Howdy all...
I had posted here before, I acquired a boat with a freshly built 350 mercruiser that, after a few days of scratching my head, finally pulled to find out the crank was smoked due to oversized bearings and oiling issues... whoops. Anyhow, got a fresh motor that was built properly, stuffed it in, rebuilt the carb, went through everything and it ran good. I took it out on the lake with a 17" pitch prop (15.75" dia), the boat is a 1982 four winns and the outdrive is a pre-alpha mercruiser. Anyhow, with that prop it would max out at around 4000rpm / 35mph on plane and run around 165 deg F. When idling however, it would overheat.. we hit it with an IR gun and the heads were around 170 or so by the spark plug and the intake manifold was around the same, so I questioned the sending unit and gauge.
Since the motor was such a whack job, I decided to pull the lower unit and found that the impeller was 'set' in shape, although not burnt up terribly. Either way, the bottom housing seemed to have melted a little, so I replaced the entire assembly (bottom housing, upper, impeller, etc), threw a 21" prop on it (for faster top speed) and took it out tonight on the lake. Now, it does the exact opposite--it overheats on plane but runs cool when idling. I noticed that it would run around 150F or so, but as soon as I get on it to get out of the hole and plane, it slowly will start creeping up until I lay off it, I never let it get past about 215 or so. In an attempt of desperation, I tried to remove the thermostat, but it wouldn't come out, so I "beat it open" with a screwdriver. In this state, it would run at about 130 deg F at idle and still overheats quickly when I start to plane--but I did put about 2 hours on it idling and had no problems.
The block is an 010 chevy 350, good shape, it had antifreeze in it prior to being put into the boat and was flushed--the current cooling system is raw water only (oh, and this is has ONLY seen fresh water, great lakes). I checked the power steering cooler/exchanger, and it's completely clean (new). I also checked to make sure the water hose from the transom is clear and that the passages in the water neck are clear. The only thing out of ordinary that I found was that the drain plugs on the bottom of the exhaust manifolds had "grime" in them, and were plugged. I jammed a screwdriver up there and they cleaned out and started to drain properly.
As far as I can tell the water pump (belt drive) and impeller are working properly, as I would expect with them being new. Any ideas? I'm wondering if gunk in the exhaust manifolds would cause this... I plan to remove the risers tomorrow to check. I also wonder if the ball spring/valve assembly on the front of the water neck is not working correctly, and if this could also cause the overheating? To note, it only takes about 30-45 seconds when I put load on the motor and open the throttle before it starts to creep up rather quickly. I think the higher pitch prop is putting more load on the motor and would explain why it's overheating now prior to when it had the 17" prop on it and bad impeller.
I had posted here before, I acquired a boat with a freshly built 350 mercruiser that, after a few days of scratching my head, finally pulled to find out the crank was smoked due to oversized bearings and oiling issues... whoops. Anyhow, got a fresh motor that was built properly, stuffed it in, rebuilt the carb, went through everything and it ran good. I took it out on the lake with a 17" pitch prop (15.75" dia), the boat is a 1982 four winns and the outdrive is a pre-alpha mercruiser. Anyhow, with that prop it would max out at around 4000rpm / 35mph on plane and run around 165 deg F. When idling however, it would overheat.. we hit it with an IR gun and the heads were around 170 or so by the spark plug and the intake manifold was around the same, so I questioned the sending unit and gauge.
Since the motor was such a whack job, I decided to pull the lower unit and found that the impeller was 'set' in shape, although not burnt up terribly. Either way, the bottom housing seemed to have melted a little, so I replaced the entire assembly (bottom housing, upper, impeller, etc), threw a 21" prop on it (for faster top speed) and took it out tonight on the lake. Now, it does the exact opposite--it overheats on plane but runs cool when idling. I noticed that it would run around 150F or so, but as soon as I get on it to get out of the hole and plane, it slowly will start creeping up until I lay off it, I never let it get past about 215 or so. In an attempt of desperation, I tried to remove the thermostat, but it wouldn't come out, so I "beat it open" with a screwdriver. In this state, it would run at about 130 deg F at idle and still overheats quickly when I start to plane--but I did put about 2 hours on it idling and had no problems.
The block is an 010 chevy 350, good shape, it had antifreeze in it prior to being put into the boat and was flushed--the current cooling system is raw water only (oh, and this is has ONLY seen fresh water, great lakes). I checked the power steering cooler/exchanger, and it's completely clean (new). I also checked to make sure the water hose from the transom is clear and that the passages in the water neck are clear. The only thing out of ordinary that I found was that the drain plugs on the bottom of the exhaust manifolds had "grime" in them, and were plugged. I jammed a screwdriver up there and they cleaned out and started to drain properly.
As far as I can tell the water pump (belt drive) and impeller are working properly, as I would expect with them being new. Any ideas? I'm wondering if gunk in the exhaust manifolds would cause this... I plan to remove the risers tomorrow to check. I also wonder if the ball spring/valve assembly on the front of the water neck is not working correctly, and if this could also cause the overheating? To note, it only takes about 30-45 seconds when I put load on the motor and open the throttle before it starts to creep up rather quickly. I think the higher pitch prop is putting more load on the motor and would explain why it's overheating now prior to when it had the 17" prop on it and bad impeller.