Yamaha Sterndrive 5.7l Overheating

[Marty082270]

Hi everyone, I bought this boat new in 1991. I know the cooling system fairly well. I have replaced the raw water pump impeller 6-10 times over the years. This year I replaced the raw water pump impeller and took the boat out. It ran fine with no cooling issues. We ran the boat for over an hour. We took it out the 2nd time ran it for over an hour, no issues. I took it out the 3rd time and the temp went to 200, I shut it down. I removed the raw water pump impeller, looked fine but put a new one in. Removed the power steering cooler, found a few rocks and impeller parts, cleaned it out and checked flow, it is fine. Took it to the lake, it ran fine for 30 minutes, stopped for 10 minutes, started it up and it overheated. Took it all apart, all hoses off, checked flow/backflush from raw water pump to drive unit, flow seem unrestricted. Replaced the engine circulation pump and removed the thermostat. Took it to the lake and it overheated within 5 minutes. I'm assuming my next step is clear hoses to see if I'm getting air. My plan is to try this next, any suggestions would be greatly appreciated.

 

[Scott Danforth]

have you checked the flow out the exhaust?

overheat is always a lack of cooling water flow. either no flow in..... or no flow out.

 

[Marty082270]

have you checked the flow out the exhaust?

overheat is always a lack of cooling water flow. either no flow in..... or no flow out.

I used a garden hose when I had the hoses off and water came out of the back of the boat when putting the hose on each exhaust cooling hose inlet.

 

[Scott Danforth]

That is not checking. Did you at least use a 5 gallon bucket and a stop watch to determine flow in GPM?

 

[cyclops222]

What did you use to get the accurate 200 F temperature readings ? The temperature instruments may be going bad also. If truly 200 F exhaust ? You will see STEAM coming out of the exhaust water when idling. Been there.

Few rocks & impellor parts ?? How can a thermostat handle that stuff & work ?