Coolant Change Procedure

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
A partial piece of an answer

Several threads have touched on this, and here is just one-

There does not seem to a definitive answer to the question, tho.

More recently than 2008, and I do not recall the year, a mechanic friend of mine brought over his answer to the potential problem of air pockets. It was a 12 volt pump with a five gallon bucket, and a handful of small hoses and pipe pieces and clamps.
Basically, he opened up the hose connection from the engine to the hot water heater and put the hose from the new 50/50 mixture into it, pushed by the pump. Another hose link ran into an empty bucket to catch the old coolant. In a short time the new coolant was pumped in and the old was pushed out. With some minor spillage the engine hose connections were re-established.

I did not use this scheme last year for our new Beta (also a Kubota block) and just poured in the coolant at the "high point" at the top of the heat exchanger. We have not seen evidence of any air locks. There are probably internal differences in all of these blocks, even tho the external parts are so similar.

I recall that when I opened the petcocks on the lower part of the old M25XP to drain out the coolant before we lifted it out, it seemed like less came out than the manual indicated for its capacity. I ended up capping all of the hose ends to reduce the mess when we lifted and tilted it, going up and out thru the hatch.
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Advanced Beginner
Blogs Author
As I'm sure you know, there's a drain-valve on the starboard side of the M25 block, low.

I put a length of hose onto that (5/16" ID, IIRC) and drained coolant out through that valve into a jug. Opened up the manifold cap to let air into the system.

As Loren notes, it gets a lot of the coolant out, but not all.

When I flushed mine, I drained it, filled it with new coolant, bled air out and ran the motor for a while.... and then drained it, filled it with new, bled it and called it good.

I figure coolant is relatively cheap, and by doing it that way I was (relatively) sure that new coolant made it through all the hoses, the water heater elements, etc.


Christian Williams

E381 - Los Angeles
Blogs Author
OK, thanks. Getting my tasks lined up:

From the Universal manual for Uni 5432/M40:

Universal Manual.jpg

#2 "Block coolant drain"

Cooland block drain.jpg

#3 Petcock lower drain

Close petcock.jpg

#4 "Remove pipe plug under manifold" won't be easy given the restrictions of the E381 engine compartment)

My E381 has an Isotherm water heater plumbed to the heat exchanger. The previous owner put two valves in, as you can see. Both are usually open, but give options for isolating the heater.

water heater valves.jpg

The water heater is about 2 feet below the level of the manifold pressure cap.

I wonder how I will know if I have flushed the entire system, including the heater?

Maybe I should replace the coolant with distilled water for the flushing cycle, so I can be sure when it runs clear.
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Member II
Had to changed coolant recently in my m25xp. Ended up with a nasty airlock. Nothing worked. In the end I took the petcock out and filled thermostat housing with coolant using a syringe and that did the trick. For the next time I will be replacing that POS peacock with a normal ball valve.