6BTA Thermostat Replacement Procedures

[Scott A]

Probably a stupid question, but I’m learning my 6BTA as I continue to work on it. I plan on changing out the thermostat for a high flow 160 degree replacement. Boat tends to run at 195 degrees, so going to a lower thermostat hopefully will give me some more margin on the temperature. I just pulled and had serviced the heat exchanger and the aftercooler. All looked good. Also eliminated the fuel cooler. When I removed the heat exchanger, I drained most of the coolant out in that process but did so when the engine was cold. If I go after the thermostat and replace it without refilling the coolant, getting the engine up to temperature and then draining the coolant again, am I in for a mess when I pull the thermostat? Second question, is there any special procedure for getting the coolant back in the engine other than filling it up and then letting the engine idle and filling the reservoir tank as she heats up with the cap off?

Also, an unrelated and strange question. Periodically, after the boat sits (length of time doesn’t matter and it is intermittent), when I get up on a plane the first time, the engine will begin to overheat. If I stop, put the boat in reverse and back down hard, it runs fine after that. I assume the intake is getting clogged somehow. It is a standard screen angled intake cover below the waterline in the far aft of the boat. Any other thoughts?

Topic

[Scott A]

I just ordered the parts off of Amazon. Got a brass mini ball valve with male on one end, female on the other and ordered a 1/4″ Hose ID to 1/8″ Male NPT Straight Brass Fuel Fitting fitting to go with it. Set up was $16 and got it overnight with free Amazon Prime shipping. I plan on taking the plug in the hole now and plugging the ball valve end. When I drain it, I’ll just put the fitting back on it and slip a hose over it.

[Scott A]

I think the drain plug is a 1/8th npt.

[JimmyK]

I’m stealing this drain idea…going to order the parts today…do you know off hand the size of the drain fitting? I think it is 1/4″npt?

[Robert]

Coolant drain

Not directly related to your question. I have a minimum of space under and around my engine. The first time I drained the coolant, it was a messy affair. So I added this drain valve. I have since found it very useful. . You can easily control and direct the flow.

(The picture taken after I removed my safety wire)

[Rob Schepis]

They make the positive flow screens with angled holes (drilled on a 45). Is it possible you have those screens and they are installed backwards?

[Scott A]

One of these strainer types.

[Rob Schepis]

What do you have under the boat for raw water intake?

[Scott A]

“If you do the backdown before getting on plane the first time will she still overheat?”

Yes. I have had that happen. It is a pretty aggressive back down. Whenever it happens, if I do back down hard, it clears the problem. Sometimes have to back down twice, if the first back down was not aggressive enough. Just really strange. Temp drops immediately. It is clearly an impeller running dry. Happens even with a brand new impeller.

[Rob Schepis]

First Question: Go ahead and pull the tstat – you’ll be good.

Second Question: The engine will not “heat up” at idle no load. You go out with the cap loose and do a light load mid rpm type run to get the temp up enough for the tstat to open and then let her cool down so you can replenish the level. Do it again. Could take 2 to 3 hot/cold cycles to level off. On the initial refill do so slowly and squeeze the coolant hoses. That should eliminate excess air. And don’t fill the expansion tank, keep it down a good inch or so below the neck base.

Unrelated/Strange Question: That’s going to be a tough one to diagnose on a forum….If you do the backdown before getting on plane the first time will she still overheat?

https://www.sbmar.com/articles/changing-the-cummins-marine-thermostat/

[Author]

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