, the most common causes of rusty coolant are:
  • Blown head gasket (oxygen in coolant).
  • Leaking cooling water pump seal (sucking air into coolant).
  • Stray current (electrolysis) from a bad ground somewhere, generally around the cooler (because it is insulated from the engine and the pumping creates a current). Check anode for certain.
  • Seawater in the coolant. Pressure test the cooler.
For it to get that rusty it usually requires a specific aggravating cause, not just poor AF or a long change interval. This is ALWAYS the case in cars--if the coolant is rusty, there is a mechanical cause. In fact, there should NOT be enough oxygen in the engine to create orange rust (it should be black/brown ferrous rust, not orange ferric rust).
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