The coolant system has two "bleeder" hoses designed to take air out of the system. One of them is on the top of the thermostat housing, the other is on the passenger side top of the radiator. So don't worry about it.Do you have to drain the coolant when doing this? If not, doesn't that leave air in the system?
If you can see what color yours has, add the same type, green or possibly yellow. Do not use the orange DexCool for GMs.
Sidenote: The proper size O-ring to use is actually a METRIC o-ring. Remember that Ford converted all its cars to Metric, not to mention the Focus was originally designed in Europe, where metric is the standard. After reading some people used #203 and some used #202, I checked into the metric's and there happens to be a standard metric size right between a #202 and #203. Its a 7mm ID, 13mm OD, 3mm cross-section. I got mine in Viton and of course have some left over. It went back in easily and haven't had a leak since. If you need one, PM me and I'll drop one in the snail mail for you.
Sorry, I'm now out of the o-rings I bought, they can be ordered from places like McMasterCarr.
If it's coming from the housing, it is probably a cracked housing or bad gasket.my 2000 ZX3 is bleeding coolant and the thermostat housing is drenched. how can I determine what the cause of the leak is?