omc overheating high rpm

[Jerry San Diego]

I have a 3.0L omc with a cobra stern drive pushing a 21 foot Bayliner. I have been having problems with overheating. It does not overheat if I run it under 3000 rpm. After running above 3000 for a few minutes it overheats. Max rpm should be 4200-4600 which it reaches but will overheat. Can anyone help me understand what may be the problem I recently replaced the thermostat so that should be OK.. Would an old or partially melted impeller cause my problem or any can you give me more ideas.<br />thanks.

 

[jlinder]

Re: omc overheating high rpm

The impeller on the cobra is very easy to get to and replace. If you have a question about it I suggest you just replace it.<br /><br />Sorry, but aside from that and the simple suggestion to check the fan belts for slippage I don't have much for you.

 

[Bondo]

Re: omc overheating high rpm

A Half Beat Impeller would produce these symptons.......

 

[trog100]

Re: omc overheating high rpm

its 99% likely to be poor water (coolent) circulation caused by "whatever".. u have already gone for the most obvious and cheapest option.. the thermostat.. the impeller probably comes next.. then possible blockages of some sort.. just keep going till u find the cause..<br /><br />i will probably get ripped into for saying this but being as boats dont go thru lots of warm up short trip cycles u could as a temporary expedient try removing the thermostat completely just to see if it helps.. assuming u wish to keep useing the boat till u get it fixed for real..<br /><br />trog100

 

[rodbolt]

Re: omc overheating high rpm

the t-stat is fine. if it works at low speed it will work at high speed. next do the pump with a complete pump kit. check the pump housing for warpage. if the problem persists check the riser for blockage.

 

[pra100]

Re: omc overheating high rpm

The Impeller (water pump in drive) is very easy to get to and change.One other thought to keep in mind.If impeller is shot,it is made of rubber and those little pieces off of it have to go somewhere when it comes apart.If you replace the impeller and it still over heats there are some small passages that those rubber pieces can get stuck in,such as oil cooler.

 

[jlinder]

Re: omc overheating high rpm

Is there a reccomended replacement interval for the impeller?<br /><br />As Philip A. says, you would like to replace it before it breaks off and possibly clogs passages.<br /><br />If there is a reccomended replacement interval my guess is that yours is overdue, making the impeller replacement a no brainer

 

[Jerry San Diego]

Re: omc overheating high rpm

Thanks to you all for your responses. I will be replacing the impeller and see if that resolves the problem. If not I will be back to get more information as to how to replace the complete pump and check for blockage.<br />Thanks Again

 

[Lou C]

Re: omc overheating high rpm

Like rodbolt says, when you do an impeller on a Cobra do the whole job, replace the gasktets, seal, housing and the metal cage. Make sure the hose that goes from the housing to the upper unit is clear (allows the pump to prime). Make sure also the plastic housing seal is in right, because if it sucks in air when the boat is on plane it could overheat. How many seasons do you have on your manifold and riser? Salt use? that would be the next item.

 

[Jerry San Diego]

Re: omc overheating high rpm

I took the boat out of the watr and looked at the impeller. It looked good so I put it back in. Still have the same problem at hight rpm. How would you check the riser? Just take it of and look at it and see if water flos thru it OK ir should I so something else? Also need assistance on how to determine if there is blockage some where in the outdrive/ Engine- Any assistance is appreciated.<br />Thanks again.<br />Jerry

 

[jlinder]

Re: omc overheating high rpm

Jerry,<br /><br />With all the work to pull it out of the water and remove the impeller, putting it back in was not a good idea.<br /><br />The impellor is a wear item, and not all that expensive. Once you had it out just replace it.<br /><br />Think of it this way - the impellor is aobut $30. If it has a 5 year life span, that is $6/year. If it is 4 years old you have about $12 of life left on it. (Insert yoru own figures to come up with a more accurate cost). Is $12 worth it?<br /><br />I don't mean to sound rude, but I would rather be safe than feel bad if I get stuck or have to cut a day of boating short due to a problem.

 

[Bondo]

Re: omc overheating high rpm

It looked good so I put it back in.

I've got to Agree with Jack, "putting it back in was not a good idea."..............<br />It Doesn't have to be in Several Pieces to be Junk............<br />What about the Gaskets..??.. Did you Reuse Them as well ???<br /><br />I Suggest you Buy a Manual for Your Application,+ Read It............<br />The Manual will tell you Which Hoses come from/go to Where...............<br />By running Tests, disconnecting different hoses in different places, you can figure out Where it's Plugged.......<br /> <br />Good Luck...

 

[fireship1]

Re: omc overheating high rpm

Hello, I agree with Jack. I would have been a good idea to replace it when you had the chance. Hopefully you put it back the same way it came out. If you flip it over and allow the blades to turn in the opposite direction you will have a problem. Impellers tend to wear in and the blades develop a "curl" to them. This is why they loose pressure over time. They don't make a good seal in the pump housing anymore. If you try to reverse them the impeller won't last long, the blades might sear off or you will have worse cooling than you did before. Do yourself a favor. Change the pump completely. Housing, seals and impeller. You already changed the thermostat, so you are good there. Put your hand on the risers when running. If they are way too hot to touch something isn't right. You can pull them off and check for excessive rust, corrosion and clogging with rust and scale. They have two passages that are sperate from each other. One for the exhaust and one for the spent water that cools the exhaust riser and gets dumped overboard. Water should stay in the "wet side" and not leak into the exhaust side or you will have ingestion problems and slowly kill your engine. Impellers, manifolds, risers...all consumable items. Sooner or later you will HAVE to replace. If you boat in fresh water you can get sometimes 10 years out of manifolds and risers. If salt water 5 years is usually tops. Impellers must NEVER be run dry even for seconds or they will be shot! Always use a flush/test adapter (earmuffs) when running on land. And change the impeller every other season. Sorry for such a long post but I wanted to be thorough Boating isn't cheap but your service manual and this form (that features great people)! With a touch of common sense and you can save some serious money and be able to enjoy this wonderful hobby. Skip the maintenance and you will pay dearly! There is no better way to say it. Good luck!

