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Winterization Monsoon 350 - The Thermostat myth revealed?

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I've spent several hours reviewing the many great winterization techniques and tips......... smart bunch of dudes.

The most common recommendation is "when flushing with antifreeze make sure engine is up to temp to ensure thermostat is open allowing block to fill OR remove Tstat completely".......... but are we being mislead?

You've all seen this great video by PPT "Boat Winterization - Indmar Manoon 350" (https://www.youtube.com/watch?v=ezZJq3nn1Z0) but has anyone read its comments regarding the Tstat...

"Mark Jones - The Marine "Open" style cooling systems work on a pressure balance. Therefore if the cooling system is full when the antifreeze is introduced then it would NOT enter the engine block until the thermostat opened. HOWEVER since the system has been drained the pressure is low on the under side of the thermostat and therefore antifreeze WILL circulate throughout the block, pump, and manifolds regardless of the thermostat position until the system fills completely. Read More on our website at https://www.perfprotech.com/blog/articles/warm-manifold-cooling-tips

This makes 100% sense otherwise if the block was pre-drained and the Tstat isn't yet opened............. does the block remain dry until the Tstat gets up to temp? That can't be right........

In my older Indmar 350 I would pre-drain the cold engine of water and immediately flush (flush pro) with antifreeze WITHOUT running up to temp. The block would always be filled with AF.

My goal here fellas is not to mock the two most common recommendations #1 Run up to temp or #2 Remove Tstat, it's to save everyone a little extra time to enjoy a few more beers with the those few remaining sunsets. So if anyone (preferable a qualified Engine Tech) can confirm "open" style cooling system it would be much appreciated!


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Mark Jones - The Marine "Open" style cooling systems work on a pressure balance.

This is something I've wondered about, is an inboards block water jacket pressurized while the engine is running? I know this is done with outboards with an open cooling system so how about the inboard? I've not owned an outboard, I don't know how it's done, but I would guess the outlet is restricted and the impeller just pumps against it.

I do know I had a loose hose clamp on the big recirc hose and water was spray out under significant (5-10psi?) pressure. What I don't know is if that pressure is also in the block???

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For years with I/O's I would drain the block and manifolds and then put the muffs on with antifreeze and feed it in. Never had a problem for 15+ years. I started taking the stat out after hearing horror stories and getting more expensive boats. Not sure it taking the stat out is necessary - but I think it is easier than disconnecting and reconnecting a bunch of hoses - and you know for sure that the block and everything else has been flushed clean of water. The temp in my pole barn gets down to zero or below every winter.

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Looking at diagram - T-Stat open lets hot water out of the engine to be replaced by cool lake water. If engine is empty, the engine will fill first before the water 'overflows' out the risers/exhaust due to the internal passages & flow preferences of the T-stat manifold.

There is slight pressure in the motor, but not significant - I had a leaky knock sensor and got a little spray, but nothing significant. The cooling system is open, so extra pressure exits thru the exhaust risers.

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I'm always baffled by this procedure or more so on the thought that goes into doing it this way. I'm guilty of doing my engine old style but it certainly isn't hard to do or that time consuming. And based on some of the methods and lengths people are going to, to make sure they're safe it just seems a hassle to me. Let alone the mess that's made running the engine with antifreeze coming out the back and trying to catch it.

My method (short version) block only.......

Pull drain plugs (one each side, this is not hard to do and most people are doing this anyway)

Disconnect hose between exhaust manifolds and drain (most people are doing this and this truly is simple and easy to get to)

Remove top hose connection going to the engine water pump, remove both hoses at the thermostat housing going to the exhaust manifolds (this is the hardest part but it's still at the top of the engine right in front of you).

Pour antifreeze into the hose going to the engine water pump until it comes out the thermostat housing. Block fills from the bottom up. No chance of air pockets and you know the block is full. This should take about 3.5 gallons of AF.

Pour the remaining AF equally into each hose going to the exhaust manifolds. This is just to make you feel better as all the water should have drained out when you disconnect the hose connecting the two manifolds. Put your hoses back on.

Done. No buckets, no mess on the hull or in the drive way, you use less AF and you know it's done right.

I guess there's a certain cool factor to the bucket method and if you like doing it that way I certainly think that's fine too. I personally just never saw the advantage of doing it this way unless you were maybe a dealer and were set up with a tank to feed and catch the AF.

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Thanks for your response, I agree the 'old style' is simpler and less messy.

However, as my VLX has a heater core, shower and ETX Cat Cooling the Antifreeze (propylene glycol) flushing method allows me to kill 7 birds with one stone...... all at one time!

