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relyt

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Have a 3.0 OMC cobra 1990. The temperature is 3/4 high on gage immediatly. After 15 -20 min engine gets rough in idle. Last year my boat overheated badly, block was red hot around plugs, black smoke. So hot, the gas would ignite and keep a couple of pistons shooting. Whats the bad news. lol
 
Boy, something is definitely wrong here.

What did you do to the engine last year to cure the overheating?

First thing is a compression check, then an internal coolant pressure test.

Are you in Salt or Fresh water, Raw Water or Closed Cooling system?

Bruce.
 
trying to find out if there is anything I can do, to stop the over heating. The gauge goes about 3/4 of the way up fast, Buttends to stay there. The over haet started after I let it idle for about 10 min in the water. Black smoke, banging and then wouldnt turn off. Kept shooting.
 
Wouldn't turn off sounds like pre-ignition, that is from overheated carbon deposits in the Combustion Chamber, acting like glow-plugs.

The overheating could be caused by restrictions within the cooling passages, stopping the cooling water from actually getting to the hot-spots within the engine.

Black Smoke indicates to me that it is running extremely rich.

Bruce.
 
When you replaced the Impeller, were there any vanes missing? If so, they could be inside the water passages, blocking everything.

Sounds to me like you need to strip the whole cooling system down, including the engine, and physically check each and every orifice, passage and chamber, for any obstruction, and don't forget the Exhaust Manifolds and exhaust system.

The blockage, if there is one, could be at the start, middle, or even the end.

Bruce.
 
There were vanes missing from the impeller. If / when I start the tear down, should I replace most seals at the same time? Thanks for the feed back Bruce.
 
I would be replacing seals where I could, but when pulling down, try and locate the missing parts of the impeller.

This way, you will know when to stop, and start putting it back together. With the Vanes, it is like reassembling a Jigsaw.

You will be surprised just where they go. When I stripped mine down, there were vanes missing, but as I was totally rebuilding the engine, and the leg, I never did find any vane pieces. They must have simply become ingested, and exited out the in small pieces, out of the system. I thought I might have found one, but, not a scerrick.


Bruce.
 
the_tassie_devil said:
I would be replacing seals where I could, but when pulling down, try and locate the missing parts of the impeller.

This way, you will know when to stop, and start putting it back together. With the Vanes, it is like reassembling a Jigsaw.

You will be surprised just where they go. When I stripped mine down, there were vanes missing, but as I was totally rebuilding the engine, and the leg, I never did find any vane pieces. They must have simply become ingested, and exited out the in small pieces, out of the system. I thought I might have found one, but, not a scerrick.


Bruce.
Click to expand...

i've checked some of the lines , no vains. I just changed the plugs and I noticed starting from the left; plug 1 and 2 had a lot of oil on them and 3 and 4 where very dry. What do you think?
 
Sounds to me like there is a real need for a total rebuild.

Badly oiled plugs is a sign of bad Oil Control Rings, and having some good, and others bad, this is a result of internal problems.

First thing I would be doing before stripping down would be a Compression Check. You did do a Compression Check didn't you?

Bruce.
 
That has been my feeling all along. I was hoping something else. My "Mechanic" didn't do a compression test. He should have, anyways!! I believe its time for a new one. I appreciate all the info and I will be getting that compresion test done. Hopefully not to bad.. Thanks again
 
Do you think its wiser to get a mechanic to do compression test or am I able to do it? Are the tools needed easily accsessable?
 
Anyone can do a Compression test, but basically you need two people. One to hold the gauge, and the other to operate the Starter Motor, via the Ignition Switch, or starter button.

The cheapest gauges are ones you simply press into the Spark Plug hole, and hold whilst the engine is being cranked over at least 4 rotations.

All Plugs must be out, and the coil lead removed from the coil, and the throttle held fully open the whole time.

Removing the coil lead saves nasty shocks, and the throttle held open saves fuel entering the cylinders to give false readings.

After you have done the "Dry" test, squirt a few squirts from a pump oil can into the cylinders, and do the compression test again.

This is the "wet" check, and readings gained from this test, when compared to the "Dry" test, will indicate if Valves are leaking, Compression Rings are not sealing, etc.

Have fun, and once you have completed the tests, you are on your way to becoming self-taught.

Bruce.
 
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