15hp johnson 1988 no thermostat

I have a 1988 15 hp johnson. I just got the motor and it was overheating after I would run it full throttle for a few minutes. The tell tale turned into steam. So I put in a new impellar the old one was bad I peed better but still had the same problem. I than pulled the powerhead and cleaned out the cooling system there was a blocked port about1/8th in dia. that was totally clogged. I also checked to see if the exhaust was leaking into the cooling system it wasn't. Put everything backtogetherruns great pushes water fine I can go high speed fine but when i lower my rpm the pee stream turns back into steam and I have to turn the motor off and start it up again at an idle to get the pee stream back. The motor does not have a thermostat and i was told that this might be the problem to my overheating because it needs the back pressure of the Tstat in order for it to cool properly. I would like to get some conformation before I dump even more cash into this motor.

Re: 15hp johnson 1988 no thermostat

It sounds like you still have a restriction somewhere. At high rpms you have more water pressure force because the impeller is turning faster. When you shut it down quickly that force stops and your engine empties of water pretty quickly. I personally would not run an engine without a tstat because it always runs "cold" thus using more fuel. You can put one in yourself with a couple of sockets and they aren't that expensive. Make sure that you replace the gasket when you replace the tstat.

Re: 15hp johnson 1988 no thermostat

Did you replace part #39? This seal is important for adequate pressure/flow and much too often neglected.

We will not do a water pump job without replacing that seal.

http://shop2.evinrude.com/Index.aspx?s1=auhbllg7f8p8f216ahk14vpc47&catalog_id=0&siteid=1 Edit: That hyperlink did not fly. Go to your applicable outboard/gearcase to view the waterpump/drive shaft seal.

Did you pull the head. You may be dealing with a bad head gasket as this will compete with -rather against - the water pump.

A lack of a thermostat in that motor will not cause it to overheat.

There is also a cooling port that runs across the top of the block. We have designed a repair to deal with this problem that requires a small hole in the upper left exhaust housing cover and a #14 solid wire. We can get into that later if needed. It is simply sealed with a screw and does NOT require exhaust housing removal.

Re: 15hp johnson 1988 no thermostat

We did not replace that seal. When I pulled off the lower that seal was about an inch above the impellar housing. I just slid it back down and pushed it into the housing. Is it suppose to be a tight fit? I believe I cleaned out the cooling port that is on top of the block. It was packed with crap. But i removed the exhaust housing to get to it. I was going to try to remove the lower today and double chevk the impellar plate and the housing. I will replace that seal as well if i can find the part.