Cooling maxed out

[sonador]

I recently removed the thermostat on the A4 in my 1978 Catalina 30 and closed off the bypass tube from the T-fitting. This should make all of the coolant run through the engine block and therefore deliver maximum cooling. I've been running the engine at ~2000 RPM and the temperature reads ~180. If I rev higher, then the temp goes up. So, it appears that I have no reserve cooling capacity and am therefore worried that when the seawater temperature rises this summer I'll be overheating.

Here's a few details of the cooling system. It was converted to fresh water cooling early in it's life so has not had the insult of seawater running through the block. Very recently I changed the coolant pump impeller for routine maintenance. And lastly, this cooling system is set up with the modification that provides hot coolant to a hot water tank heat exchanger directly from the thermostat housing output. This configuration blocks off the bypass tube to the thermostat housing and reroutes it through a 3-way fitting. This setup is described by Don Moyer somewhere on this website (I'll find the citation if anyone is interested).

So here's my laundry list of what might be wrong. I'd like to diagnose and fix this as quickly and cheaply as possible. Any suggestions on the order in which to tackle this problem will be greatly appreciated.
1. Coolant pump isn't pumping coolant at fast enough rate. How can I tell?
2. Sea water pump isn't pumping fast enough. It looks like plenty of water coming out the back.
3. Temperature sensor is not accurate.
4. Passages in block are clogged leading to inefficient cooling.
5. Engine is producing more heat than it should. That's probably a long shot.
6. On that last note, maybe there's a restriction in the exhaust. How can I tell?
7. Heat exchanger is corroded and therefore inefficient. It is almost 30 years old.

Any other ideas?

Sonador

[Kelly]

Sonador,

Your cooling setup seems very similar to mine (minus the hot water tank), and seems to have very similar results. Just a couple of comments:

My temperature readings are the same as yours. I suspect that my problem is a poorly bled fresh water circuit (coolant). This would mean that my fresh water pump is running into air and circulation is far from optimal. I plan on inserting a bucket of coolant on the intake side of the water pump so the pump can draw directly from it and I'll be able to see the returning coolant dumping back into the bucket.

Otherwise: since you mention "plenty of water coming out the back" let's put "maybe there's a restriction in the exhaust" and "heat exchanger is corroded and therefore inefficient" pretty low on the list of suspects. Your fresh water circuit could be restricted, either in the block, in the hoses or in the exchanger, but this too would be unlikely given the nature of the fresh water/coolant mix. I recently disassembled all of the above and there was no significant blockage anywhere.

Just to be clear, does your coolant run to a sea water heat exchanger after exiting the hot water heater? If not, then once the water in the tank is hot, you wouldn't get any more cooling action (don't be offended, I know this is obvious...)

Kelly

[tenders]

Gee, I was going to say that a 30-year-old heat exchanger would be my first guess for reduced cooling capacity like this. Not so much because of corrosion (although there certainly could be that) -- but because of scale buildup.

How's the water heater working? If it's working well, seems like the problem is outside of the coolant loop.

I'd do an acid flush on the raw water side, and not feel bad if the heat exchanger had to be replaced.

[sonador]

Well those are two excellent suggestions. Actually the reason I took the thermostat out in the first place was that I couldn't get the air out of the coolant. The engine would run fine at 180 for a while and then very suddenly peg the temp gauge at 230. The time constant was too fast for liquid, so I surmise that an air bubble got trapped near the temp sender. I thought that topping off the fluid from the highest point in the block, i.e. the thermostat housing, would have taken care of any trapped air. Kelly's suggestion to run coolant from a bucket will not only assure that I get the system filled air free, but will also allow me to see how fast the coolant is flowing. Does anyone know the GPM rating on the pump at say 1000 rpm engine speed?

As for the heat exchanger. I'll flush it with acid next time I'm at the boat. It's certainly due for a cleaning. I don't know why it didn't occur to me to do that.

And yes, the hot water heater is separate from the sea water/coolant heat exchanger. I can feel the pipes getting hot going to the tank but have not yet tried the water. That's on my list after I get the oil pressure gauge working and the charging system up to snuff.

Thanks for the good suggestions.