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OMC King Cobra cooling issues.

B

BK59

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HI, I have a 1992 OMC King Cobra with a 5.8l ford engine with a closed cooling system. The issue I am having is, after running the engine for a while the coolant overflows the heat exchanger causing engine to get warm. The engine is a new long block and I normally run it a 2800 to 3000 rpm for extended periods of time. The temp usually runs between 160 and 180 degrees. When I notice an increase in temp I shut it down and fing about a half a gallon of coolant in a jug I run the overflow hose into. I have replaced the heat exchanger cap, thinking it maybe the issue but took it out today with the same results. If anyone has run into this problem and has any insight, please let me know.
I am a boat guy and have worked on many boats in my life but this one has me baffled.
Thank you in advance for you help
 
I'm assuming that the exhaust is raw water cooled, are the manifolds and elbows clear and allowing good raw water flow out of the exchanger to keep the coolant at a normal temp? I'd check that first.
Next, pressure test the cap you have now, if it doesn't hold pressure that can happen, also test the exchanger for leaks.
The other thing that can cause that is leaky head gaskets, pushing combustion gas into the coolant. Very common on older Subaru EJ2.5 engines. With those, at low speeds they wouldn't overheat, but if driven faster, they would often overheat, pushing coolant into the recovery tank. Another sign was small black bits of carbon in the recovery tank. For that do a combustion gas test on the coolant. Might also want to do an oil analysis, see if potassium and sodium show up in the oil, they are from antifreeze, another sign of leaky head gaskets.
 
louc said:
I'm assuming that the exhaust is raw water cooled, are the manifolds and elbows clear and allowing good raw water flow out of the exchanger to keep the coolant at a normal temp? I'd check that first.
Next, pressure test the cap you have now, if it doesn't hold pressure that can happen, also test the exchanger for leaks.
The other thing that can cause that is leaky head gaskets, pushing combustion gas into the coolant. Very common on older Subaru EJ2.5 engines. With those, at low speeds they wouldn't overheat, but if driven faster, they would often overheat, pushing coolant into the recovery tank. Another sign was small black bits of carbon in the recovery tank. For that do a combustion gas test on the coolant. Might also want to do an oil analysis, see if potassium and sodium show up in the oil, they are from antifreeze, another sign of leaky head gaskets.
Click to expand...
Only the risers are raw water cooled. The manifolds are fresh water cooled. Manifolds are new last year when the new long block was installed. Oil is clean and shows no sign of coolant. I will try and do some pressure testing next. Thank you for the info.
 
We run in saltwater mostly. I don't recall if I changed the risers when I did the manifolds. It will run for 30 minutes @3000 rpm before losing any coolant.
 
Most exhaust manifolds are at the end of their service life after 5 years in salt water...some see less, a few, a little more.

As Lou suggested, I'd think the exhaust elbows are a very good place to start. If they are clogged, the raw water cant flow thru them at a high enough rate and overheating usually occurs...and it only gets worse from there...
 
Agreed, you need good flow out of the elbows to have adequate raw water flow through the heat exchanger to keep the coolant at the right temperature.
One question though. So you have a full closed cooling system, is this running off of the standard Cobra impeller in the upper gear housing?
Also, I'm wondering how they could have set it up with antifreeze in the manifolds in a full closed system. The standard OMC/Volvo manifold design has a water inlet on the bottom of the manifold front and rear. For you to have manifolds on the closed cooling circuit, there must be an inlet (bottom of manifold) and an exit (upper portion of manifold). To my knowledge, OMC or Volvo never had manifolds designed like that. Merc does, and you can set up a Merc with full closed cooling for that reason. If they are not set up this way, there will be hot spots in the manifold. In fact the hottest area will be where the manifold and elbow meet. Perhaps that is overheating the coolant as it is about to return to the heat exchanger. The coolant should exit the engine, then travel to the lower section of the manifold, then to the upper section of the manifold, and then back to the exchanger.
I also am not sure, if you are using a standard Cobra impeller in the upper gear housing, if this flows enough raw water to keep a full closed system cool enough, because now you have the extra heat load of the exhaust manifolds.
If you can post up a pic of your system, I am interested to see how they plumbed it.


Here's your exhaust system, if these are the manifolds you have, I think trying to use them in a full closed system, might be the cause of your problem with hot running.
For a comparison, look at how Merc sets up a full closed system, the manifolds have a low mounted inlet and a high mounted return.
I think the only correct way to set up a full closed system on an OMC or Volvo engine, is to somehow adopt Merc manifolds to it.
So a solution might be, just put the manifolds on the raw water side of the system, that is how every OMC or Volvo I have seen with closed cooling is set up, half system only.
 
Should have said Elbows, not manifolds, in post #5...

Lou - based on your pics, I'd agree that the 'half system' is the only way to add Fresh water cooling to to the Factory OMC exhaust. Trying the Mercruiser approach on a ford block is something I would discourage .... they only made the log style exhausts and they left a lot to be desired...I think they are hard to come by as well...The sierra parts have the extra pipe plugs "on top" but don't know about the gasket options offered.

of course, if the engine in question has aftermarket parts (or a kit added) probably need lots of pics to get all the details.
 
I replaced the risers however the old risers looked great. They came off a freshwater boat, but I replaced them anyway. Now I have a different problem. I am getting water in the bellows/gimbal area. Is there a gasket for mounting the outdrive on the King Cobra. I have a regular Cobra mounting gasket but they are two different animals. I was not able to test the cooling system out fully yet.
 
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