81 OMC Inline - Overheating at High RPM's

[pls5792]

Changed impeller, Checked Head and replaced exhaust manifold, Removed/Replaced Thermostat, checked recirc. pump. and Motor still overheats at 1.5k+ rpm's. Boat will run for an hour at normal temp. at lower rpm. <br /><br />Checked inflow at hose connecting intermediate housing and exhaust manifold. Checked flow from exhaust manifold to thermostat housing. Noticed there is no flow directly to thermostat housing from intermediate housing. Could this design cause the water to "pre-heat" excessively? If "inflow" was to bypass the exhaust manifold and connect directly with thermostat housing would this cause additional problems?<br /><br />Can the recirc. pump be functional but yet not strong enough? Any other ideas?

 

[Boatist]

Re: 81 OMC Inline - Overheating at High RPM's

I am a little confused. I have a 1980 4 cyl inline 3.0L 181ci 140 HP motor. On my motor the water comes from the impeller up and thru the intermediate housing then to a rubber hose to the thermostat housing at the front of engine. This hose supplies water to the engine and also flows arrossed the thermostat to the exhaust manifold. The circulation pump sucks water from this area and pumps into the engine as well as circulating water in engine. Ecess water flows acrossed the thermostat to the exhaust manifold then riser and out the outdrive. When the thermostat opens circulation pump suck cold water and pumps into block and hot water flows out the thermostat and thru manifold, riser and out.<br /><br />Hose coming from the intermediate housing to thermostat houseing should have a good steady flow of water. This means impeller is ok and pumping water. Hose from thermostat houseing to exhaust should have the same flow, when thermostat closed will be cold water but when thermostat open will be warm water. If the flow is good at this point try running a hose from the thermostat houseing over the side instead of too the exhaust manifold and see if motor cool right down. Do not run too long as have no cooling to exhaust manifold. If the motor still runs hot with good flow here then either the circulation pump is bad or the thermostat is bad. The circulation pump input hole could be cover with a gasket or plugged with something. If it is plugged then no cold water will enter engine and no hot water will come out of the thermostat when it opens. If engine cool right down then have a plug in exhaust manifold output to riser or in the riser ristricking output flow.<br /><br />Good luck and let us know what you find. With all you have done sounds like been working on for a long time. Love to know what your find.

 

[pls5792]

Re: 81 OMC Inline - Overheating at High RPM's

Thank You for the prompt response, "Boatist."<br /><br />The scenario you described on the hoses of your boat is correct, according to my manual. My Boat is a 1983, nonetheless, when viewing schematics I noticed that my exhaust manifold did not match the year. Consequently, searched numbers stamped on Head Cover and found it to be a 1981. Unfortunately, this year has a different manifold that brings water across the manifold before entering the thermostat housing. I was thinking this may not be necessary because water will still flow from the block, as you described. <br /><br />I figure to take it one step at a time. Your description of the recirc. pump being plugged is a good possibility. Of course, my visual inspection did not consider this possiblity (duh). This is the only part left to Replace and being lazy and cheap, it wast the only part that was not replaced....YET. I know that I should have started with the obvious. Will keep you posted, thanks again.

 

[pls5792]

Re: 81 OMC Inline - Overheating at High RPM's

Just changed recirc. pump and still overheating. Steady flow through thermostat and to recirc. pump but not sure of flow beyond that. Can anyone tell me where water should be exiting from the stern drive? How can I check the "out" flow through the block? How about the "out" flow through the stern drive?

