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1990 OMC Cobra 3.0 overheating @ low speed

guido2

guido2

Regular Contributor
The impeller has been replaced, engine was rebuilt with new water pump because the original engine had overheated too. The boat owner didn't replace the impeller until after the rebuild. At mid to high speeds the water temp stays below 160. At low speeds the temp will rise over 200 unless the operator speeds up to cool it down. The original exhaust manifold is still being used. Any suggestions? Thanks in advance.
 
Ok, thanks. Do these exhaust manifolds ever crack from overheating? Or are they pretty "bullet proof"?
 
maybe and the head gasket can go also, Take the hose off the t-stat housing the long one from the transom, put water to the drive and start the boat hold the hose straight up you should have 12-15 inchs of water. 3-4 seconds is all you need quick shut off the motor.

Good water to the t-stat the blockage is at the t-stat or after.

No good water to the t-stat then it's before the t-stat.

Capiche?
 
Ha! You know a little Italiano. Good blood.....we love to work hard. Will follow your tests and check back. Oil looks good. Thanks, Tim
 
Going to lake to test now,will give you report. Garden hose would "force feed" so I will idle for a moment at the dock and test impeller output there. Have been swamped with work, wish I could concentrate on one thing at once. I'm primarily an outboard and automotive repair, but doing this job for a friend who has been frustrated with it. Thanks so much, Tim
 
Ok, idled @ dock and raced up to 180. Put in reverse and worse. Won't overheat with slow foreward.....go figure. Checked output and pretty low. At idle about a slow garden hose flow. Figured bad pump so back home and took apart. Owner had replaced impeller but housing was a bit melted by the stainless steel insert. Also noticed a bit warped where it seats on baseplate. Blew back through supply line with air and great flow. Also blew down from pump inlet and had good flow of air out water intake screens. My guess is a bad pump housing and replace impeller again. Seems the stainless steel pump housing insert is distorted too. Impeller has suffered some already. What do you think? Thanks, Tim
 
Listen make believe i just slapped you lightly on the back of the head. The impeller was all busted up at one time? If yes i bet it was cause the impeller housing was melted. It was melted cause someone ran the drive with no water going to it. Capiche?

Sooo look for the pieces ! Take the hose off the transom the pieces could be clogging that hose or fitting on the other side of the transom.

No pieces there then they might be in a heat exchanger if you got one follow the hose.

T-stat housing overflow hole might be clogged up. not clogged up or the t-stat if clogged up or frozen clossed or something.

Then follow the thought here, Take the hoses off the engine mounted circulating pump. Only other small holes are in the riser.. If not finding no pieces then ya gotta take off the riser, it good ? Clogged up or fugazied ?

Say that it's good then stick ya face down the tube could be a flapper / shutter fell down or melted off and is blocking the exhaust.

Make believe i'm waving both hands at you, go find the clog, and post back.

Good luck keep us posted if you get in a jam, and or fix it .
 
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Thanks.....so to avoid another head slap, I'd better not only replace the pump but find the blockage. Ok I'll look closer. Seems as soon as you give the motor a little rev, it will pump water.....otherwise low output.
 
Ok, new kit installed and pre lubed with lite synthetic oil. Tested at dock with output hose off at thermostat housing. Result, no water flow at all. What the h_ _ _ did I do now?
 
Is the little crack in output side hose a problem? Whats happening here? Had low water flow before, now nothing. Checked for any blockages or leaks but all ok.
 

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The hose kinked at the transom inside? I'm bing nice today so no kick. Try putting water backwards down the long hose from the t-stat back cobble together the garden hose and the motor hose, and see what water comes out the back.

Jeez your getting a bum rap wit this chittt.
 
Thanks, great water flow both ways and when water is applied to the motor imput, it comes out under stern drive unit at base of transom. No thermostat, the owner took it out.
 
In my third photo you see the hole in the little hose. Is that just a pressure relief hose when the thermostat is fully closed? C'mon techs......this guy has been fighting this overheating problem for 2 years and its desroyed one 3 litre motor already. HELP!
 
When you put the water pump kit in did you replace the impeller again? The reason I ask is because there are questionable ones out there. Last year I went through 4 impellers on my twins, one brand new one in the spring quit working the following weekend the other stopped working mid-summer. Yet when you pull them they look brand new? Anyway just a thought if you still have the other new one to swap them.
 
Jeez i wish i could touch it . You think the water is pushing or sucking thru the slit huh? How much could that f--king thing be ?

Got a dealer near you ?

I mean if everything else is good ...
 
I used the new impeller that came with the kit. I could try the old one and use it in the new housing to see what happens. The old impeller does seem more flexible.
To "Chief" I say no I'm not too worried about the little slit in the relief hose, but this boat has the best dealer in northern Minnesota puzzled too. The owner has put thousands into this problem already. Hell, I should be able to figure it out with a little help from "my friends". You want me to go at it on my own?
 
The split in the tube in the third picture, is that the pressure hose?

The reason I think it is getting enough water to cool the engine at speed is that the prop is forcing the water into the inlet, and the engine's own water pump is turning sufficiently fast enough to suck the necessary coolant past the split in the pipe, and into the engine.

That split is way too big to allow sufficient flow to go through the hose, unless there is the vacuum from the Water Pump upstream to assist.

Bruce.

PS. I may be wrong, but I love pictures.

PPS. I don't even know what a Cobra looks like, the marine version that is, so you can tell me to shut up if you like.
 
Taz alot like a alpha.

That hose got to go like a posted how mush is that f--king hose. Actually i got so many stikin size hoses i would have cut one to fit.

And if i didn't have it my friends got all kinda size rubber hose, i would have went over and cut a piece.
 
Ok, the split is in a tiny hose venting from the output lobe on the water pump. The little hose terminates in the lower housing thus venting a small amount of the pumps output back to the lake. For the "Devil" don't be afraid to give advice, I appreciate any I can get. Sometimes even poor advice can spark a solution.
I did check pressure between water pump outlet and thermostat housing and there are no leaks on the output side.
There has to be a solution. Hell, we put a man on the moon..........didn't we?
 
Also I hooked water supply from house to engine intake at thermostat housing and it cools perfectly. Could there be a problem with pump rotation direction?
 
Looking at your pic above with the impeller inside the housing you've got it in the right way and it should turn CCW, have you changed the impeller again yet? Also (dumb question) but are you absolutely sure that the water pump shaft is turning? Very rare but have heard of the shaft breaking inside, everything still works fine except for the water pump. If all else has failed I would be removing the drive and separating the halves to inspect the entire water passage area.

cobra.jpg
 
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