This is not a good problem to have:

• It will interfere with the water pump and cause a drop in pump pressure.
• The reason it is foaming is because you have two coolants reacting together, some of the old stuff which may be off (sour) and the new one which has just been put in the radiator system.
• A cocktail of this type will eat through things like timing covers, cylinder heads, water fitting, and so on in absolutely no time at all. Warning guys it is very very aggressive.
• One of the best ways to combat this is to disconnect the heater tap hoses and stick the garden hose into heater tap hose and completely and thoroughly flush the entire system out. It may take as long as 25 to 35 minutes to get all the old crap out.

Then start again by putting a fresh batch of additive that is suited to that vehicle in the cooling system.

Now that we are talking about coolant additives, one thing that has become abundantly clear is that many mechanics have gone away from generic brands of coolants and are back to using the specified manufacturers coolant e.g. Holden coolant for Holden’s, Ford coolant for Ford’s, Volvo coolant for Volvo’s.