Over Heating OMC Cobra 4.3

[Geodude619]

Okay, well first off I am new here and just gotta say its an amazing community for people to come together and help out! Glad to be apart of it. I am kinda new to the boating life so I do apologize if this is a dumb question. Now on to the issue. I have a 4.3 omc cobra that is over heating. I have checked the thermostat housing and all the hose leading into it as well as out. It is not clogged and there is water running through there. I have also removed the impeller and checked that out and it seems all good (previous owner just replaced it). I did not replace the impeller kit because it looks good. Now, the elbow exhaust/exhaust manifold get super hot when running(cant even hold onto them). Iv been told that maybe the Thermostat might be bad? Anything else it might be? What can I do to check? Now this might be the dumb question, is seems to run cooler on muff (neutral) then in water (actually in gear) not sure if this is normal, or maybe i just did not run it long enough on muffs for it to heat up. Any advice is greatly appreciated. (any advice you think a new guy needs Thanks in advance

 

[Lou C]

First of all what year is this engine? If it an early one let's say 1986--1990 they used a one piece manifold riser unit that look like Batwings. They had 4 outlet ports in the riser area that can get clogged by rust, you can remove them and rod them out and then flush them with fresh water. The later models have a more typical 2 piece set up just like Volvo Pentas of the same vintage. So first make sure your risers are not clogged. The thermostat will not cause the exhaust to overheat right away because until the stat opens (160*) all the water goes out the manifolds. At idle they should be between 85-100 degrees which is just a bit warm but you can keep your hand on them. Have you tested the thermostat in a pot of hot water? It should start to open at about 155-160 and be fully open at 180 or so. To test the impeller, you put the boat in the water and with a helper, you disconnect the hose from the transom mount to the thermo housing at the thermo housing. Hold the hose straight up and have your helper start the engine just briefly. You should see a stream of water 2-4" tall. If not then the impeller is either worn, the wear plate is worn, or there are old impeller bits in the power steering cooler on the rear of the engine or in the thermo housing. These are the basic things to check. When all is right with the system, the engine should run at 160-175, the manifolds should be at about 100 at idle, and after coming off plane, the highest you should see is about 135*. Get an IR temp measuring gun, it will be your best diagnostic tool for cooling system issues.....

 

[Scott Danforth]

welcome aboard.

did you replace the raw water pump and perform all appropriate preventive maintenance on this new to you boat?

 

[Geodude619]

Thanks for the quick responses!! It is a 1991 OMC engine. I have not yet tested my stat, to be honest im not 100% there is one in there. I removed the housing the other day and did not see one in there. I will check again, and also check the risers. I will do that test for the impeller just to cross that off the list but im almost sure it is working good but does not hurt to try . To check the impeller, it must be in raw water or can it be with the muffs on garden hose?

No i have not changed the raw water pump, when I purchased the boat the previous owner had done alot of things so it has newish parts but i can do that as well.

Sounds like it is going to be the risers, water seems to be traveling through the thermostat housing and hose, but maybe just not through the manifold, hence the reason why its getting really hot?

 

[Lou C]

To check the impeller output the boat should be in the water , water pressure from the garden hose on the muffs will mask a weak impeller.
You can just back your trailer in the water and put the drive down you don't actually have to launch it as long as the water at the ramp is deep enough.

 

[Geodude619]

Oh ok, i thought so, then il do that, i will check the risers make sure there is no blockage and il check stat and them test impeller, fingers crossed, thanks so much!

 

[Scott Danforth]

dont check and assume the PO did the maintenance. replace the raw water pump and know it was replaced.

boat ownership 101, new to you, do all the maintenance items

new fuel filter
new oil filter and oil change
new gear lube
new bellows
new raw water pump
new plugs, cap, rotor, and wires
new belts

compression test should have been done prior to purchasing, however re-do it and document the numbers for future reference

 

[Geodude619]

Got it. and awesome, thanks for that, that was going to be my next question, what should I do myself to be on the safe side. Thanks for that!

 

[Geodude619]

Trying to post pictures but its not letting me, but updated, i removed risers and everything seems clear, should i remove manifolds as well and check?

 

[Scott Danforth]

Picture upload tutorial in general rules forum

 

[MRS]

When over heating you should always change the impeller first the whole kit gasket metal cage and impeller and the cover. If you see any melting on the inside cover that means it slipped from the impeller being over heated. 99 percent of the time that will fix it. I know been there done that if that does not do it then you can look else where. Just saying what I have been trough.

 

[Geodude619]

Awesome yea thanks for the advice, well i removed the Risers and the manifolds do seem alil glogged, i ran water through the right side manifold (facing the back of boat) and water came through. Did the same to left side and no water came through. Im assuming this is the issue. Only problem now is i cant remove manifold. Ine of the bolts seems stuck inside.

 

[Lou C]

Get a can of Kano Kroil and start spraying it. Try not to strip or break the bolts holding the manifolds on.
My 4.3 used studs and nuts holding them on, no problem getting them off and done it a few times. Yours had studs & nuts or bolts?
http://www.crowleymarine.com/johnson-evinrude/parts/56042.cfm?mdl=3LC7CU
based on the parts diagram yours should have studs & nuts, easy to get off.

Also if no water at all is getting through first take off the thermostat housing, and make sure all the passages inside are open and not clogged. You can disconnect both ex manifold feed hoses and rod out the water passage in the stat housing with a thin screwdriver or stiff wire. Have someone briefly start the engine with the muffs hooked up to the drive and make sure water shoots out of both sides of the thermostat housing holes that feed those manifold hoses....

 

[Geodude619]

Yes sorry they have studs and nuts, but one of the studs on each side does not come out, the nut comes out but the stud is stuck in there and doesnt let the manifold out, there is quite a bit of blockage in manifolds so im gonna try to vacumm out and see how much i can get out. The first thing i did when it over heated was open up theromostat housing and everything was clear in there, looks like when water gets to the manifold thats where it stop. Causing risers to heat up maybe?

 

[Geodude619]

Does anyway happen to have a water flow diagram so i can understand a little more how the cooling system works? Im assuming water passes through the side "cracks/openings" in manifold and risers and big opening in middle is exhaust, is this correct?

 

[Geodude619]

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