I'm just back from my boat and did some work on the engine. Mine is a D2-55F (the revision matters - the heat exchanger is different; for example, while previous revisions had seawater come in at the front and out at the back, mine had both connections at the back and the front is only covered by a sealed cap).
I started by pumping off the cooling fluid through the filler cap, using the Pela 6000 vacuum pump, the same thing I'm also using for oil changes. This works great. I cleaned the filler cap and inserted a new seal (
https://www.parts4engines.com/volvo-penta-expansion-tank-sealing-ring/).
The remaining fluid came out by opening the tap mentioned in the thread here. To prevent a mess, I attached a hose to it. And yes, you only need to unscrew the bottom part.
Using lots of fresh water I flushed everything. Also, I removed the expansion tank and cleaned it as good as a I could.
To get to the front facing cap of the heat exchanger I loosened the screws of the generator and pushed it towards the right after removing the belt.
Also, I removed the hose connected to the outlet of the seawater pump and the hose connecting to the heat exchanger right next to the MDI box.
The MDI boxes two screws holding it in place also came out and I moved the box out of the way. Speaking of: I would not worry about the missing screw. Use a bigger washer and tighten the MDI box.
The pipes weren't too clogged and there was only minor incrustation on it (also some in the cap itself) which all came off pretty easily.
I used a long metal rod with a suitable diameter and pushed it through each and every tube. Except for three it all went smoothly, the three required some pushing and a bit stuff came out eventually.
To get to the back of the exchanger I removed the two seawater hoses connected to it and the one going to the exhaust elbow.
That side of the exchanger was almost completely clean.
The screws came of pretty easily. I used a long Allen key that would also work when slightly angled. This allowed me to reach the screw at the bottom of the rear cap without detaching the engine from the gear box.
Before I put the caps back into place, I used two new O-rings from this set:
https://www.parts4engines.com/volvo-penta-d2-55-heat-exchanger-gasket-and-seal-kit/ (you need the big ones). I bought the set, because after watching some videos I thought it might be necessary to remove more parts, which would have required more seals. But good to have it on board now.
I did not bother getting the exchanger tubes out of the engine. They sit in there quite tightly and I could not find a reason to take them out, so I left them where they were.
In the end I filled the engine with ready mixed Volvo Penta coolant (
https://www.parts4engines.com/volvo-penta-ready-mixed-coolant-green/). Notice that the manual says that about 9.5 litres would fit in there...don't trust it. I managed to spill a good portion and it's a pain to clean up the engine. Fill in the first five litres fast, but then go slowly. I could get about 7 litres in. Then I closed the filler cap and started filling the expansion tank, which was another litre.
I checked for leaks and when I was happy, I started the engine and let it idle for a bit. Making sure there are still no leaks, I let it run until it wamred up properly. When the thermostat opens, more coolant will be used and you have to keep an eye on the expansion tank. I could add another litre or so.
