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Overcooling off plane

C

Carmel

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Hi all. New guy here. So what I have going on is on my 5.7 raw water cooled engine , when coming off plane my temp drops off the gauge and runs rich. Black soot! My temp gauge works and so does my temp sensor/switch. Replaced t stat. Ran on muffs got up to temp. 175 on gauge. ran rpms up and all works well until I bring back to idle. Then my temp drops. I checked at thermostat housing with my hand and could fell the temp drop from hot to cold just as the gauge showed. I can be on the lake running around and this happens. up down temps no overheating. Not sure what to check. Any help I would appreciate
 
There are more versions of a 5.7 raw water cooled engine than you can shake a stick at.... Some engine specific info please!!! Some engines don't run @ temp on muffs.
 
Hi sandkicker. So my engine is 5.7 GI-j 300 hp #4012257684 .
Runs to temp on muffs and in lake. Temp drops after having rpms up then drop rpms and temp drops and runs rich then works back to temp at idle.
 
Yes oem and installed correct. As for as run like this ? Well this is the first year for me to own this boat, but at beginning of year didnt notice this. I started to look into this when all the soot from captain call exhaust was on the boat. Thats why I'm hear trying to learn about if this is a problem or not. When temp drop but is very sluggish.
 
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It seems like there was another situation similar to this within the past year, and it ended up the hoses were not routed from the thermostat housing to the correct ports. Have you verified your engine's plumbing against the manufacturer documentation?
 
Hey there DJR. So there are 5 hoses that run to the tstat housing , big from circ pump to stat, med from impeller to tstat , 2 smaller from tstat to manifold . One smaller one that I think runs to fuel pump.I wish it would be something like that. But I think its impossible to mix them up.
 
So I found a diagram online of my setup and how the t stat housing works and to me is a little dump on placement of the sensor for ecu. So as the water circulates is faw water into housing and fills to full as it circulates to get hot. Extra water goes through water to exhaust manifold. When t stat opens and lets out hop water out{ this is where I am running into the over cooling } the new raw water is coming into the circulation pump past the SENSOR which is correct in that the water is cool. Thats my temp drop and after that water warms up and the sensor, Im good again. I would have thought they would have pushed [ bypassed] some hot water back into circulation to warm new raw water.
Sorry for grammar. lol
 
Sandkicker if you look ar the diagram you posted you will see next to the big hose for the circ pump there is a hole that my sensor/switch goes. Does this sound correct?
 
Been a long time since I saw once of these. But I'm guessing that temp sensor goes into the hole on the front of housing. In that location, the sensor will see temp of engine water leaving the block. Since engine is MPI, there are likely two temp sensors, one for the dashboard gauge and one for the ECM.

Still think your problem is t'stat related since contrary to popular belief, the job of the T'stat is to keep the engine warm enough, not cool. Check for obstruction in t'stat housing. BTW, I once chased a plumbing problem that turned out to be that a new gasket had been installed over the old one.
 
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Carmel said:
So I found a diagram online of my setup and how the t stat housing works and to me is a little dump on placement of the sensor for ecu.
I agree.
The sensor should sense temp prior to the thermostat's location where it would receive a more consistent reading, regardless of engine RPM.


So as the water circulates is faw water into housing and fills to full as it circulates to get hot. Extra water goes through water to exhaust manifold.
The raw water cooled engine's T-stat housing allows most all seawater to bi-pass actual engine cooling demands.
Only that which is needed to cool the engine (via thermostat's open position) is allowed to enter and leave the engine.
In other words, there will always be a significant amount of cold seawater (blue arrow in) circulating within the T-stat housing mixing chamber.

Example only:
raw water cooling Tstat housing.jpg


When t stat opens and lets out hop water out{ this is where I am running into the over cooling } the new raw water is coming into the circulation pump past the SENSOR which is correct in that the water is cool. Thats my temp drop and after that water warms up and the sensor, Im good again.
As you know, the engine circulating pump is charging the cylinder block and cylinder heads with coolant.
The actual thermostat holds back coolant (i.e., seawater) on a "on-demand" basis. In other words, the thermostat will only allow coolant to return to the T-stat's mixing chamber as it reaches and surpasses the temp rating.
A good working thermostat doesn't know nor care at which RPM the engine is operating at.

I would have thought they would have pushed [ bypassed] some hot water back into circulation to warm new raw water.
Look at my image above.
Blue = incoming cold seawater
Red = seawater that has removed engine heat.... it is being held back by the thermostat.
Yellow/Blue = a mix of tempered seawater and cold seawater on it's way to the circulating pump.
Red/Blue = tempered seawater that is heading to the exhaust system.
Click to expand...
 
With 1000's of VP powered boat on the water without your issue, I would say there is something wrong with your installation. IE, Wrong thermostat, thermostat backwards, thermostat housing no longer good.

One last thing, did you check the incoming cooling water stream for air bubbles? This is done by placing a clear hose between the pump and the t-stat housing. Air in the cooling water will crack your cylinder heads.
 
So I took impeller off and back flushed the line and tighten all fittings. Ran on muffs and couldn't get it to do it. Temp drop that is. So Chris, I think I was getting air in the system from a fitting. I need to get it out and run it to make sure but on the muffs it was good.
 
My OMC has that same exact t-stat housing (VP took it from OMC actually) and what I have found is that it is common every now and then for a small piece of rust from the cast iron intake to lodge in the stat keeping it from closing all the way, then you get the overcooling when coming off plane or at idle. Mine would go down to 100-120* vs 160-175*. You will all laugh but I came up with a 'solution' instead of removing the housing etc every time that happens...run it so it gets to normal temp (ie 160 on the dash gauge)....and if you shoot it with an IR Temp gun...150-155 on the intake right under the housing...this tells you its open.......then get a small hammer and tap all around the t stat housing...while the t stat is open...this vibration can jar the rust flake loose and you don't have to remove the housing, clean it out etc to get it to run at normal temp. If its a chronic problem then drain some water out of the engine remove the tstat housing, and scrap out as much of the flaking rust as you can from the intake manifold..in my experience...that's what causes the t stats to stick...one little flake of rust holds it open.....
 
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