OMC cobra Cooling- good flow to little to no flow

[rmlansberry]

Hi all- I am looking for insight on the cause of a problem I have with a Chevy 305 V8 with OMC cobra outdrive. I started winterizing my boat in the driveway the other week. Had the hose hooked up to muffs and had the boat running for about 10 min. I had good flow (or what I believe compares to good flow by seeing others' videos online of boats on muffs) of water going through the system and coming out under the outdrive. The engine got up to temp and stay there for several minutes as I was working the fuel stabilizer through the fuel lines. I turned off the engine to swap antifreeze for water to run through the system. Long story short, I got sidetracked in the garage for a few minutes. When i came back out, i had my friend on the boat start the engine (even though no water or antifreeze was hooked up). Boat ran for about 20 seconds dry before I realized what was going on so we shut it down. I hooked hose back up to muffs with water and started engine. Did this for about 20 seconds and noticed no hot water coming out the return after passing through the cooling system, so we shut it down. Thermostat never went above stanard operating temp or 160 or 180 (forget exact guage number). My thought was I burned up the impeller and that was reason for no flow. Took boat to mechanic next day bc I was traveling for work and gone all week and an overnight freeze was expected before i would have a second chance to winterize at home. Told mechanic what happened and asked for new impeller and winterize. Mechanic checked impeller and said it was actually ok and not missing significant pieces that might potentially be lodged. After installing new impeller (housing and plates were fine) mechanic noted poor flow still. Pulled water hose from t-stat and found it to be sputtering or sometimes no flow. Said reason has to be bad seals in the outdrive between upper and lower unit because its sucking air somewhere. Said its a 3-4 hour job and a pain to pull and split outdrive and change seals in the OMC Cobra. Also said might open a can of worms splitting outdrive as its an '88 (freshwater boat). Asked me if I ever overheated all summer and I said no, never... Mechanic said likely because outdrive is sucking water from bad seal area in lieu of air when on lake preventing overheating. I'm going to monitor on the ramp next spring with submerged drive down and pull t stat hose again to check flow. Mechanic also said flow seemed to get weaker as engine ran longer and got hotter. Questions are why would this be? Why did I have good flow before i accidentally ran it dry for 20 seconds, and then lost flow, when impeller was fine? And new impeller didn't help? Could running dry for that short of time damage something else that might be causing this problem? I'm curious if this situation was the same as over summer when I operated without any issues... And not sure how anything else could have been damaged as a result of my stupidity that would cause poor flow? Any estimate on cost of replacement seals and how long the repair would be? I'm worried next spring when I back trailer in, start, and pull hose off therm its gonna still be poor flow and need a pulled outdrive repair. Any idea why I didn't overheat, even when on plane, this summer if the pump flow was the same as it is now? Thanks for reading and any comments. This will haunt me all winter as I just stored the winterized boat today.

 

[Scott Danforth]

Do not try to suck antifreeze up thru muffs. You need to drain the block and manifolds. If you want to add antifreeze then, use a funnel thru the t-stat lines.

Running the impeller dry for 20 seconds toasted the impeller.

 

[rmlansberry]

Thanks for the response Scott. That's how the winterize procedure ended up being completed. Could running it dry for 20 seconds have hurt the seals in the outdrive? Like I said, I've never had overheat problems over the summer but had poor flow even after new impeller install. New impeller install performed by a certified boat mechanic. I'm curious if flow will be OK when entire outdrive is submerged under normal operating conditions, because I've never had an issue before.

 

[Lou C]

No Title

I've owned a Cobra for a while and have seen some of the issues you can have with the impeller/cooling system. And I really doubt, that you running it 20 min on the hose caused those seals on the water tube to leak. I had a bad overheat on the water, and even then, those seals were still OK.
I have split the Cobra to clean out marine growth in the lower unit water intake area and to check those water seals (salt water, not easy, got a system/trick to doing it see the pic below...) and its not a hard job. Fresh water boat should be easy.
I think you need to do some more investigating and make sure the impeller was installed right, and that there are no blockages in the lower unit or the hose that runs from the transom mount to the thermo housing.
Your problem was actually a blessing in disguise. If you were able to suck in the AF, if your thermostat did not open all the way, you might have had a block full of mostly water and just a little AF. And could have had a cracked block/heads in the spring.
Back to the problem. When you de-winterize the boat, you need to check the impeller. What I do is when running it on the water hose I leave the plastic cover off the end of the upper gear housing and look around the impeller housing. If there is any water leaking around it, that o ring seal is not sealing and you are losing water and its sucking in air. There is a little bleed hole/nipple on the impeller housing, that must not be clogged, it helps the impeller prime. When you run it, pull the little hose off the nipple, make sure water squirts out. If these 2 points are OK then you need to go further. I would remove the impeller housing and then test for flow. Hook up your muffs, hold them tight to the lower unit and get a helper to turn on the water. You should see water spurting out of the opening in the impeller mount on the right hand side. If not, it could be those water seals but I doubt it. Next, take a garden hose in the boat and disconnect the hose that goes from the thermo housing to the transom mount, at the thermo housing. Stick a garden hose it it and have a helper hold it in. Then you turn the water on and go to the rear of the boat, water should be spurting out of the left hand opening in the impeller mount. This tells you if that hose and the power steering cooler are clear. You might also look at the passages in the thermo housing just to be on the safe side, the internal rust sometimes makes them smaller, but not likely on a fresh water boat.
Next take a good look at the impeller, the housing wear plate and the cage in side. I would use a genuine BRP/OMC housing, cage and impeller. Sometimes the after market ones do not work as well. Sometimes that little cage shifts just a bit and blocks some of the openings. If the wear plate is scored, the impeller will not pump well. It is not a hard system to repair, honestly a lot easier than a Merc Alpha where you have to split the drive just to replace the impeller.

 

[rmlansberry]

Thanks Lou. It's gonna bother me til Spring. I don't think there should be a block from t stat hose and PS cooler to impeller housing because the old impeller didn't disintegrate and get sucked up the cooling tubes. Worth checking for a block anyway. Just blows my mind I didn't have this problem all summer, then I ran it dry without water to muffs for 20 seconds, and then lost flow. Perhaps it's been poor all along but fine in normal operating conditions (I.e. Lake). I hope that's the case. I plan To first flush the system at home. Next, take to lake and back down the launch ramp, lower the leg with boat still on trailer, and disconnect hose to t stat, start engine, and check for flow. Crossing fingers ... But then curious if I'll lose flow running wide open on plane (again never had a problem in summer pulling a tube etc).