[colsen]

My cooling system appears to be plumbed incorrectly, but before I go off and re-plumb it I thought I'd see if anyone here knows of a reason that it should be left as is.

So I have an old 5411 which appears to have been converted from raw to fresh water cooling some time ago. In the three years I've owned it, I have yet to see the temperature gauge move. The gauge works, the sender works, the thermostat works, both raw and coolant pumps are working. The engine just never gets hot.

After much speculating, testing, and head-scratching, I believe I know what the problem is: The recirculating hose is plumbed right back into the heat exchanger input. The effect is as if there is no thermostat installed at all.

Hang on, I'll post some pictures to try to illustrate the question.

 

[colsen]

This is the original raw-water cooling system shown in the owners manual:

And after some research this is how I think the fresh-water conversion should be plumbed:

But this is how mine is currently plumbed:

Here's a pic of the actual thermostat plumbing in question:

Any idea why it would be done this way?

 

[capt jgwinks]

You're absolutely correct. I have the converted 5411 as well. The correct setup is as you show it. The line from the top of the thermostat housing goes to one end of the exchanger, and the line from the bottom of the t-stat goes to the other end via the mixing tee. The third line from the tee goes back to the pump. Do you still have the 140 deg thermostat installed? With fresh water cooling a 160 or 180 deg t-stat should be used to allow the engine to come up to proper operating temp.

 

[colsen]

Excellent. Thanks for the confirmation. I do still have the 143 degree t-stat installed. I vaguely recall reading somewhere that hire temps could shorten the life of the impeller. I'll see how it goes with the cooler t-stat, maybe try a warmer one later.

 

[JimsCAL]

Yes, you want the recirculation water to bypass the heat exchanger. I wish the 5411 on my previous boat had the fresh water cooling. The raw water cooling was a pain for many years and eventually killed the engine.

I would consider going with a higher temperature thermostat. I find it hard to believe that an additional 20 degrees of water temperature would have any significant impact on the impeller life. The improvement in the operating conditions of the engine would seem to worth it. Impellers should be changed every year anyway.

 

[colsen]

Well I finally had a chance to redo the plumbing, and it was a big, fat fail. The engine slowly warmed up to ~145, and then kept on going. I shut it down before it hit 200.

The hoses got very warm initially, but then when it started overheating, the hoses had cooled down. It's like the circulation stalled when the t-stat opened.

Investigating now to see if I need to burp or bleed it or something.

 

[JimsCAL]

Yes, check for air in the system. Also double check that the hoses out of the thermostat housing are not swapped. That would cause the water to bypass the HX when the thermostat opens rather than sending it into the HX.

 

[colsen]

OK. Pretty sure the hoses are not swapped. The hoses get warm pretty quickly after starting the engine, so it seems like that part of the circulation is working right.

It's acting as if the path from the t-stat housing to the HX is blocked, but I'm pretty sure it isn't.

If bleeding air doesn't work, I guess I'll try running it with the t-stat removed, just to verify that the coolant is flowing freely through that path (if I remove the cap, I should see coolant flowing into the expansion tank).

BTW, I read somewhere last night that there are two little holes in the t-stat's outer ring that allow a small amount of coolant to bypass it, but if the t-stat is not oriented so that the holes are in an open area then they don't work. I think it's meant to provide constant cooling to the exhaust pipe in a raw water system, not sure if it would matter for fresh water.

 

[JimsCAL]

It's acting as if the path from the t-stat housing to the HX is blocked, but I'm pretty sure it isn't.

Note that the thermostat would do this if the hoses were connected backwards. I am pretty sure the previous post saying the hose from the top of the thermostat housing should go to the HX inlet and the one from the bottom should go to the tee at the outlet. You should be able to tell where the water is going by feeling which hose is hotter when the engine first runs and comes up to temperature. The hose going to the tee should initially warm up first as the coolant is recirculating and the one going to the HX should be cooler. Then as the coolant heats up, the hose going to the HX should get hotter as the tstat opens and sends more water to the HX and less to the recirc.

 

[colsen]

Note that the thermostat would do this if the hoses were connected backwards. I am pretty sure the previous post saying the hose from the top of the thermostat housing should go to the HX inlet and the one from the bottom should go to the tee at the outlet. You should be able to tell where the water is going by feeling which hose is hotter when the engine first runs and comes up to temperature. The hose going to the tee should initially warm up first as the coolant is recirculating and the one going to the HX should be cooler. Then as the coolant heats up, the hose going to the HX should get hotter as the tstat opens and sends more water to the HX and less to the recirc.

Yep, all that seems to be working correctly (until the t-stat opens anyway, and then it stalls).

I finally had a chance to have another go at it this afternoon, with better (?) results. First, I replaced the recirc water pump impeller. Next, I double-checked that the t-stat wasn't upside down (it wasn't, and isn't possible anyway). Then I installed a back-flushing port on one of the hoses to allow forcing air out of the lines.

The result was that it warmed up to about 125-130 degrees (according to the Teleflex gauge anyway), and stayed there regardless of rpm or load. The hoses got warm in the expected order.

I think that on the previous test run there must have been air in the system, and it just wouldn't circulate once the t-stat opened.

But with a 143 degree t-stat, I would have expected a higher temperature. So things are better, but I'm still scratching my head.

 

[JimsCAL]

Sounds good! Only thing you seem to be concerned about is the temps running a bit low. May only be accuracy of the temperature gauge. In any case, its better than running the engine cold like you were before. If you can find a thermostat with a higher temperature rating (like 160 to 180F) that fits the housing, that would be even better.