94 yamaha 150 overheating

Hello and thank you has anyone who tries to help. I have a 1994 Yamaha 150 outboard that began overheating. A little backstory first, I've had this boat for about 4 months and it has always ran perfectly until a couple weeks ago when my lower unit gave out and I had to limp it back to the dock. I found another used lower unit that I was able to purchase. Being a dummy and not thinking to look over the water pump parts I threw that lower unit on to my motor and cranked it up. Once running it took about 25 or 30 seconds to start peeing and then in less than 2 minutes the water coming out of The Tell Tale what's steaming and too hot to touch. I immediately shut it down as soon as that started and pulled the lower unit back off. Once I got the housing off of the impeller I saw where are the rubber on the impeller had been scored up like it was run dry for a short amount of time. I ordered a new water pump kit and replace the impeller, and the base plate in gasket. Once it was all back together I fired it up and the same thing is happening. Once again I pulled off the lower unit this time to find nothing is wrong with the impeller and everything appears to be working fine. I pulled off the water jackets and they were all very clean which came as no surprise because supposedly this motor was completely rebuilt less than five years ago. I also bought two new thermostats and replace them which make no difference in this issue. I'm hoping someone can give me some advice, maybe something I'm overlooking so I can get this situated get back on the water. I should also note that there is steam or white smoke coming out of the vents about the prop, and it's a lot more then I've ever seen in the past. The other thing I want to mention is that the Woodruff key appeared to be filed down a little bit by whoever worked on it last, and I was unable to remove it. When putting the impeller on to the key it seems to fit on properly but I am unsure if there's any possibility Indian color can be slipping on the drive shaft do do not fully engaging the Woodruff key and causing not enough water flow?

It's possible that this has everything to do with changing the lower unit, and it's also possible that this has nothing to do with that (because you've only owned the boat for 4 months.. so this is a problem that may have come up even if you hadn't replaced the lower unit).


Considering the age of the engine, the length of time you've owned it (which is short), and the overheating combined with the white exhaust smoke, the very first thing I would do is check for good compression in all the cylinders. It's a cheap and quick test that could save you a ton of time and money here. In general, what you described sounds like a blown head gasket or some other kind of internal damage (which would mean you want to look for a new engine rather than spend anymore time and money fixing up this one). But if compression is ok, then there are a few other things to check:


Look for clogs and kinks in the cooling lines just to be sure, but this is usually a bad thermoswitch or a bad CDI unit (assuming compression in all cylinders checked out ok). I would definitely buy the service manual from Yamaha, because that will tell you how to test the electrical parts, and how to do the compression test.
Good luck.

I thank you very much for the response. Yesterday afternoon I took my lower unit to my boat guy and as soon as he walked up to it and got to look at the impeller he told me that it was shot. I proceeded to tell him that I had just put it on less than a week ago and that the engine had been barely run do to it still not cooling properly. Once he pulled the impeller off the shaft to inspect it he asked me where I got it from which, I told him I got it off Amazon as a kit. He proceeded to tell me how the impeller I got was junk and got me a Sierra impeller he had in stock. I went home and put it all back together and Wallah everything was working just as it should. Engine still running great, cooling system running all well again with lukewarm water coming out The Tell Tale and a nice strong stream. So if anyone is reading this after purchasing a water pump kit online that is not a Sierra brand or Genuine Yamaha parts don't try to save a couple bucks because everything else is garbage and will cause you headaches.

Dahoff89 said:

I thank you very much for the response. Yesterday afternoon I took my lower unit to my boat guy and as soon as he walked up to it and got to look at the impeller he told me that it was shot. I proceeded to tell him that I had just put it on less than a week ago and that the engine had been barely run do to it still not cooling properly. Once he pulled the impeller off the shaft to inspect it he asked me where I got it from which, I told him I got it off Amazon as a kit. He proceeded to tell me how the impeller I got was junk and got me a Sierra impeller he had in stock. I went home and put it all back together and Wallah everything was working just as it should. Engine still running great, cooling system running all well again with lukewarm water coming out The Tell Tale and a nice strong stream. So if anyone is reading this after purchasing a water pump kit online that is not a Sierra brand or Genuine Yamaha parts don't try to save a couple bucks because everything else is garbage and will cause you headaches.

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Very good.

Good to know that my next impeller change doesn't have to be the expensive Yamaha OEM kit. I used a Sierra oil filter, but hadn't tried their impeller yet.

Yep! My boat guy told me to use genuine Yamaha or Sierra parts from here on out and it would be fine. Can't wait to get back out on the water this weekend!