1988 305 with alpha 1... Overheating at idle, cools down on plane.

Hi everyone, I've been around boats most of my life but this is "my" first boat. I just bought a 1988 Rinker 240 festival cuddy... 305 with alpha-1. The boat is originally from Michigan and trailered here to Florida a few years ago. It has sat and never touched the salt water. The engine and sterndrive are in really good shape and not rusted so I believe it. The previous owner says the impeller in the outdrive had just been replaced before it was winterized 2 yrs ago. I put it in lake tarpon last weekend and is overheating at idle, but when you get on plane it will cool down. My train of thought is that the impeller in the drive is pumping water up to the engine, but the engine mounted water pump is inefficient, and needing the additional rpms to pump enough water to cool it. I have turned wrenches most my life is an ASE master Tech, But boats are quite a bit different.

Any thoughts as to why it is cool on plane and overheating at idle? How can I test the water pumps to verify correct operation. I pulled the top plate off the thermostat housing, and the thremostat was wide open. I ran the engine to see how much water flow I had at idle (on the hose) .... I did have some flow, but honestly expected more. I'm not one to throw parts at a problem...but I'm leaning towards the engine mounted pump... I assume this pump could not prime itself sitting in the water without the help of the stern drive impeller so the stern drive I pump must be doing the pumping.... Additionally what are the balls and springs in the thermostat housing for?

Thanks so much!

Chris

Replace the impeller in the outdrive.

A rubber impeller sitting in the housing in the outdrive for two years in florida weather is done!!!!

Motor water pumps rarely if ever fail!!!!!!!!!!!!

Impeller cost ~ $25.00

if things are not broken do not mess with them.. curiosity will kill your boat!!!


You are trying to over diagnose a simple problem. replace the impeller. Also look very closely for ZEBRA mussels left behind from Michigan in the water pick up on the lower unit and incomming water stream. (seriuos problem in the great lakes)

I'm a little surprised the impeller would go bad after only 2 years. I change my Alpha 1 impeller usually every 3-4 years without issue.
However, once it starts to get warm at idle, I know it's time.

to kghost's comment above, I also wouldn't rule out the quagga mussel thing. They can block an intake pretty fast if the boats left in the water for long.

if the impeller achieved a "set", the fins would pump poorly at low RPM's but at high RPM's, centrifigal force would throw them out against the pump housings, sealing and improving pump performance.

Ayuh,.... The Impeller is Always the 1st place to look, in Any overheat situation,.....

Hi Chris,
Kghost, Bondo, and the others are spot on with this issue. It sounds about 100% like a typical raw water pick up impeller and/or housing problem. I have been around and owned boats for 38 years, and I have seen this over and over again. I recommend you change the entire raw water pump....the total kit...housing and all...and my bet is that the problem will go away. As mentioned, it is extremely rare for the engine mounted circulation water pump to fail.....Let us know what you do. Good luck!