Radiator (over heating)

Hello, just fitted a new radiator today and now it overheats? pretty sure i did everything correctly but can't work out why it is doing this can anyone help i need to drive. Any help would be great thanks.

The fan is working but the temperature just keeps rising. I ran the engine after filling the coolant up until it reached correct temp then stopped it let it cool down then topped up and put caop back on but on my fairly short drive home the temp just kept rising.

Why was the rad replaced, old one over heating?

Is it really over heating, i.e. boiling up and looseing water?

I replaced the old one because it had a leak and it had over heated a couple of times when in traffic (loss of water, steam, boiling the whole lot) now it boils up and liquid is being forced out of the blue cap. Not sure what to do desperate to get it sorted it's my daily

I bought a new rad and the small plastic pipe that returns to the expansion tank was blocked/hadnt been tapped from the factory so had to make the hole myself. worth checking

ahhhh… poss the previous leaky rad and loss of coolant has caused more serious engine problems (or the engine already had problems that finished off the old rad?).

Its not unknown for the rad caps to fail and not hold pressure.

I'm pretty sure the engine is fine gonn put the old rad back on and see what happens

Right then tried the old rad and seem to be getting the same syptoms. I'm now guessing that it's me being a retard and not doing something right I'm sure 100% it's all connected up properly but perhaps I'm getting an air lock somewhere hence the overheating?  When I'm running the engine with the blue cap off should liquid be blobbing over the top and should I keep this going till the fan kicks in? I've never done this before so bear with me. I'm starting to get a bit eggy now as at the moment it looks like i won't be going to Stonor park

All sorted now with the new radiator i ended up getting a new expansion tank cap and replacing the thermostat and now it's better than ever before thanks for the help (especially Jseaman for the how to guide)