 

[Boatist]

Re: 81 OMC Inline - Overheating at High RPM's

Might be able to help but need to know if it is a stringer and also hp. I am gessing it is a 120 hp stringer but different than the 140. If you had the OMC service manual you would have saved a lot of time and money by not replacing all those good parts.<br /><br />Water exits all of the 1980 stringer models the same way. The input is different on the 120hp.<br />Water coming out of the head and thru the thermostat goes thru a rubber hose to the exhaust manifold. Then goes thru exhaust manifold up thru riser down to the bell houseing thru the bell houseing to the intermediant houseing thru the intermediant houseing out to the outdrive then down and out the prop. If engine is tilted will run out of the imtermediant houseing and not go down thru the outdrive to the prop.<br /><br />Does your motor have one or two hoses from the thermostat houseing to the exhaust manifold??<br /><br />From the front of the engine are you sure the hole to the left of the thermostat is open, not covered by a gasket?? This is where water is sucked into the block. If this hole is blocked will get no cool water into the block.<br /><br />If you pull the output hose off the thermostat houseing (hot water) and run it over the side do you have good flow and does engine cool down fast?? With this hose off can only run for a short while as no cooling to exhaust manifold.<br /><br />If low flow or no flow there what happeneds if pull input hose off the thermostat houseing. Do you have a good strong steady flow??<br /><br />All above only applies to a 1980 omc stringer. Probably same for the 1983 stringer but if it is not the stringer then may not apply at all. <br /><br />Good luck

 

[pls5792]

Re: 81 OMC Inline - Overheating at High RPM's

Yes, it is a 120 hp stringer. I have been self educating myself on this boat since none of the local shops will touch it. The last mechanic took over 2 months to change the exhaust manifold. I figured I could learn the entire boat and motor and stern in 2 months.<br /><br />Unfortunately, I have the "OMC Manual" but it is not specific to the 120 hp. especially the exhaust manifold. Upon reviewing the schematic for parts on the 120, I noticed their to be a "heat exchange assembly." This fits between the housing cover on the manifold. The housing cover has 2 hoses connecting to the thermostat.Furthermore, the hoses connect to the same side of the thermostat but "over and under."<br /><br />Another problem I found was that the picture on the schematic did not match what I saw on my motor. I had a mechanic change the exhaust manifold and it seems that he cut and plugged the existing "assembly" instead of replacing it. I am going to try and replace this item also. Then I will proceed with the other flow tests.<br /><br />The exhaust manifold is chambered and I am guessing that one side is incoming and the other is outgoing, but what is in the center? This is where the "heat exchange assembly" fits in. What is its purpose?<br /><br />Never say die!

 

[Boatist]

Re: 81 OMC Inline - Overheating at High RPM's

The heat exchanger is to keep salt water out of the engine and it changes everything as to how cooling system works. I have not delt with a heat exchanger yet.<br /><br />Manual I am looking at is for a 1980 OMC 120hp. I belive it is the same as your 1983 but you will have to be the judge.<br /><br />Ok on the 120 impeller cooling water comes up thru the intermediate houseing then goes into a rubber hose and into the riser. Then thru the riser to the engine side of exhaust manifold thru the exhaust manifold and out thru hose closest to engine. If you pull this hose off should have a good flow of cold sea water. Normaly this hose would go to top hose of thermostat houseing then water would go down into the circulation pump hose and into the block. From the block would head up into the engine head and out thru the thermostat when open, into the bottom hose on thermostat houseing. From there would go thru bottom hose to the farthest hose away from block on exhaust manifold, thru exhaust manifold side away from block into the riser down and thru intermediate houseing and out there or if engine down then down thru outdrive and thru prop.<br /><br />Since you have a heat exchanger will work different. First thing that I would check with a heat exchanger is is the engine full of coolent. Just like on a car here must have coolent in engine for it to cool. The heat exchanger is your radiator and works just like a auto radiator except pump cool water thru it to cool instead of blowing air thru it like on a car. If have any hose leak loose freezz plug (any engine coolent leak) then will run out of coolent until leak fixed and engine refilled.<br /><br />Again I have not worked on a heat exchanger but here is how I think it should work. Cold sea water should come from the front of exhaust manifold and go to a fitting on heat exchanger, cold water should flow thru cooling pipes in heat exchanger then out back to the exhaust manifold fitting side away form engine then thru manifold and out.<br /><br />Hot engine coolent should come out of thermostat houseing lower hose thru the heat exchanger where cooled then back into the thermostat houseing upper hose then sucked into circulation pump and pumped into bolck up thru the head out the thermostat when open and then starts same trip again.<br /><br />Hope this all makes since and helps you fix your overheating problem. Also hope will come back and tell us what fixed your problem. I know you will find it soon.<br />Good Luck

