OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

[hughesra]

I have an OMC Cobra 4.3L stern drive that is overheating.

I started it up for the first time of the season yesterday (using hose and muffs) and I noticed that the water pump was leaking some water - the water pump is about 3 years old. I opened up the water pump and looked at it, it looked okay, still spins and rubber is still flexible. I cleaned it up and put new sealant on both sides of the metal gasket and closed it up.

Today, I put the boat in the ocean and started it up. It started to overheat.

I pulled it out of the water and put the hose and muffs on it again. It did not overheat - the water flowed through and was exhausted properly.

I put it back in the ocean and it started to overheat again.

I pulled it out of the water and opened up the water pump. It still looked okay. I noticed - inside the water pump cavity - that the port side water intake was full of water and it was fresh water - meaning that it was still there from when I used the hose/muffs (so no ocean water was getting in).

I am not familiar with what is between the intake screens and the water pump - but I have never flushed the raw water cooling system. I keep the boat in the ocean all summer, so there is build up of barnacles/mussels.

- does the raw water always enter through the 4 screened holes (per side) that the muffs cover? or when the motor is in the ocean, does it also enter through the slots above the prop?

- does it make sense that the water intake (inside the water pump cavity) should stay full of water? Why does it not drain - is there a one way valve or could it be blocked?

I am thinking that the water intake is blocked somehow and the water pump is not strong enough to suck up the water.

Is there some way to open up the bottom of the leg - I have never done this before - and will I be able to clean out the intake area?

Thanks in advance

 

[Joshua Nichols]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

Gotta year?????I wanta say blockage. in the intake screen.. You might have some little oceanic critters living in there.. The water hose and muffs produce a pressure that will force water up past em.. Change the dang water pump.. It's a maintenance item 3 years old???:facepalm: Can you fix it? Can you turn a wrench and read instructions?? If so.. It shouldn't be that tough..

 

[hughesra]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

Yes, it is 1989 GM motor in a 1990 FourWinns.

1. I think you might be right about the blockage in the intake screen. I have never cleaned them out. I did not look closely at them yet - can you tell me - can they be removed - I remember seeing a screw next to them, but I think that they are inside the openings? Or do I just spray the hose on them?

2. I did look at the water pump, it looks okay, but maybe I will change it anyways. Yes, I do know how to change it.

3. Is there an easy way to flush out the whole raw water cooling system? Do I have to take the exhaust manifolds out to clean them or can I just flush water through the whole system? I do not have any special hose connectors.

Thanks

 

[cr2k]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

Water pump impeller is the first line. Should be changed every other year, yours is 3 years old and way over due.

 

[Lou C]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

If the water passage is not draining, then there is a good possibility that you have small barnacles in the water intakes, take a good look with a flashlight to see what's in there, I have had to break them up with a small screwdriver (like a jewelers screwdriver), remove the impeller housing, lay the drive on its side and hook up the muffs and hose to blow them out. The older Cobras required you to split the drive to remove the water intake screens, the newer ones they could be removed with out splitting the drive. When you run it on the water hose, the water pressure is probably forcing enough water in there to keep the engine cool, but in the water if you have restrictions, your water flow may not be enough. Did you try disconnecting the raw water in take hose with the boat in the water, and briefly have someone start the engine, to see how much water is coming out of that hose? Should be a good stream pushed upward. I replace the impeller ever season (probably overkill) and clean out the water intake area as best I can, and I don't have overheating problems with my Cobra....

About the screw by the water intakes, if it is not right in the water intake screen assembly, you may have the older style, do NOT remove that large Philips head screw that is near the water intake screen on the lower unit....the screw is in the groove leading to the 2nd intake hole from the bottom, that means you have the older style.

Post a good pic of the lower unit and we can tell for sure, but I think the newer style ones with the externally removable screens started in 91 or so.

 

[hughesra]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

Thanks guys, but I have still not fixed it. I bought a new water pump and installed it but I need to wait until tomorrow until I try it out.

One question I have - I was reading the manual - I can not determine if there is a raw water engine water pump (other than the impellor water pump on the leg). The manual talks about an engine water pump, but I think it is for the fresh water (closed) cooling system. I have a closed system - both a fresh water cooling system and a raw water system, with heat exchanger in between. Does anyone know if there are 2 pumps on the raw water system - the impellor on the leg and another one on the engine?

 

[Joshua Nichols]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

The engine pump is on the engine... Belt driven and in the middle...

