1989 OMC Cobra 350 overheating problem

[A1957]

I’m having an overheating issue I can’t seem to resolve. It started after I lost an impeller in the sea water pump.
i replaced the impeller, stainless plate and the pump body sleeve. I removed the drive so I could use a large bottle brush to go through the gimble housing to the oil cooler, removed the oil cooler and used small bottle brushes to clean it out and I ran the large bottle brush through the hoses before and after. I got what I think is most of the impeller debris.
It‘s all back together and when I start it with the water supply disconnected at the thermostat housing and running the engine on a drive muff, the water coming out of the hose rises about 1-2” out of the end of the hose. Less than the manual says it should be.
Did I miss something?

 

[Scott Danforth]

did you back-flush?

my guess you still have bits of the old impeller in the system.

yes, water should rise about 10"

 

[Lou C]

I would carefully remove the impeller housing and disconnect the raw water intake hose at the thermostat housing. Stick a garden hose in that hose and back flush the hose and see if any more impeller bits come out. Then remove the stat housing and test the stat in a pot of hot water, it should start to open at 160*. Then make sure all the ports in the stat housing are open including the bypass port. Lastly have your elbows and manifolds ever been replaced? If not they can get clogged with rust and may need to be replaced.

 

[A1957]

I didn’t feel like I got all of the impeller with just the back flush which is why I disassembled everything. Good thing to because I found more chunks that the hose didn’t dislodge. I did replace the thermostat when I had the housing off to check for debris. The stat is rated for 180, not 160.

I’m 99.9% sure there is no blockage anywhere. It’s been back flushed, forward flushed, and had long brushes run through every passage and hose from the sea water pump to the t-stat housing. So I’m too the point that I’m wondering about why the first impeller failed. Could something be amiss PRIOR to the sea water pump?

Is there a difference in what I should see coming out of the hose at the thermostat housing when running on a hose vs being in the water?

And 10” of water column? Yikes, that double or more then the manual says. I was looking for 3-4”. I’m getting 1-2” at idle.

 

[Lou C]

That is the wrong stat, a 180* stat is too hot for raw water cooling, the standard stat for OMC Cobras was 160*. This is fine in freshwater but in salt water I use a 140* stat now, I was able to find one that fits the same OMC/Volvo stat housing (for a V/P appliction, the 7.4 liter V8).
The true test of the impeller function is with the boat in the water. The water hose provides pressure so it can mask a worn impeller. When I used to use the 160 stats my temps ranged from 155-175 now with the 140 stat it ranges from 140-150.
Make sure that the small bypass port on your impeller housing is open, this allows the impeller to prime as the drive is lowered into the water.
I don't know where the 10" column of water came from the standard in the shop manual I have is 3-4", directly from OMC.

 

[A1957]

The impeller part number I bought was the OEM number from the factory parts book, 0383307. It’s not a Sierra or off brand replacement. My boat is 100% fresh water.
The port on the water pump housing does **** water when the water is turned on so the pump is being primed. And the water coming out of the hose doesn’t have any air bubbles.
Could any of the impeller parts gone the other way and be in the lower drive causing restriction prior to the water pump?
I’m baffled.

 

[Lou C]

Well if you remove the housing to back flush, then hook up the muffs to flush from the bottom up, see if anything comes out of the other port in the impeller mount. I doubt it but I guess you never know.
One thing I always do after replacing a Cobra impeller is run it on the water hose with the cover for the tail end of the upper gear housing off. Then I check to make sure the bypass fitting spits water and I make sure there are no leaks around the impeller housing itself. That o ring gasket can shift out of place when installing and that's what you will see. I use a thick marine grease (OMC triple guard) to hold it in place while installing.
Before the impeller went bad what were your running temps?
I'm wondering how you wound up with a 180* stat.

other causes of overheating are bad water tube gaskets in the lower gear housing. To change these you have to split the upper and lower units. If you get a healty spurt of water when flushing from the bottom up to the impeller mount they are probably OK. I had an issue with marine growth growing on the plastic water intake screen that caused mild higher temps (like 175-185) I could not stop stuff from growing on it (salt water, moored boat) so I just removed it for good. No problems after that. Now I run at a cool 140*, after coming off plane it may rise to 145* but it doesn't even hit 150*.

 

[A1957]

The gauge in the boat doesn’t have numbers, just hash marks. It has always run at about the 1st mark, about where a quarter tank would be indicated on a fuel gauge.
Put the thermostat is not causing low water volume. That the issue I’m trying to resolve.
I did run water with the muff with the pump off and water flowed out of the right hand port freely.
Here is a video of the water volume at the t-stat hog at idle on a muff.

 

[Lou C]

Keep in mind that idle speed will affect impeller output just like it affects oil pressure. It sounds like its idling a bit slower than normal. Set your idle at 600 and then look at it again.
Something there does not add up. You are saying that it ran at the 1st hash mark which is probably 140 or so. But you have a 180* stat, which by design will be the minumum temp you are running at after warm up, unless its not closing all the way. What I have found with the OMC is that if you use an IR temp gun to measure temps, the area right underneath the thermostat housing on the intake manifold reads approx 10* lower than the dash gauge at least on mine it always has.
BTW engine looks immaculate. Fresh water must be nice. Here we are always fighting rust! We have to change exhaust manifolds & elbows at approx 7 years to avoid rust through and big problems (water leaking back into a cyl), and its a good idea to replace the cyl heads at 15 years since they can rust through behind the valve seats, unless you have closed cooling. When I repower this boat it will for sure have a half closed system.

 

[A1957]

The only reason I think it’s a 180 degree stat is that’s what I saw on the internet associated with the part number. I’ve looked further this AM and other sites refer to it as a 160. I’ll pull it back out and check it in a pot of water.
And I’ll check the idle speed and water flow again to make sure it’s where it needs to be.

 

[A1957]

I just talked to the shop where I bought the T-stat and they tell me it’s a 160. They have a 180 option but virtually never sell it. So where on the web it says an OMC/Johnson part number 0383307 stat is 180 degree, it’s wrong.
Phew.
Another shop has suggested that the grommets on the water tube between the upper and lower half’s of the drive may be bad and if so I’m sucking some air and will never get enough flow.
Does this pass the smell test?

 

[hugh g]

I just talked to the shop where I bought the T-stat and they tell me it’s a 160. They have a 180 option but virtually never sell it. So where on the web it says an OMC/Johnson part number 0383307 stat is 180 degree, it’s wrong.
Phew.
Another shop has suggested that the grommets on the water tube between the upper and lower half’s of the drive may be bad and if so I’m sucking some air and will never get enough flow.
Does this pass the smell test?

Yes. I had to replace them one time on my Cobra due to an overheat.

 

[Lou C]

I'd test it as I described above. A bad overheat or loss of cooling water will usually toast one of them.

 

[A1957]

I took the drive back off the boat and split the upper/lower halves. The rubber grommets on the water tube were hard but looked good. I don’t think that was the loss of water pressure.
I put in new grommets, new orings (3) on the little housing around the driveshaft and and a gasket under the housing with the water tube. I also replaced the intake screen while I was in there. The inside of the housing where I could see was very clean. The the bolts came out easy and all in all its in great shape for its age.
It’s ready to go back together but I can’t seem to get the driveshaft and the water pipe to seat. My home made stand let’s the bottom move around too much so I have a buddy coming tomorrow AM to help.
What I don’t have is a pressure or vacuum tester so I’m going to have to wing it and hope I’ve got it sealed up. Fingers crossed.