I have a 1989 OMC Cobra 4.3L stern drive. I am experiencing an over heating problem (I never let it get past 200) and have done the following: Checked impellor and it looked brand new. Checked flow from water line to thermostat (from impellor) and was flowing well while idling. Checked Thermostat housing for blockages and installed new thermostat. Pulled water pump and inspected impellor and looked for blockages – none found. Flushed water through engine with garden hose under thermostat housing and confirmed there was no blockage as water flowed out both ports for the water pump. I feel I have traced the complete cooling route and am baffled why I still am seeing the temperature rise. I can sit at idle at the dock and it won’t over heat – stays right @ 150 degrees. I’ve even taken it out of gear and revved it to 2k rpm and still the temperature stays at 150. I can take the boat out and run it a slow speed 1- 1.5k rpm and it will stay at 150. But as soon as I get it on plane and am running over 2k rpm the heat starts to ramp up slowly. Once I being it back down to a slow speed the engine will immediately start to cool back to the 150. I’m really scratching my head here. I had a boat mechanic tell me to take the thermostat completely out...I haven’t tried that yet...what are your thoughts about that? Any help or suggestions anyone can offer will be greatly appreciated. Sorry for the long post

rob - welcome to the forum. you can try to remove the thermostat and see if that helps. however, it sounds like maybe not enough water is making it's way through the system to cool the engine. when was the last time the risers were removed and checked for blockages? is this a salt or fresh water boat?

Bruce,

Thanks for the quick reply. I dont think the risers have ever been removed. This is a freshwater boat. How hard is that job? I have a manual (actually an online manual from Seloc). I didn't see that in any of the trouble shooting points.

Yea, but when the boat is idling isn't when your having your problem, is it?

Or another way, flowing "well" at idle doesn't mean it flows "well" at a higher RPM while under load.

And do you know for sure that "well" is what it should be?

2suns...thanks for the reply. Yes you are right this isn't where I'm having the problem, but what I tried before I took it out. I do know what it should be at idle...the rule of thumb is a 2-4" column of water coming out of the hose pointed straight up...I had about 2".

Any suggestions?

How old is that impeller that looks brand new?

rob - the risers are usually not that difficult to inspect. on a salt water boat that is raw water cooled, yuo can only expect 3-6 years out of the risers and manifolds. for a fresh water boat, you should get much more, but you still need to check them every few years and i'd say yours are due. removing the risers is not very difficult (it' just basic wrenching) but getting the nuts and bolts to turn after so much time will be a challenge. use a little penetrating oil first and let it soak for a bit. once the risers are removed, check the interior passges for corrosion or blockages and visually inspect the manifolds for the same. a flashlight comes in handy here. it may be possible to have them cleaned, rather than having to replace them. before you start, though, get part numbers and check to see if there are gaskets that will need to be replaced and have them on-hand before dissassembly. if you;re doing the work yourself, the parts are not that expensive IF you can find them. i had a 1993 OMC King Cobra that i could get risers for, but manifolds were only available from sweden and by the time both were changed, it was a very expensive repair bill.

There is also the possibility that the circulating pump, (water pump on the front of the motor) has a broken impeller shaft. This will usually turn at slow speed but not at faster speeds. It is not real common but does happen. Any drips at all from under the front pulley?

I pulled the water pump and checked the shaft and impellor...they looked good.

2suns: I've never changed the impellor...owned the boat for 10 years

Bingo!

The lower unit was overhauled (not by me) a few years ago and it may have been replaced then. I will however replace it now and see if it makes a difference. I'm really starting to feel it's an obstruction problem in the exhaust though.

If you haven't changed the impeller in 10 years, the blades will be nearly hard as plexiglas, and won't flex. Found a pic online, not the best but will help me explain---

Notice how the impeller is off-centered in the housing? In normal operation this is a 'positive displacement' pump. To accomplish this, as the impeller rotates the blades must be able to freely flex in order to keep constant contact with the edge of the housing. If the blades are old and stiff as yours will be, what happens is the blades compress in the tight spot but won't flex out in the wide area, and the blades can't seal themselves. The result is that your positive displacement pump is now a centrifical pump, and not a very efficient one! It will still pump SOME water (such as enough to cool an engine while idling) but at higher speed there's not enough impeller blade to move enough water to keep the engine cool. (but given time the blades will eventually flex back, and when you inspect it, it can look "brand new"...)

Not to say you don't have some rust blocking water in the risers, but my money is on the impeller...



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I just 'doctored up' the impeller pic-- if the blades are stiff and won't retract in operation, this is kinda what it might look like inside the housing, the blades would stay bent and won't seal, drastically reducing pumping efficiency...

yep...I'm going to replace it.

Thanks for all the advise guys...

What FR said.

I've ran into this on other stuff, not boat related. A pump will perform fine, until you put it under a load, like in your case, increased RPM. Put some head pressure on it and the volume of water moved falls flat. So while your 2-4" stream of water out of your hose looks good, as soon as you get a restriction, the impeller may not have enough oomph (technical term) to move the same amount of water. Make sense?

I suppose if it was mine, I'd replace the impeller and t-stat first. All the riser stuff that Btoran mentioned is also a good idea, too.

Good luck. Let us know what you find.

Yup, two to three years is alot to ask of an impeller. And for the little bit extra it costs I would get the OEM replacement. Some of the aftermarket impellers have had issues.

If you have an 89, there are no separate risers to check, those are the one piece 'batwing' manifolds that OMC used from 86-90 or so. You can take em off, and look at the exhaust opening, there are 4 openings that the water exits from, make sure they are nice and open and there is no flaking rust. If they are plugged with rust you have a problem, because they are not available anymore, unless you find a set on ebay. There is a conversion using Ocso manifolds and risers being sold by one of the marine retailers that will work if you need it but it's pricey. I have em on mine, put a new set on in 2004 and have a spare set for my next change in few years. If I still have he boat at the next change interval (here in salt 4-6 years) I'll go with the Osco conversion.

Other point, DO change the impeller (very easy on a Cobra) BUT make sure that the gasket on that impeller housing seals well, if it doesn't, when the boat is on plane, the upper gear housing is out of the water and the impeller housing can suck in air and lower the water output. When you check it at idle, the upper housing (where the impeller lives) is under water so you would not notice this. It does match your symptom of OK water flow and normal temps at idle and low speed, and overheating when the boat is on plane.