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OMC Cobra 3.0 bursting water circulating hose

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dchable2007

New member
I have an 1992 OMC Cobra 3.0. In preparing for the season I changed the impeller and thermostat. When I put in the water and started it, it ran for approx 30 seconds and water sprayed out of a slit in the water circulating hose between thermostat housing and water circulating pump. I put on new hose but I could see it expanding and shut off before it could burst. I checked the impeller and it was seated properly. I pulled the intake hose at the thermostat housing and it gushed water when the engine started. I pulled the new thermostat and checked it on stove and it opened fine. I pulled water circulating hose and water pumped out of thermo housing. I loosened belt from altenator and the water circulating pump turns freely and feels like bearings are okay. Looking for ideas? Have new circulating pump on order to try.
 
Most likely a blockage somewhere. Try backwashing the system. disconnect the hose where the problem starts and watch for any debris flying out.
 
Thermo in wrong? What kind of hose did you use?When you changed the impeller ?Did it comeout in pieces or whole with all the vanes? There is a blockage somewhere. The motor has a built in bypass and it shouldn't build up pressure. Is it pumping any water out the drive?
 
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Master mechanic said:
How you gonna backwash the riser on the boat ?
Click to expand...

Ha good point. I'll be more specific with the steps that I took when looking for my clog. Maybe it will help you out?

Ran the boat and no water was being pumped through, so I disconnected the line coming in from the impeller, ran it again, had water pumping. Great!

Connected that, disconnected at the thermostat, ran it, no water. Crap!

Disconnected the line from the impeller again and backwashed water from the line right before the thermostat, and I got a bunch of gunk that was clogged at the oil cooler. After that I decided to disconnect more lines and run water from my house through the hoses to see if there was anymore clogs. I pulled the thermostat out and ran water through every hose. I opened up the bleed valves on the block to ensure water was coming through them. I disconnected the lines at the manifold and backwashed water through them, then ran water into the manifold to ensure it was coming out. Everything looked great!

Hooked everything back up, ran the boat, problem solved.

I guess what I'm saying is to go further down the line and disconnect hoses at the next connection and run water through. If it doesn't go through, your problem lies somewhere in there. If it does flow through, it's probably further down somewhere.
 
If I'm following you, you are talking about the circulating pump "suction" hose.
The large diameter hose that joins the T-stat housing port to the Circ pump.... correct?

A flexible vane sea water pump is not capable of generating much pressure, so I'm not quite understanding how this little pressure could threaten this suction hose.... especially with the circ pump pulling suction on it.
We normally do not see this hose expand unless it's in a closed loop system, such as a Marine Closed Cooling system, or an automotive cooling system.
In these systems, this hose may see up to 10 to 12 psi or so.

However, I would agree with the guys..... I'd pull things apart, and see if I had a major restriction somewhere.
Exhaust manifold to elbow sea water transfer ports, for example.


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.........................

You and I were both posting at exactly the same time, so I did not see your recent post.

dwillems said:
I guess what I'm saying is to go further down the line and disconnect hoses at the next connection and run water through. If it doesn't go through, your problem lies somewhere in there. If it does flow through, it's probably further down somewhere.
Click to expand...
Yep.
Always follow the sea water path from the point of entry completely on through every component, until it reaches and exits the exhaust system.

When we do this, instead of blaming the impeller or thermostat, it's much easier to locate the problem. :D


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RicardoMarine said:
If I'm following you, you are talking about the circulating pump "suction" hose.
The large diameter hose that joins the T-stat housing port to the Circ pump.... correct?

A flexible vane sea water pump is not capable of generating much pressure, so I'm not quite understanding how this little pressure could threaten this suction hose.... especially with the circ pump pulling suction on it.
We normally do not see this hose expand unless it's in a closed loop system, such as a Marine Closed Cooling system, or an automotive cooling system.
In these systems, this hose may see up to 10 to 12 psi or so.

However, I would agree with the guys..... I'd pull things apart, and see if I had a major restriction somewhere.
Exhaust manifold to elbow sea water transfer ports, for example.


Thanks for the info. It is the U-shaped hose from the thermo housing to the water circ pump. Hose may have been old and weak. I replaced hosewhen I started engine, after 20 seconds it started to expand so I shut it off. Just so I understand correctly, water comes in through impeller hose from transom to the thermo housing. If the engine is cold, water bypasses thermostat and exits through exhaust manifold. Thermostat opens, water passes through thermo housing to water circ pump (U-shaped hose) into engine block, back through thermo housing and out exhaust. Tonight, I will run house hose into exhaust inlet (from thermo housing) to see if water comes out of outdrive.
Click to expand...
 
Thanks for the info. It is the U-shaped hose from the thermo housing to the water circ pump. Hose may have been old and weak. I replaced hosewhen I started engine, after 20 seconds it started to expand so I shut it off. Just so I understand correctly, water comes in through impeller hose from transom to the thermo housing. If the engine is cold, water bypasses thermostat and exits through exhaust manifold. Thermostat opens, water passes through thermo housing to water circ pump (U-shaped hose) into engine block, back through thermo housing and out exhaust. Tonight, I will run house hose into exhaust inlet (from thermo housing) to see if water comes out of outdrive.
 
Thermostat in correctly. I initially repaired u-shaped hose with rescue tape but it burst immediately. Then tried a 1-3/4 ID flexible radiator hose just to test, it expanded so I shut off engine. Ordered a used hose from ebay because part no longer available new. It is in good shape so I installed it but don't want to run engine until I research more. Is the bypass you refer to in the thermo housing? Seemed to pump water out drive in the water but not when on garden hose.
 
dchable2007 said:
Is the bypass you refer to in the thermo housing?
Click to expand...
In terms of the sea water supply, yes, the raw water cooled engine T-Stat housing does divert most all sea water onto and out the exhaust. This is because engine cooling demands don't quite equal the heat removal capability of the sea water being supplied.

What cooling water the engine does not require, leaves the T-stat housing and onto the exhaust manifold(s), and eventually out with the exhaust.

Engine cooling demands are determined by the open position of the thermostat.
IOW, the circ pump charges the block with cooling water, and it can only exit on an "As Needed" basis, predicated by the thermostat.

If we sent all supplied sea water through the engine first, and if the thermostat is doing it's job (holding back coolant), the exhaust would become starved of cooling water.
The mixing chamber within the T-stat housing that takes care of this.


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Last night I disconnected the hose from thermo housing to the exhaust manifold on the thermo housing end. I attached a garden hose and ran it and water passed through the exhaust and out through the outdrive. I don't think it is blocked. I decided to start the engine again to see what happens. I loosened the clamp on the U-shaped hose so it would not burst. Ran okay for about 30 seconds and then water spayed from end of intake hose from transon at the thermo housing. One thing I didn't mention before is that engine has been making a squealing and rattling sound when starts up. I thought it was the belt but thought I should mention. I still think it is water circulating pump.
 
Move the filter . Mine is on the bulkhead. Same engine as yours.

The hose is steel reinforced the one your using i don't know.

I burst the u-shape hose also my t-stat was frozen closed.
 
I replaced the water circulating pump and it seems to have done the trick. Still need to get into the lake to test thoroughly. Thanks for all your suggestions.
 
Can you give us an update with the new circulating pump? What was the condition of the old pump? I have exactly the same problem bursting the hose and I took out the circulating pump but looks fine to me. I will start a new posting but it would be nice to see your problem was finally solved with the new pump.
 
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