Mercruiser 5 L mpi cooling system

A

affa

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Hi,
I started the engine without the drive installed to heat up the oil before changing it, it come some water out in the exhaust and it was much white smoke, I thought the water would circulate inside engine when thermostat was closed? It has been winterised with antifreeze, my concern is if something’s gone wrong in the cold? It’s not closed cooling, and it’s the type where you can remove a blue plug in the bottom to drain the water from the engine
Regards
Affa
 
Scott Danforth

Scott Danforth

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Since you did not have cooling water flowing to the motor when you ran it, you burned all the rubber bits in your exhaust

the white smoke is most likely steam and burning rubber

If you drained the block, heads, and exhaust manifolds prior to adding antifreeze, you should have been protected
 
A

affa

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Ok thanks, the rubber in the exhaust, is that the flappers?
 
Scott Danforth

Scott Danforth

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flappers and the rubber exhaust couplings. both rated to 250F max. however without water running, your exhaust goes from 200F to 950F
 
J

Jmunk

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How long did you run it without a hose feeding in water?
 
A

affa

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Not long, the riser’s wasn’t hot, maybe for 5-10 minutes, and it came some water out,
(that’s why I wondered, didn’t think it would come water out when I had no water feeding),
It was winterized to the top with antifreeze
 
Last edited:
Scott Danforth

Scott Danforth

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affa said:
maybe for 5-10 minutes,
Click to expand...
5-10 seconds is max to run without water. at 5-10 minutes, you definitely burned up the flappers and most likely the rubber connectors.
 
J

Jmunk

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No water came out due to the fact there wasn’t any water pressure to push it out. Best case it’s just the flappers and rubber couplers that need changed. Worst case the engine overheated.
 
alldodge

alldodge

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If motor uses a Bravo drive than also the impeller is burned
 
A

affa

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It’s an alpha drive,I’ll check the rubber parts
 
A

affa

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Are there any problem using the boat with worn out flappers?
 
Lou C

Lou C

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Well, it's hard to say, but if the static water line is just high enough with no safety margin it is possible to get water up the exhaust if you slow down quickly. They are there for a reason....
The pix I posted above are to show what happens during an overheat, vs a good flapper.
 
Scott Danforth

Scott Danforth

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usually the burned flappers drop down the Y-pipe, plug exhaust causing the motor to run like crap, then the flapper pin wears a hole in the y-pipe and you develop a leak in the boat.
 
Lou C

Lou C

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I had to pull the outdrive to get the rest of the burned up flappers out they were in the exhaust passage, lucky the pins stayed where they are supposed to be.
Not sure if this applies to the Alpha or Bravo drives but on OMC Cobras and Volvo SXs there are water tube grommets that will also get overheated and softened/melted, due to a lack of cooling water. So splitting the drive the check all that, is wise after an overheat, or else you can have an overheat that you can't figure out!
 
Scott06

Scott06

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affa said:
Are there any problem using the boat with worn out flappers?
Click to expand...
its not ideal as they put them there for a reason, but have had no issues with a boat where when I changed them it was obvious they had been not working for years.
 
Lou C

Lou C

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It's impossible to generalize because every boat is different, some have more than enough static water line height, some are marginal, if the driver is prone to rapid slowdowns, if a heavy swim platform is added, etc. all those are factors. If you REALLY want to know, learn how to measure the static water line of your boat!
 
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