  • Pre-drain water (block/hoses/v-drive/heater/manifolds)
  • Attached hose from AF pump to Quick Flush Value (www.quickflushvalve.com)
  • Close seacock
  • Start AF pump and engine
  1. Gets engine up to temp for oil change
  2. Flushes heater core with AF
  3. Flushes shower with AF
  4. Flushes ETX cats with AF
  5. Flushes engine with AF
  6. Get gas stabilizer into system

My only unknown is the max temp for propylene glycol and will heating it degrade the freezing properties?

Here's a great video showing the basic process https://youtu.be/vbr9mHKc8WM.

Lastly #7, I pump the colleted AF into the ballast tanks for extra protection and pump lube.

Cool eh?

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Don't get me wrong I think it's cool just too many moving parts going on for me (engine up to temp, shower being drained/filled with AF, heater lines drained and filled with AF).

For my shower and heater I went the quick disconnect method that was posted on here. I basically use the shower to pump the AF through the heater. As long as this method is working for you, that's great.

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I drain all hoses and reconnect, also drain block.

I toss either a sump pump or a bilge pump into a 5 gallon bucket. Fill it with Pinkie. Out side the boat on the ground.

I hook up a piece of garden hose and an old style hose spritzer end that has a male thread on the end. I screw on a male garden hose end onto that. Sounds complicated but really isn't. The hose spritzer is my shut off valve.

I disconnect the heater hose connection on the manifold and hook it up on the end of the spritzer and pump pinkie through the heater hose and into the block. When pinkie comes out of the manifold I know the entire block is full.

Then I pull the 2 hoses that go to the exhaust manifolds and squirt pinkie into each of them. About a half gallon.

No spilled Pinkie in the boat from missing the funnel!

No voids in the block.

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I've never done the antifreeze thing, but when I had a direct drive, because of the severe angle the engine is mounted in the boat, I would pull all drain plugs and hoses, then park it on a hill for a couple minutes with the engine level. The idea was to drain the water at the rear of the block/manifolds. This was probably overkill but was cheap and easy...

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I appreacite all the comments.

Here's in southern Ontario, Malibu dealers (and other marines) all add AF as part of their winterization service. It's great to just drain and forget, but adding AF seems to be norm here.

Having said this, dealers charge about $1500 (winterize/oil/trans/ballast/impeller)......... I can do same thing for $300.

No matter if you use AF or not, engine should be run upto temp for oil change......... I figure why not just run on AF in the first place.

Might sound complicated but it's really no big deal.....

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Having said this, dealers charge about $1500 (winterize/oil/trans/ballast/impeller).........

$1500? Holy ***! You can get a new engine after just a few years of doing it yourself. That number alone should eliminate the hesitation for those that are afraid to do it themselves.

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At the end of the day.....just make sure ALL water is drained. Personally I add AF (old habits hard to break) but know many who don't and have never had issues.

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  • 2 weeks later...

I prefer using antifreeze on my Malibu VLX / Monsoon 325....What I like about antifreeze is that it prevents rust and keeps the seals & impeller "lubed" up. Back in the 1980's, I worked at a shop that sold Supras and we simply pulled all the plugs, hoses, drained the water and pulled the impeller, putting everything in a bag on the steering wheel. We never had a problem, although we had some DIY customers who skipped removing the impeller and a few of those guys burned 'em up in the spring on the first dry run, getting chunks of rubber in the cooling system...Not good. Regardless, we changed impellers out every 2 seasons, religiously. To me, aside from the rust issues, the impellers just seem to last longer using antifreeze.

Here is what I do...

  1. Run engine & change engine oil, as well as transmission & V-Drive, if necessary.
  2. Drain the Engine & Manifolds and then reattach everything, just like in the PPT winterization video (Great Video).
  3. For my heater, I just blow air into the lines.
  4. For the antifreeze, since I don't have a pump for the antifreeze (like in the video) I use a 5 gallon container with a hose/spigot attachment. I'll set it on the doghouse, above the engine, and just let it get sucked in to the intake hose.
  5. I use -100 Antifreeze. I know it's overkill, but it only costs a few bucks more.

The -100 let's me sleep better at night, just like throwing a trouble light in the engine when the first fall freeze comes and the temps dip below freezing, for a few hours, overnight.

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I installed a pressure gauge in the drain hole one summer and observed pressure readings of 4 to 10 psi. I also leave my block dry during the winter and change my oil cold using a long drain interval.

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