Got a neighbor with a Perkins 4T 236 in a Cat rubber tired excavator. Since it has a Perkins,I thought this was probably best place to ask about it. He's had this unit a couple weeks & seemed to run ok,but started to push coolant out the overflow last night.I went over this eve & had a look. Appears thermostat is stuck open as temp was same on both sides(shot with temp gun),never got over 130degF.We had it on high idle & after a few minutes it started to push coolant out overflow.Some bubbles appeared too.The coolant is a milky colour too, but doesn't seem to be oily. He said it just showed up milky after last night when it first pushed out the overflow tube. Coolant was nice green colour before that.Once we brought RPMs back down to an idle, it drops down in rad some & hardly any bubbles.I was thinking possibly a head gasket,but wanted some other opinions before I tear the head off this thing. Thanx in advance