 

[pls5792]

Re: 81 OMC Inline - Overheating at High RPM's

This "Heat exchanger thing" is small and fits into the exhaust manifold where the water lines connect. The name was derived from the parts schematic.<br /><br />Nonetheless, I tried the flow tests and when engine was fully lowered the water flowed out the prop. fairly steady.<br /><br />While checking the water flowing through the hoses I noticed the temp guage was redlining at about 220 degrees. Not thinking I continued testing the water flow and noticed the water was hot but it did not burn me. Crudely Checked the temperature sending unit but need a digital ohm meter for better check. Any ideas for another test. I was going to try heating up motor again but inserting a cooking thermometer into thermostat housing for temperature comparison.<br /><br />Sooner or later, One way or another.

 

[pls5792]

Re: 81 OMC Inline - Overheating at High RPM's

Yes!!!!!! Finally, the overheating seems to have stopped! I believe the original problem seemed to have been the temperature sending unit. <br /><br />For the sake of closure, I will attempt to recount the history of this "overheating." Four months ago, I bought this boat from a neighbor. He had just bought a 24' cabin cruiser and couldn't bother troubleshooting the 18' 83 Cobia with new overheating problems. He told me that he had "just replaced the stringer when the overheating started. I already done the stern but she still is overheating at higher rpm's. Probably needs a new head." For $750 dollars with the manual, I figured it would be best to "close the deal" as quickly as possible before he had second thoughts. This is all I had to "go on." Having only owned a small john boat with 5hp kicker, I figured it best to send my boat to a pro. Unfortunately, this was my first mistake. The shop started by telling me "we don't ususally work on these"; they kept the boat for 2 weeks only to tell me "the tilt and the starter works." Still clueless, I took my boat to a "friend." He said he does not work on those but "might know somebody." Another week passes and my friend's friend calls to ask the history and I tell him the story. Another week passes and a different friend of my friend calls for some background, too. Now, I am starting to wonder. Well this guy takes 2 weeks with all payments before repair to discover the exhaust manifold is cracked and will need to be replaced. Takes another 4 weeks to do the work. When I finally get out on the water, the overheating at high rpm's starts again. This is when I decide to become a boat mechanic and start troubleshooting for myself. <br /><br />I have rebuilt several VW's and a few Lawn Mower. Furthermore, I am a engineer by profession and extremely mechanically inclined. As a began with the impeller, I immediately realized why nobody will work on these. In addition, the guys at the various marinas acted as if I had some type of plague, trying to repair this boat. This attitude only makes me more committed. Now, I feel as though I could rebuild both motor and stern, should I desire. It would take some time but I could do it. <br /><br />Anyway, back to the boat, I think the temperature sending unit went first with the previous owner. He did not have in his thermostat and when his motor overheated the guage was innaccurate causing overheating and the exhaust manifold to crack. He replaced the impeller, too, but found the "overheating" still present. I replaced the exhaust manifold, not the head. The motor at this point was not overheating but the temp. guage was bad. I replaced the unit to find the engine now running at about 140-150 degrees and back to normal with the thermostat. A lesson well learned and a path I am happy to have traveled. <br /><br />It sort of felt naked not knowing my boat the way I know my car and other "toys." Sincerest "Thank you" to Boatist Member 8614 for your patience and insight.

 

[Boatist]

Re: 81 OMC Inline - Overheating at High RPM's

Glad to see you got it going.<br />Now time for the fun.