 

[Jerry San Diego]

Re: omc overheating high rpm

Thanks again to all for your input- after reading your posts it does sound kind of dumb to put the old impeller back in and for the low cost to replace it- I should have done so, thanks for bringing me to my senses. I do run it in salt water only. The age of the risers and manifols are unknown to me but probably are pretty old> I have had the boat about 1 1/2 years and it is a 1989. Doubtful the previous owner replaced much of anythng. I will replace the impeller and pull the riser off and look at it. Also check the manual and start checking hoses. It is still hard to understand why it will only heat up at high above 3,000 RPM. <br /><br />Thanks again for all your assistance, will keep you posted when I get it finaly figured out.<br /><br />Jerry

 

[Boatist]

Re: omc overheating high rpm

I bet it will be a clogged exhaust mainfold / riser. When they start to go bad rust and scale flake off on the inside and plug the holes between the manifold and riser. It will have enough flow at slow speed to keep engine cool but at high speed can not pump enough water thru the clog to cool the engine.<br /><br />I have a 1980 OMC Stringer and have changed the Impeller 2 times. Both times when doing other work in the area. Each time I did not see any problem with the impeller, but I never turn the engine with out a wet impeller and I do not run the outdrive is sand or mud. When my manifold went bad I had the same symtoms as you have right now. When I pulled the riser the manifold and riser holes were completely plugged and I do not know how it cooled at low speed but it did. I cleaned the rust and scale out and it ran cool right at 170 degrees. I should have changed the manifold right then but did not. About 6 months later I got a low rpm miss on the number 4 cylinder. Did a compression check and found the cylinder down to 90 psi. This was due to the bad manifold allowing water to seep thru into the exhaust passage. Since the number 4 cylinder is the lowest water ran back to the number 4 exhaust valve and rusted it so it did not seat correctly. Had to replace one valve but all the rest were fine. Did grind and polish all and clean the head up but been fine since.<br /><br />Good Luck

 

[lakelivin]

Re: omc overheating high rpm

Several have already suggested picking up a service manual if you don't have one. I'd suggest you get the OMC OEM one (& a parts catalog) rather than Clymers or Seloc. I've got Clymer's, but needed to get the OEM one as well, and there's no comparison. Wish someone had told me that before I bought the Clymers in the first place. <br /><br />There's a used 1989 one on eBay for $39. I'd of bought it last week, but needed it in a hurry & the sellers were out of town & couldn't ship in time. I bought a new one on eBay for $10 more (I did snag a cheap used parts catalog for $8 though).

 

[trog100]

Re: omc overheating high rpm

a cooling system should be able to pump more water around the engine than is needed to keep it at its proper operating temperature.. the job of the thermostat is to make sure it dosnt do this.. it restricts the exessive water flow and only lets enough go around to keep the engine at its desired temperature.. the thermostat should never be fully open for very long if is it there is a flow problem elsewhere..<br /><br />u have a situtation where the stat is fully open yet for whatever reason there is isnt enough water flow going around the engine to cope with anything more than light loads.. 3000 rpm and below..<br /><br />without going into how often u should replace impellers it dosnt sound like the impeller is the problem in your case.. that leaves a restriction somewhere else in the system..<br /><br />a duff impeller would tend to pump less the slower it goes and probably cause overheating at idle as well as higher rpms.. with a restriction of some kind it dosnt matter how fast the impellor goes only a limited amount of water can get pumped around the system.. in your case enough to cope with light loads but not high ones..<br /><br />the other confusing thing perhaps is that with a car engine high revs and light loads can go together but with a boat they cant.. high rpm means high loads and lots of heat to be gotten rid of..<br /><br />trog100<br /><br />ps.. when u visualize just how restrictive even a fully open thermostat is u can get some idea of the kind of blockage u are looking for.. quite a big one to be honest..

 

[Jerry San Diego]

Re: omc overheating high rpm

I have decided to pull both the riser and manifold and check them out- Sounds like a prudent step and one of the easiest thing to do at this point- besides I can clean them at the same tme even if they are not real bad. Looks like all it should cost is a few bucks gaskets and a little elbow grease. I really ony want to take it to a shop if I have too. Very expensive out here. An hour from a mechanic is about the same as an hour with a lawyer-- Your assistance is invaluable to someone like me-- I do have a minual but at times it is comfusing. <br />thanks again. <br /><br />Jerry

 

[Don S]

Re: omc overheating high rpm

If you are thinking of cleaning the manifolds and risers (the water passages) to remove all the rust and scale, let me throw one point your way.<br />The reason they plug up is rust and scale from the manifold itself flaking off and plugging the passages, this usually means the manifolds are dead. By cleaning them, you stop the last line of defense and warning (Engine starting to get hot) to make them run cool again. The only problem to this is the web of cast iron is thinner and will now get holes in it and probably allow water to get into the engine without any warning.<br /><br />Basically if enought of the inside of the manifold or riser has corroded enough to plug up, it's enough to require replacement.

 

[Jerry San Diego]

Re: omc overheating high rpm

Thanks Don:<br /><br />I will check prices of new ones, they are definately probably due.