fresh water cooled overheating

[mainefisher]

I just started my fresh water cooled A4 on the hard using a pail and hose for the raw water intake. I have used this system for years. The raw water side seems fine with good aspiration of the water and good flow out with the exhaust. On the fresh water side (antifreeze), the hose coming from the exhaust manifold to the heat exchanger is very warm and the return to the pump is very cool and doesn't change as I run the engine. The temp has reached 190 (last year it never ran above 140, even under load) and would continue. I have clamped the bypass hose without change. Any suggestions as the the next diagnostic steps. The heat exchanger is 3 years old from MMI and has run flawlessly. I did pull the engine forward to replace the stuffing box this spring, but replaced hoses as before.

Thanks

[hanleyclifford]

Since the antifreeze is being returned to the pump cooled, as it should be, the exchanger is doing its job; I would begin to suspect the temp sender is telling a fib or the ground is bad.

[mainefisher]

Hanley, Thanks for your comment. I can actually "smell' the hot engine and see the needle on the temp gauge move. I actually question whether the fresh water pump is functioning adequately. I plan to remove the fresh water hoses at the heat exchanger and see if they have both suction and output in a test bucket. Sound reasonable?

[thatch]

mainfisher,
Have you checked for a stuck thermostat?
Tom

[mainefisher]

I did clamp the bypass hose between the thermostat and the exhaust manifold to see if there was any drop in temp and it just kept rising.

[thatch]

Mainefisher,
After the engine coolant leaves the the pump it either is allowed to go through the engine or through the bypass hose depending on whether the thermostat is open or closed. It then travels through the short hose between the theromostat cover and the exhaust manifold. Pinching off this hose would stop all coolant flow to the block and temp would rise. It really does sound like a stuck thermostat or a blockage in the block unless of course your waterpump isn't doing it's job.
Tom

[hanleyclifford]

If the antifreeze is being returned to the pump cool, the pump must be functioning at least partially. There may be a blockage in the engine which restricts the antifreeze flow enough to cause overheating. Is this a new or earlier model A4?

[mainefisher]

Thank you, Gentlemen. Does it seem reasonable to disconnect the two freshwater hoses at the heat exchanger and put the inlet hose in some water and see if the freshwater pump is working? If it is, then I will attack the thermostat housing. Absolutely nothing on my Sabre 28 has easy access!!

Tony

[hanleyclifford]

That is reasonable and will give you the opportunity to examine output from the block - look for impeller debris.

[mainefisher]

Probably be Tuesday before I get to it. Hope for the best!! Thanks

[rigspelt]

I agree: sounds like the heat exchanger is doing its job, cooling the hot engine effluent before sending it back to the water pump to re-enter the engine. Could verify overheating with a laser sensing thermometer on the hoses and block.

I'd also wonder about a flow problem after the heat exchanger. This started after moving the engine forward and then hooking everything back up again. That created an opportunity to introduce a blockage, though I can't think of any specific examples to chase. Water pump impeller intact?

[mainefisher]

Problem solved and fixed. Disconnected the two fresh water hoses at the heat exchanger to test and see if the fresh water pump was actually pumping. When the hoses were removed the outlet port at the base of the exchanger didn't even drain the antifreeze which was readily visible at the fill cap. Removed the heat exchanger and flushed it out with a garden hose. Ran clear and unable to capture any obstructing debris. Re installed and running cool as usual. So in conclusion appears that the problem was some obstruction in the heat exchanger. Thanks for everyones comments and Don's reassuring guidance on the phone this am. The idea of trying to remove the freshwater pump in my engine compartment was beginning to convince me that kayaks are really more comfortable than I remembered!!

Tony