King Cobra overheating on plane, cannot find the issue

Blentine

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hello new member here, please bare with me! im out of possible solutions with my first boat. I have a 1993 four winns 201 liberator with a 5.7 350 small block chevy. the outdrive is an King Cobra and has had this issue since I bout the boat. a mechanic has had it for over a year as of may and rebuilt the outdrive from ground up, yet still has overheating condition.
to help diagnose the problem myself, I drove the boat- finding that it only overheats once on plane. coming off plane the boat cools down to proper operation tempatures. to further narrow the problem, I put a clear hose from oil cooler to thermostat housing on top of block.

As soon as I hit 1800 rpms, the minimum rpm that the boat will plane on, I start to observe about 50/50 air to water being pumped from the outdrive to the engine.
when off plane when there is water behind the transom and the outdrive is submerged, the clear tube becomes solid water and operates normally. once boat is on plane and part of outdrive is above the water, it begins sucking air and thus overheats.

all pickups and O-rings visible when case is split have been checked and replaced. incredibly stumped here and desperate for help please! on my third impellor since I bought the boat.

P.S. the liberator has dual exhausts out both sides of boat, so there is no way it is sucking exhaust. thanks, Brandon
 

Grub54891

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Interesting. I shall watch this thread to see what happens. I'd wonder if the impeller housing itself is not sealing completly,when on plane,it could suck air easier then when low in the water. A new housing,and it should be fine,unless where it seats is compromised.
 

HT32BSX115

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hello new member here, please bare with me! im out of possible solutions with my first boat. I have a 1993 four winns 201 liberator with a 5.7 350 small block chevy. the outdrive is an King Cobra and has had this issue since I bout the boat. a mechanic has had it for over a year as of may and rebuilt the outdrive from ground up, yet still has overheating condition.
to help diagnose the problem myself, I drove the boat- finding that it only overheats once on plane. coming off plane the boat cools down to proper operation tempatures. to further narrow the problem, I put a clear hose from oil cooler to thermostat housing on top of block.

As soon as I hit 1800 rpms, the minimum rpm that the boat will plane on, I start to observe about 50/50 air to water being pumped from the outdrive to the engine.
when off plane when there is water behind the transom and the outdrive is submerged, the clear tube becomes solid water and operates normally. once boat is on plane and part of outdrive is above the water, it begins sucking air and thus overheats.

all pickups and O-rings visible when case is split have been checked and replaced. incredibly stumped here and desperate for help please! on my third impellor since I bought the boat.

P.S. the liberator has dual exhausts out both sides of boat, so there is no way it is sucking exhaust. thanks, Brandon

Howdy,


Welcome aboard!

Well, as always, we might need a few things. It always helps to post the actual model number of what you have. OMC called SEVERAL engine/drive packages "King" Cobra and they were VERY different.

The model number will look like :
574APEAMH 1992
574APLAMH 1992
574APRAMH 1992

Notice the 1992 model year. OMC didn't offer a 5.7L engine in 1993, so you may have a 1992 engine and drive unless someone swapped in a 5.7L GM engine, flywheel housing etc...... (1993 was Ford 5.0/5.8L engines)

You can view model numbers and parts lists at http://epc.brp.com (email can be left blank)

Now, in general, if you have overheating at ANY higher speeds and power settings, it's likely due to poor cooling water flow either FROM the raw water pump, associated hoses, and/or out through the exhaust manifolds/risers. (If the water gets in, it MUST get out!)

Looks like you have identified a possible problem already. bubbles in a clear hose from the raw water pump to the T-stat housing indicate it's sucking air at the raw water pump (is it drive mounted or engine mounted?)

That year and model *should* be a dog-clutch Cobra (drive-mounted raw-pump) but in more 20+ years, Someone could have swapped the drive with anything.......

I would start there.

Regards,


Rick
 

Blentine

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Thanks for the welcome

I'm sorry my mistake I believe it is a 1991 model four winns, that aside, someone in the past as swapped the adapter plate for another cobra out drive- enabling new water impellers to be ordered.

The exact impeller for the king cobra I have is N.L.A (no longer available) so the adapter plate allows for (still available) impellers to be used

And yes the pump is mounted on the top of the out drive itself, just above the water line!

I will try to post some pictures as I progress hunting this issue, thank you rick for your input!
 

Blentine

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Also that is correct I would have poor water flow through the all speed and rpm ranges if there was restrictions, but does flow very well.
But the thing that is stumping me is that it the water flow is strong and plentiful at speeds from idle all the way up to just before on plane.

Once on plane, (and at any trim) you can see the air begin to flow through the clear tube and progressively displace all the water in the tube within seconds.

Once this begins to happen must come off plane after a few seconds and get water flowing again in fear of burning another impeller / overheat engine
 

bruceb58

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Mar 5, 2006
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The fact that it is just before plane is suspicious and makes me think its the connection between the lower and upper unit. Way before you get up on plane, the water pump is already out of the water.
 

HT32BSX115

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The exact impeller for the king cobra I have is N.L.A (no longer available) so the adapter plate allows for (still available) impellers to be used
All the more important to have the correct model number I suppose... OMC had 2 (main style) drives in 1991, the Dog-clutch drives and the "Interim" cone clutch drive. According to BRP, if yours is a 1991 model, it uses impeller 0983895 (which is available)

The one that is NOT available was used on the "interim" cone clutch drive .........
(0987176, replaced by Volvo 3854072 and actually it IS available.........Marine Engine dot com has 10 of them for $110!!) But I digress

I'm with Bruce, you have a " leak" in the drive......... when it gets up out of the water and exposed to air it "sucks" (air, that is!!)

The "bubbles" are the tell-tale sign
 
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Grub54891

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Agree with the above comments. Just like poking a hole in a drinking straw below the fluid level. It'll be fine till you get below the leak.
 

Lou C

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Nov 10, 2002
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When the mechanic split the drive did he replace the water tube gaskets and make sure there was not a hole corroded in the water tube.
When I do a Cobra impeller I run it on the water hose with the plastic cover off to make sure water is not leaking around the impeller housing. If water leaks out air can leak in
 

jerryjerry05

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May 7, 2008
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Remove the drive.
Put in some kind of stand. Needs to be straight up and down.
Install muffs and duct tape them in place.
Try to seal as good as possible.
Turn water on slowly.
As the water goes through the unit it should only come out where it couples to the transom/pivot assy.
If it leaks ANYWHERE else? That's the problem.

Sometimes when the upper and lower drives are separated the rubber seals could get pinched/cocked and allow it to suck air.

Also when the drives off check the hose from the bell housing to the transom.
They can wear and suck air.
Drive off insert hose in the water passage on the pivot housing and turn on the water. There should be no leaks until it comes out the exhaust.

How old are the manifolds/risers?
 
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