6BTA5.9 M1 Coolant drain and replace

[James DeCristofaro]

I have twin 1991 6BTA5.9 M1 250hp engines in a trawler. About 2020 hours each, run well. Purchased boat December 2017. The coolant is a swamp brown green with a slight residue in it (I pulled a small pill bottle sized sample from the tank. I don’t know how old it is and I’d like to change it.

I’m not sure but as I understand it the engines have to be run up over 180 F to open the T-stats so the entire plumbing circuit is open and drains thoroughly. Today – in northern Virginia Chesapeake Bay – I ran the engines at the dock for a couple of hours and couldn’t get them up past 175 F, at rpms from 900 to 1600, and even tried a small load pulling against the dock lines for a while – temp still would not go up.

I’ve gone through all 762 Seaboard posts and all the on line articles – coolant, thermostat, etc. I have located the coolant drain on the low end of the coolant pipe below the raw water heat exchanger. I still have questions.

Do the motors need to be up over 180 F to drain / flush / replace coolant? Or can I just open the drain fitting and all the existing coolant will drain out?

How is a fresh water flush performed? By simply running low pressure fresh water into the radiator style cap opening on the tank above the fan housing with a low pressure garden hose – and out the drain plug? Is that with engine running – or not running?

What about Restore? I’ve read everything from it’s great to it’s not worth the trouble. And if I can’t get the motor up over 180 F, is there any point to using Restore?

And what about making sure there’s enough coolant in the engine, without air pockets? Is the solution simply a slow pour with the new coolant? Can the engine be run with the cap off and adding more coolant as it circulates, like a car engine?

And what about the loop to the hot water heater – how does that get flushed?

Right now I’m going to hold off on any thermostat work. I’ll do that next year with some help. But I would like to replace the coolant this year before winterizing.

Appreciate any feedback /experiences. Thanks.

Topic

[James DeCristofaro]

Thanks for the replies.

I’m getting when draining and refilling slow and patient is important. And good to know I can run the engines with the filler cap off and add more coolant as needed. It also seems there’s no way to get the T Stat to open (if it did in fact not open) without taking the boat out on the water under load – unless maybe I replace the coolant during the summer when the seawater is warm? If flushing with fresh water and/or chemical flush, that makes it quite a project if having to leave and return to the dock several times. If anyone knows how to get the boat up to higher temp while in the slip please let me know.

I found this post online on Boatdiesel..com, which helps put things in perspective:

Tony May 9, 2003
“From experience, I would say that the “B” series Cummins is not the easiest engine to fill with coolant.. Seems too easy to air lock or underfill….My solution has been to add about 5 gallons of coolant on the six cylinder engine that has the full width expansion tank( most all engines since 1990 or so). This usually brings it up to about 1/2 to 3/4 full in the tank.. I then run the engine at about 800-1200 RPM until I get about 150 degrees with the pressure cap off (idling out of the harbor works fine if conditions permit).. Then I fill the tank up and close the cap…Seems to be the best system for me… The C’s are not as bad but I still run them with the cap off to “de-aerate” before I fill to the top……In all cases, watch your temp on the first hard run, and the next day, pull off the cap and top off…The plastic remote expansion bottle should be about 1/2 full when cold.. It’s also best to have the plastic bottle tank at close to the same level as the tank on the engine…If it’s higher, you can’t check the engine tank level without siphoning from the bottle (I guess you could pinch the hose)..Too low, and it may not work properly…”.

And for whatever reason, my engines don’t have expansion bottles/plastic remote bottles, just the athwartships semi rectangular heavy metal expansion tank and a clear 3/8″ plastic hose from a fitting on the filler cap expansion tank collar down to the bilge below the engine.

There’s another piece here I realized I haven’t mentioned.

Below the heat exchanger is a long tube running fore and aft about 2-1/2″ diameter and the same length as the heat exchanger. I believe it’s called the water transfer tube. At the bottom side of aft end of the water transfer tube is the pipe plug – where the coolant is drained.

After I ran the engines for over an hour at the dock, I felt the tube. The forward end of the tube was very warm to the touch – and the aft end of the tube was cold.

If the thermostat was opening, wouldn’t the entire water transfer tube be very warm?

[Stephen Oliver]

I’m no expert like Justin, but I have just done both engines.. Mine were not swamp coloured coolant like yours, but I basically drained them cold, be patient.. it can take a while to drain out. I then filled with fresh water (slowly).. and ran under load – you might need more than idle in gear.. especially with cold seawater.. once I was happy it had circulated – I then dumped that (after the engines had cooled some) and refilled very slowly with a 50/50 mix of some red coolant we get over here (aus) its heavy duty diesel stuff rated for 1mil km.. or 8 years.. when I say I filled it slowly – i filled it really slow – then ran the engines up to temp – with the cap open – and watched.. then filled as required. I then left the boat for a few days – went back and added whatever was required (which was almost nothing) I also added coolant direct to the expansion bottle.. but only about a 1/3 full. They have been good since then. No special burping required and a lot less hassle than I was expecting (having read up on the subject)

Also re the water heater.. as its part of the coolant circuit – as long as it works and heats up – then in theory it should flush automatically..

Steve

[Justin Riege]

Are you sure that thermostat isn’t opening? Also there are 160F thermostats floating around. We have been selling them for a while. You could be at a steady 175 with the T Stat open.

As far as the other stuff, I would drain it, fill with clean water, run it until T Stat opens, this will flush everything out. Drain again and fill with new coolant.

Fill it slowly, and run again until the T Stat opens, then top it off and be done with it. Go fish enjoy the boat.

[Author]

Replies