I drive a 2000 Ford Focus SE with the 2.0 DOHC Zetec engine. There has
been a light brown foamy substance in my coolant tank. From what I've
read, it is probably oil in the coolant. There is no water or coolant
in the engine oil though. Anyway, yesterday I drained and flushed the
system as best I could without a water hose. Anyway, I removed the
coolant tank to get some of the gunk out and there was that foamy stuff
in the three hoses connecting to it. What would be the best way to
clean the foam out of the system? Should I use a bottle of that
radiator flush? I believe the foam is restricting the flow of coolant.
Would it be horrible to drain the coolant out, add the bottle of flush
and fill with water, then drive it for a little while? I figured the
driving would increase the pressure in the cooling system and possibly
remove more foam. Any advice?

The cause of the oil contamination is still in question. I tried to
pressure test the system at home but the tool didn't have an adapter to
fit my tank. So I hope it is not a head gasket. I see a small leak
around the thermostat housing that bolts to the head. Could that cause
the contamination, or would it almost have to be the head gasket?

Well thanks for any help anybody can provide.