Yamaha 25 Overheating/outboard 101

[nodima]

I have a 1993 Yamaha 25hp 2 stroke engine that has an overheating problem. Upon taking the engine out of winter storage the tell tale was not flowing. The water pump was replaced. We took the boat out to a local lake, after a short period of time, the engine lost power. At first it seemed likely to be a fuel issue, so I went over the entire fuel system and carbs, which did not fix the issue. <br /><br />Now I suspect that the issue is cooling, as the motor will work correctly again after a cooling off period of a couple of minutes, but only for a short time. Also I noticed that the first run is the longest, and the last time out I also noticed an indicator light was on while this was happening.<br /><br />Last time out, the tell tale water was very warm. From reading the manual and posts on this board, I know the tell tale is not necessarily a good indicator of anything but flow. If I have a good flow of water in the tell tale, does that mean that water is going through the powerhead, or does the tell tale split off prior? <br /><br />When the water pump was replaced the impeller was completely gone, it is possible that pieces of it are now downstream in the cooling system.<br /><br />I installed a new T stat tonight, but have not yet run the boat to see if this is the issue. I did test the two T Stats in a pot of water, and the new one seemed to work better than the old one, but not so much better that I am convinced that this will be the solution. <br /><br />Assuming the new T Stat is not the answer, what is the next logical step to take to solve this issue? Can I flush the cooling system? If so, how does one go about this task?<br /><br />Thanks<br /><br />nodima

 

[jim dozier]

Re: Yamaha 25 Overheating/outboard 101

It is certainly possible that the impeller pieces are lodged somewhere and blocking flow. It is also possible that the overheating episodes have started a leaking headgasket.<br /><br />I would recommend that you drop the lower unit again, and remove the thermostat and stuff a garden hose with a rag for packing into the thermostat housing to backflush the powerhead. Put a bucket under the water tube (where the water pump attaches) and check for any pieces of old impeller that might flush out. Then I would clamp a garden hose on the water tube and flush it out the other way. Put the thermostat cover back on (temporarily without the thermostat) and with the garden hose still clamped on the water tube check the quantity of flow through the engine and out the exhaust.<br /><br />Also do compression test on the engine.

 

[nodima]

Re: Yamaha 25 Overheating/outboard 101

Follow up. Removed the lower unit, flushed the OB from both top and bottom. Had several pieces of impeller fall out. I don't think I got it all however. When Flushing from the T-Stat end, the pisser flows fast and furious, but less water than I would expect comes back down the water tube and does not seem to have a lot of pressure pushing it. When flushing upward from the water tube, the pisser flows slower. <br /><br />Both ways water seems to be coming down from the holes alongside the exhaust going down the shaft. <br /><br />My guess is that I have not gotten all of the pieces out. <br /><br />Any other things to try before calling the local dealer and waiting a month or so? <br /><br />Thanks again

 

[jim dozier]

Re: Yamaha 25 Overheating/outboard 101

Sounds to me like there is still a blockage of some sort in there. If all was OK I would think that you would have a good pisser stream and lots of water coming out of the exhaust with the hose clamped on the water tube (at least as good as a normal engine pissing at idle). I think its good that you were able to flush some of the pieces out though. <br /><br />On some engines there is a grommet at the top of the water tube where it joins the powerhead that can get piched and block flow. Check with the exploded parts diagram or somebody who knows this particular engine. It might be worth removing the water tube if possible and replacing the grommet. Maybe you can put it back together and check it again. If not it may be time to pull the head and check the cooling passages and install a new head gasket. Pulling the head is relatively simple if no bolts break off. Use a torque wrench to tighten the head bolts to spec.

 

[nodima]

Re: Yamaha 25 Overheating/outboard 101

Thanks for all the help, Jim. I went back to it again (third time is the charm) and was able to dislodge more pieces of the impeller, and now have normal flow throughout the system.<br /><br />FWIW--I could not fit the male hose fitting into the T-stat housing, so I cut the hose, then built up the outside of it with electrical tape. This allwed it to wedge into the housing, and enabled me to get more pressure into the system, and ultimately fixed the issue. <br /><br />Took it out last night for a spin on the lake, and it worked beautifully.<br /><br />Thanks iboats.com!!!

 

[jim dozier]

Re: Yamaha 25 Overheating/outboard 101

Way to go! It often seems that determination is the key to success with outboards. Its not usually rocket science that gets it, just elbow grease.

 

[dajohnson53]

Re: Yamaha 25 Overheating/outboard 101

Don't ya love it when things work out!

 

[rodbolt]

Re: Yamaha 25 Overheating/outboard 101

would be even easier to do periodic maint instead of repairs .<br /> good luck and keep posting

 

[nodima]

Re: Yamaha 25 Overheating/outboard 101

LOL. Yup. Gotta agree with you, rodbolt. This is the first engine I have actually owned myself, despite being around boats for ~30 years. I consider this a 'lesson learned'. Despite a bit of frustration, this gave me some much needed confidence and understanding of how this OB works, and that is worth a lot. Now I have to go find some stripers. <br /><br />nodima