I changed the coolant from yellow/green to the orange/red last weekend. What I did was the following:
1. With engine cold, I drained the rad – very simple job, remove the filler cap and open the tap at the bottom of the rad then leave for 10 minutes.
2. Close the rad tap, fill with plain water, add one bottle of Holts rad clean. Note that the rad drain removes only around 5L of coolant, so you still have around 5L left in the engine block and heater matrix.
3. I did not disconnect the upper heater hose and primed the system with antifreeze as Haynes suggest. Instead I started the engine for a few minutes, then toped-up again until the expansion tank was full.
4. I the drove around for 15 minutes with the climate control on HI (and all the windows open… it was a hot day), making sure that the temp gauge peaked at its normal upper temp (around 97 degrees) and that the fans cut in. It is best to drive the car using low gears, because the water pump is mechanical and the higher engine revs will help circulate the coolant faster (around 3,500-4,000 rpm will do).
5. I let the engine cool, drained, then refilled with tap water. Drove around for 5 minutes just until normal working temperature has been reached. I then let the engine cool down, drained, refilled with tap water, and repeated this whole process once again including the 5 minute drive. By doing this twice with tap water I made sure that all that was left in the system was plain water – and I checked to see that clear water were coming out of the rad, i.e. no trace of the old yellow/green antifreeze. This double-flush also removed any traces of the Holts rad cleaner.
6. I then filled the system for the last time with Vx red antifreeze – and it took exactly one 5L container to get to the full mark, as I already had 5L of tap water in the system (remember that the rad drain only removes 5L). Been driving the car since and it behaves normal with no coolant lose.
Now a few notes about all this:
a. When I say I let the engine cool down, I only left it of a couple of hours. The engine does not have to be stone cold, just cold enough to prevent any damage to the block or heads as result of sudden temperature change. Incidentally, the water drained from the rad after two hours were lukewarm. And, to be on the safe side, I filled it with hot tap water (i.e. around 60-70 degrees) again to minimise the chance of temperature change damage.
b. As I said this was the quick way. Having said that, I am all in favour of ‘proper’ flush, including the removal of the stat and flushing the block etc. But the system seemed clean and the coolant was clear, so taking off the plenum and intake manifold to get to the stat seemed unjustified on this occasion.
c. As for the Holts rad cleaner, I don’t think it actually did much. I used it because it seemed sensible to give it a quick flush while I was at it, but to be honest if I had a sludge problem I would have gone for a heftier method using a commercial product (or just Plain Caustic Soda) and leave it in the system for much longer etc.
d. You mention using distilled water in the past – I used to do this on my old car until I was told off by a friend who is a Water Treatment Engineer (she specialises in desalination plants). Apparently distilled water is very active and will attack the metal to try and rob it of Iron and Magnesium atoms. Normal water already has a small amount of metal ions in it which make it chemically stable. What you really want is soft water, i.e. water that has low Calcium contents – and not distilled water. You can do any of the following (a) check your tap water with a very cheap fishtank Calcium testing kit availed from any pet shop, or (b) use bottled water – the Calcium content is listed on the label so choose one with low Calcium, or (c) boil the tap water once before using it, this will remove any access Calcium.
So in summary this was essentially a quick coolant swap that was achieved by diluting the coolant through repeatedly draining and re-filling the rad.