Hi Philip, how did you drain the coolant? There is a coolant drain valve on the starboard side of the M25 (near the oil dipstick). The other places where water can remain might be the heat exchanger, hoses and the manifold itself. There is a drain plug near the low end of the manifold (port side aft). If you get all of these you should be good to flush & replace the coolant. If you don't get the lion's share of the fresh water coolant capacity accounted for draining from all these places, re-post, I sure someone else will have more suggestions.

One more thing, the M25 coolant capacity you quoted probably does not include what's in the hot water heater or its hoses.

The following was posted by Stu Jackson, I think on sail net (thanks Stu ,) and may be of assistance :

I know this refers to burping the system but it relates to your drain issues as you are obviously doing the reverse of "filling the system"

Re: Engine overheating problem
« Reply #6 on: September 02, 2008, 02:25:11 AM »
Bill, you need to properly BURP the engine's freshwater cooling system.

Here's a story about another issue related to the heating/cooling system: [url]http://c34.org/bbs/index.php/topic,3769.0.html[/url]

Just make sure you fill the hot water heater loop and I don't bother to use the thermostat's bleed valve. If you only use the bleed valve, the engine temp WILL shoot up, stay with it. Take the HW heater hose off the bottom of the thermostat and FILL them. (See second picture below)

I also wrote a recent tech notes article published in Mainsheet:

Replacing Antifreeze and Burping Your Engine

A fellow skipper recently asked: After five seasons I am thinking of changing the antifreeze in my M25. When I did it 5 years ago I recall having trouble removing air after the change. If I slowly drain from the petcock on the block and fill thru the plastic expansion tank simultaneously with the pressure cap off on the engine would this work so I don't introduce air?

I responded: We just did this the other day when repairing the exhaust riser connection to the flange. Don't bother with the petcock on the manifold, it's hard to catch the water.

Do this, it's a lot easier. Loosen the screw on the hose clamp of the big black hose underneath the freshwater pump and use a thin screwdriver to open up the space between the hose and the elbow. Do it right from the front of the engine. You're right, take off the manifold cap when doing this.

As for the air block, it's called “BURPING” and all you need to do is take the 3/8 inch hose off the bottom of the thermostat. Pour distilled water or antifreeze or the 50/50 mix into that hose with a funnel until water comes out of the bottom of the thermostat. Loosen the hose clamp and pull the hose off. Hold that hose up above the thermostat and pour the stuff into it until the water comes out of the bottom of the thermostat - the nipple that you just removed the hose from. Once that happens, just put the hose back on the nipple under the thermostat housing and tighten up the hose clamp. Put some Lanocote on the nipple to make it easier to remove the hose the next time. That gets the air out of the loop of hose that goes to the hot water heater and avoids overheating at startup and does not require that you rev the engine while the thermostat petcock is open. You won't need to open the thermostat petcock at all.

Update August 7, 2013 - Thanks to Ray Irvine who suggested I add: "Pour your new coolant into the engine through the radiator cap on the manifold, and not through the small hoses on the T stat. You only need to burp your engine AFTER the coolant is topped off. The purpose of burping is to remove the air bubble in the hose from the engine to the hot water heater. Ray also suggested either holding the hose up high or even adding another short piece of hose to get more "head" on the system as you were burping. Thanks, Ray.

HE also made this comment more recently:

We have an M25, too. The very last place one would encourage draining the block would be from a petcock. Why? 'Cuz they have small openings (which can get clogged from debris in the coolant) and they are usually in a position where opening them will spray coolant all over and be hard to drain