Penta MD11C intermittent steaming

[StefanSG]

After coming up to temperature (with a little puff of steam) every three or four minutes the temperature gauge shoots up to the top of the green zone, and there is a small amount of steam for 10-20 seconds and then the temperature gauge shoots back down for another two or three minutes of normal operation. The engine doesn’t overheat generally, but the steam concerns me because it will add to the scaling and blockage in the manifold.
I’ll try and describe my current setup, things I’ve already tried, and things I think might contribute.
My engine is a recently rebuilt MD-11 C good compression cooling channels all cleared out in jackets and manifold. New gaskets all round, valve seals etc. Correct brand new thermostat tested. Pumps water fine although it sprays the water out of the transom.
I currently have a Vetus waterlock exhaust - the exhaust pipe loops down, around under the engine comes up around the flywheel, enters the waterlock on the starboard side of the engine and then exit the transom. I am worried that this set up might be contributing to an overheating problem I’m getting especially as the waterlock has risen up to the same level as the manifold. (I will try fixing it securely below the manifold as my next move) Perhaps a pressure or water build up in the exhaust ?
The whole engine is above the waterline by 18”
And if I get rid of the big loop, how can I get a vetus muffler in the system when the exhaust elbow is about 9 inches from the transom hole
Thanks in advance, at my wits end with this one, manifold currently off for another inspection and flush.

 

[Metalicmike]

You say that you have tested the thermostat, It sounds like the thermostat is opening at too high a temperature hence the puff of steam. Then cool water closes the the stat and the cycle starts again.

 

[StefanSG]

You say that you have tested the thermostat, It sounds like the thermostat is opening at too high a temperature hence the puff of steam. Then cool water closes the the stat and the cycle starts again.

I’ve tested it in a saucepan with the jam thermometer, it opens fully at 60 degrees. I will test it again.

SSG

 

[VicS]

I’ve tested it in a saucepan with the jam thermometer, it opens fully at 60 degrees. I will test it again.

SSG

The water lock should be at the lowest point of the exhaust system but I don't believe the fact that it isn't is the cause.

I have always believed the fluctuating temperature to be due to blockage of the cylinder water jacket passages but you say they have been cleaned.

 

[Bilgediver]

Did you clear out the drilled holes in the block on the exhaust mànifold side of each bore and did you remove the crap that accumulates in the cooling water gallery around the bore at the bottom of the block which is at the same level as the drain locks.

If mud and sand is sitting there then you get exactly the symptoms you are describing. You may be able to clear this by opening the drains while the engine is running and poking with wire . Also with the engine at rest applying water under mains pressure at the drain cocks with the thermostat removed might flush out the passages.