volvo 125B over heats out on plane

[barmahn]

I have a 83 byliner with a 125B volvo motor in it. The boat runs great when not on plane and runs great when out on plane. My issue is that the boat will overheat when out on plane. Here is what I have all replaced in it to try and fix it. I have replaced the raw waterpump and housing, the elbow on the 270 outdrive, the hose from the transon to nipple, the 3 o rings on the lower unit. If not on plane the boat will run a cool 190. I just had the heat ex changer cleaned as the guy said that there was a lot of crap in it. He said that should fix the problem. I just thing that it is not picking up the water once it is out on plane. When I tried to run the boat in a tub that goes up half way up the lower unit it wont suck the water up. But with the boat in the water it will suck it up. Take the cap off of the heat ex changer where the screen it there is no water when out on plane. Can a heat ex changer cause this issue. The boat mechanic seems to be just wanting to throw more money at it as it seems to me. I have had a few people tell me that they have had this issue with no water pickup and they put a thru hull pick up and did away with the pickup from the lower unit and solved their issue. Any other ideas?????

 

[kenny nunez]

When the engine overheats does coolant start coming out of he heat exchanger? The only other thing I can think of is your exhaust manifold could have a restriction not allowing the raw water out.

 

[ChrisinPHX]

"When I tried to run the boat in a tub that goes up half way up the lower unit it wont suck the water up. But with the boat in the water it will suck it up".

You still have a leak in the leg. I would say the water level in the tub isn't high enough. When in the tub, mark the water line with a sharpie. Then when the boat is loaded and at rest in the water at the lake, take a sharpie and mark the water line again. It will certainly be higher. Your air leak is somewhere in between those lines.
It's sucking air when in the tub from above that low water mark. When at idle or going slow on the lake, the water is above that low water mark enough for it to suck in water. Going fast, the leak is once again exposed to air essentially breaking that vacuum draw of the water, and that's why you aren't drawing in water.
I'd bet if you just installed a thru-hull you'd be fine. Next time mine leaks, that's what I'm doing.