 

[bruceb58]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

The pump on the front of the engine is a circulating pump only. It is not a raw water pump. If you aren't sure if you have a heat exchanger, you probably don't have one. You can post a picture here of the front of your engine to be absolutely sure though.

 

[hughesra]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

I think that the water pump that is on the engine (in the front middle, belt driven) is for the FRESH-water cooling system, not the SEA-water cooling system.

Looking at the engine, I see the SEA water hose coming from the transom, goes through a hydraulic fluid (for power steering) cooler, then goes directly to the heat exchanger. From there there are 2 water hoses - one to each exhaust manifold.

There are also 2 other water hoses from/to the heat exchanger, they go to/from the block and I assume they are the FRESH water cooling system - and I assume there is an engine mounted, belt driven water pump attached to them.

I do not see where there could be an engine mounted water pump for the SEA water cooling system?

 

[bruceb58]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

The hoses that go to the each exhaust manifold are coming from the thermostsat housing. That is not a fresh water cooling heat exchanger.

A fresh water heat exchanger is going to be a couple feet long and about 6 inches in diameter with a radiator pressure cap somewhere.

It will look something like what is on the front of this engine:

 

[hughesra]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

Sorry, my last post was replying to a previous post. Then I read your post.

I definitely have a heat exchanger. The motor has both a fresh-water (closed) cooling system and the sea-water (raw-water) cooling system.

My question was - does the sea-water cooling system only have the one pump - the impellor - the one mounted on the leg. I think that you answered my question when you said that the pump on the engine is not a raw-water pump. So, you are saying that the raw-water system only has the one pump - the impellor in the leg.

The reason I was asking was because I wanted to be sure that my problem has to do with the water pump/impellor on the leg. If there was another water pump in the raw-water cooling system, I thought that maybe I should check it. But it sounds like there is no other water pump in the raw-water cooling system.

 

[bruceb58]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

So...you have the big tube with a radiator cap?

 

[hughesra]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

yes

 

[bruceb58]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

The circulating pump driven by the belt on the front of the engine is what circulates the coolant on the engine side of the heat exchanger. The raw water pump on the drive is what provides the water flow on the raw water side of the heat exchanger.

When you were overheating, did you happen to see how hot your manifolds were getting? Is your coolant reservoir full?

 

[hughesra]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

OK, thanks, so there is only one pump on the raw water cooling system = the impellor/pump on the leg.

I believe the fresh water cooling system is working okay. The coolant reservoir is full. When I run the motor on the hose/muffs, I can see the temperature rise, then drop a bit when the fresh water thermostat opens. When running on the hose/muffs, everything seems fine - it does not overheat.

When I run in the ocean (no hose/muffs), the motor overheats. The exhaust manifolds get really hot (they are raw-water cooled).

Another question I had was how to properly run the engine when I am out of the water. The best way is to run using hose/muffs, but in my case, I can not tell if I have fixed the problem if I am running on hose/muffs. I believe I can run the engine if I put the leg in a bucket of water. But the problem is - how to get the leg submerged enough in a bucket. When the boat is in the ocean, the whole leg is submerged. When I try to use a bucket, I can only get part of the leg submerged - the horizontal part of the leg stops me from using a really deep bucket. I can submerge it enough to cover the 2 x 4 water intake holes, and I can just barely cover the horizontal slots/holes above the prop (not sure what these are for). But I wonder if this is deep enough - and I wonder if this is part of the problem why the water pump will not suck up the water? It is difficult for me to keep putting the boat in and out of the water to test and fix it. Any idea if the level of the water around the leg will effect the operation of the water pump?

 

[bruceb58]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

When you are in the water, you are relying on no air leaks in your pump system and that your impeller is working properly. On muffs, the pressure of the hose reduces those issues. I would start with a new impleller and a new housing to go along with that impeller.

When you are in the water, you can remove the hose that comes from the drive for 30 seconds to determine if the water flow is good or not.

When you are on land trying to diagnose with a bucket, it may be tougher as you almost need the water to be as high as the pump so you are assured it is primed. It may work with it just covering the inlet holes but not as likely. When you are on the water, the pump is above water but you have the speed of the boat forcing water into the inlet holes up to the pump.

 

[patrick53]

Re: OMC Cobra 4.3L Overheating when in water, okay with hose/muffs

I had the same problem and was able to blow through the tube which brings in raw water into the manifolds. After several repetitions of this I fired up the boat and the came blowing out. I'm assuming there I released some minor blockage which the pressure from the hose on the earmuffs was able to bypass, but the impeller didn't have the strength to overcome.