This is my first post here, I've been looking around for the past week, coincidendally since I bought my first boat in many years. I did a compression test and cooling system leakdown test (fwc) before buying the boat and while the compression was not through the roof, it wasn't too bad either. The coolant system was charged to 15 psi, and lost about 2 psi in the 20 mins or so that it took me to do the compression test. I think most of that 2 psi came out the pressure gauge connection to the heat exchanger. No water in the oil, other than a bit of condensation. Put the muffs on the leg and ran the engine, temperature climbed up to the red zone on the boat's temp gauge (this is in the sellers driveway, I didn't have my infared thermometer with me). So I'm thinking the raw water impeller needs to be changed, not that big a deal. So I bought the boat.
First thing I do is drop the lower unit and I can see right away that the engine must have been run with no water through the outdrive, as the plastic water tube is melted, when I took off the upper water pump housing, the impeller was stiff and chipped in places, but not melted, and the upper housing was melted directly below the shaft exit, where there is a groove in the housing. Intake screen is not clogged at all. I blew compressed air up the water tube (with the other end of the hose off the heat exchanger) and it is not blocked. So I get a new upper water pump housing and impeller kit, change all the parts, and put it back together. Hook up the muffs and run the engine, and the temp climbs up to 180 and sits there. This is measured with the infared thermometer, and the dash gauge is reading very close to the red zone. So I'm now thinking that someone put a 180 automotive thermostat in the engine. Pull off the thermostat housing, and there is no thermostat in there. Wonderful. Now I'm thinking I have a much more serious problem. Pull off the heat exchanger end caps, and it's not totally clogged, but not that great either. So I remove the heat exchanger, as well as the exhaust risers. The risers look absolutely perfect, no corrosion or blockage anywhere. So I get the heat exchanger hot-tanked and pressure tested at the rad shop, and re-install everything, including a 165 thermostat. Run the engine on the muffs again, thermostat opens at 165 exactly, engine temp still goes to 180. So I remove the exhaust manifold endcaps to have a look, other than being pretty black, they look fine. Remove engine circulating pump, thinking it may be an automotive version and the impeller corroded, but no, it's the marine version, with bronze impeller and it looks to be in fine shape. I blew compressed air through the block from the ports for the water pump, seemed like good flow through the block. So I put everything back together, filled up (again) with fresh 50\50 mix coolant, and ran it again. 180 again. It doesn't matter if the engine is idling or running at about 2500 rpm, it stays at 180. Obviously, this is unloaded. The temp at the thermostat housing is 180, at both manifolds 200-210, at risers 120, at top side of heat exchanger 180, at bottom of heat exchanger 120 (where the engine circ. pump pickup hose is). So it seems to me I have 120 deg water going into the engine block, heating up to 180 by the time it gets back to the heat exchanger, and that the heat exchanger is doing it's job, and that I have enough raw water getting to the heat exchanger in order for that to happen. There is a good amount of water coming out of the exhaust port in the propellor as well.
So I put the boat in the water, curious to see if the temp stayed at 180 loaded or not. Got the boat off the trailer, and stalled repeatedly trying to get into reverse. I had tested the shift system on land, and it seemed to work fine, but once in the water not so much. So now I am replacing the lower shift cable, all bellows (even though they don't seem bad at all, I'm in there anyway). That should fix that problem, but I still haven't tested the engine temperature under load. 180 seems high to me, but maybe not, as automotive engines idle at that temp, although granted most have a 180 or 190 thermostat. I'm thinking that with a 7psi cap on the heat exchanger, the coolant shouldn't boil until about 220 or 230, and maybe I'll be okay ?
Again, I have checked all hoses and ports for blockage, hot tanked the heat exchanger, replaced the raw water pump housing and impeller, checked the engine circulating pump. Nothing seems to be wrong with any of it. The heat exchanger is 4" diameter, which I see is about the right size for a ford 302. A 5" heat exchanger would provide more cooling, and perhaps I am going to have to look at replacing mine with a 5" ?
Any thoughts, suggestions or ideas would be much appreciated.