OMC Cobra 3.0 Litre overheating

[Staffan]

I've just become the proud owner of a boat with an OMC Cobra 3.0 Litre (1987)with a stern drive. At low speed everything is fine, but revving up to 1800 rpm and above gives an overheating situation which I have tried to locate the cause of without success.<br />I have:<br />- Put in a new impeller + gasket<br />- Checked the rubber tubes<br />- Taken the manifold end and 'knee' off, changed gaskets & checked for free flow and leaks between the exhaust flow & water ducts, cleared the ducts from rust flakes & debris.<br />- Removed the thermostat (out of stock at the dealers) <br />- Checked that water is coming into & out of the system.<br />- Tightened the drive belts to optimal position<br />- Checked for water in oil - none<br />- Put in new spark plugs ( old ones were incorrect type)<br />I live in Sweden and my boat is in the Stockholm archipelago. Water temp around 20 degrees C. We are having great summer weather, and I am longing to make a trip with my family in our 'new' boat.<br />Any tips?<br /><br />------------------

 

[Navigator]

Re: OMC Cobra 3.0 Litre overheating

Staffen,<br />What is your compression on each cylinder? You may have a dead or badly scored cylinder that is generating the heat.<br /><br />Nav

<br /><br />------------------<br />Sold the Force and bought my Honda...Oh how I miss those days of Screaming at my Outboard.

 

[tcube]

Re: OMC Cobra 3.0 Litre overheating

Staffan,<br /><br />Check out: http://www.sterndrive.com/cgi-bin/forum2/config.pl <br /><br />They have a forum deidcated soley to OMC sterndrives with many postings about Cobras - to include overheat problems.<br /><br />Semper Gumby<br />tcube

 

[Staffan]

Re: OMC Cobra 3.0 Litre overheating

Thanks for the tips. I will follow on the link you sent me.<br />The engine sounds OK, no misfiring or irregularities. I guess the cooling ducts in the block may be clogged by rust flakes. The water here is slightly salty, which promotes rust, I guess.

 

[Staffan]

Re: OMC Cobra 3.0 Litre overheating

Thanks for the advice Nav, I've been away for a business trip, but when I came back I checked the compression - <br />150/150/145/148 which I hear is within acceptable limits/variance per cylinder.<br />I have now decided to hand over the problem to a workshop. I don't feel confident enough to take the rest of the engine apart. I feel more aquainted with the engine now which is good for future (touch wood) breakdowns...<br /><br />------------------