Dude - bleed the radiator to make sure that it has no air in it. Usually any air in the system collects in the top of the radiator .. It will feel cold at the top and hot at the bottom. When you're feeling it, don't stick your hand in the fan..
There should be a bleed screw on the right hand side of the radiator (the end opposite to the hoses). Just open it a bit (don't take it out altogether or you'll not be able to fit it back in when all the coolant is ****ing out!
) so the air escapes. When coolant starts coming out, it's done.
If you need a new radiator, check out the rad' from the 155 Q4. It is about 6cm wider than the normal rad' you will have. On the radiator mounting plate (the rusty bit of metal that the radiator sits on
) you'll see another round hole (about 2cm diameter) which is where the radiator lug will fit, so you get an idea of the extra width.
You also have to remove the plastic blanking plate that fits in the gap between your small radiator and the headlamp, as the bigger radiator is going to go in there...
My old bus doesn't have a/c though... so I don't know if your radiator has any other hoses or connectors that the Q4 rad' has to match up to.. but I don't think they do.. all the 155 rad's look very similar and of course, they will all fit in the 155 nose.