OMC 4.3L & Cobra Cooling System Questions

e016188

Cadet
Joined
Jun 16, 2009
Messages
27
I?m hoping to get some experience feedback.

I have a 1991 I/O OMC 4.3L with a cobra drive. The water temperature gauge seems to be running a little hotter than in recent years. The temp gauge is rock steady at engine idle (160-F) and runs up to 180-185 F at WOT.
Question #1. Should I be concerned about indicated WOT temp?

I purchased and installed both a water pump kit (sierra pn 18-3348) and thermostat kit (sierra 18-3653).

I did pull the water hose from the transom at the thermostat housing inlet to run the OMC manual outlined water pressure head test.
The engine speed at idle is ~ 750-800 rpm.
I hold the water hose level with the top of the spark arrester as outlined in the OMC manual.
I use a set of muffs and garden hose attached to the outdrive.
I see a pressure head of water at or slightly less than 2 inches.
Question #2. Do I have the correct setup to run this test?

I pulled the water pump housing and impeller from the outdrive.
I plugged my garden hose into the transom to thermostat housing hose at the thermostat housing and used a hose clamp to hold the garden hose in the transom to thermostat housing hose.
I turned the garden hose on and see water flowing freely from the discharge water pump port on the outdrive.
I re-installed the water pump housing without the impeller.
I turned the garden hose on and watch water flow freely and evenly from both water intake ports on the drive lower unit.

Observations/Conclusions.
I think the water flow from the water intake ports to the thermostat housing looks ok. I think I?m at the lower end of OMC manual defined press head at the thermostat housing inlet, provided I have the correct engine speed to run this test.
I?m thinking the indicated operating temp is at the thermostat open temperature and maybe the cooling capability is reduced at WOT. I?m thinking I need to look at exhaust leaks into the cooling water supply and the circulating pump and temperature sensor.

Any recommendations/thoughts or systemic problems with OMC cobra cooling systems?
 

bruceb58

Supreme Mariner
Joined
Mar 5, 2006
Messages
30,559
Yes, your temp is too high.

Doing that test while on muffs is pretty worthless. You need to have the boat in the water to do it properly.

If this is a saltwater boat, check your manifolds and risers for blockage. Check your thermostat outlets that go to the manifolds for blockage.
 

e016188

Cadet
Joined
Jun 16, 2009
Messages
27
Thanks Bruce,

The boat has never been in saltwater.

I did inspect the thermostat housing when I replaced the thermostat. I do see some internal discoloration and some corrosion. . As I recall, the ?hole size? in the thermostat housing to each exhaust manifold is ~.375 to .45 inches in diameter. Most definitely bigger than .25 inches.

What would you recommend next?
I purchase power steering oil cooler to thermostat housing see through hose to look for exhaust gas leak into cooling water supply?
I start to look at engine mounted water circulating pump.
I verify water temperature sensor is functioning properly.

I have owned/used the boat for 9-10 years and have never had to work cooling system issues or problems. I don?t know if this provides insight in typical time/use related wear or maintenance problems.

I did pull the drive from the pivot housing ~ 5-6 years ago.
I replaced the transom support bearing.
I replaced the exhaust bellows.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
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49,967
in the water at 1500 rpm or above, the raw water pump should fill a 5 gallon bucket in about 15-20 seconds.
 

bruceb58

Supreme Mariner
Joined
Mar 5, 2006
Messages
30,559
Have you ever done a measurement with an IR temp gauge? If not, pick one up and measure your thermostat housing and/or where the temp sender is. See if it matches what your dash gauge is telling you. Also, measure what your exhaust risers are at.
 
Last edited:

e016188

Cadet
Joined
Jun 16, 2009
Messages
27
Thanks Bruce/Scott,

I now have additional ideas to troubleshoot operation. I will let you know what I find.

Jim
 

HT32BSX115

Supreme Mariner
Joined
Dec 8, 2005
Messages
10,083
The engine speed at idle is ~ 750-800 rpm.

Howdy,

Your idle RPM is a little high too (assuming your tach is correct.........many are NOT) While you're verifying your temp gage, get another tach and verify it too.

It should be 550-650. Adjust your RPM at idle closer to 550 and your clutch dogs will last longer.


Regards,

Rick
 

e016188

Cadet
Joined
Jun 16, 2009
Messages
27
Hello,

I did get time to work on the boat tonight. I did check engine idle speed and I did set at ~550 rpm. Idle speed was up at 850 rpm.

I did borrow an IR gun from work. I will buy one. Is the water temperature sensor mounted in the top of the circulating water pump? See the attached picture.

I turned the ignition key on. The temperature gauge indicates about ~125 dg-F. The engine was still warm from being run earlier in the evening. When I disconnect the lead wire to the sensor mounted in the top of the water pump the gauge does not change. When I connect the lead wire for the sensor to group the indicate temperature does not change. I'm wondering of this is the temperature sensor? If not, what is the sensor for?

Jim
 

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HT32BSX115

Supreme Mariner
Joined
Dec 8, 2005
Messages
10,083
Howdy Jim,

That is a temp "switch". Usually used to activate either a light or buzzer indicating overheat. Your temp sender is on the intake manifold to the starboard side of the t-stat housing.

Have a look at http://epc.brp.com select sterndrive and find your year and model. You can look at parts illustrations and numbers there.

Cheers,

Rick
 
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