I am looking for suggestions. My 4.6L Mustang overheats in traffic. I have replaced the water pump, thermostat, radiator, cap, heater core and cooling fan. The water flows through the block and radiator just fine. When it over heats the water comes out at the cap.
Overheating in traffic is generally a fan issue. If it doesn't overheat on the highway, that usually means flow is acceptable, and the air passing over the rad is doing enough to keep it cooled.
I would do as the above poster said and check that fan.
OK, this is out on a limb but my Mustang used to also overheat when sitting still for a while (like in traffic, drive through, etc..) and it started doing this after I had to go 'off-road' one time to avoid rear-ending someone. I ripped the front plastic piece (black thing, air dam or whatever) and I thought that was causing it to overheat...
Long story short, I had to replace my intake manifold and when I did, I replaced the 180 degree thermostat that I had in my car with the factory one (210 or so..) and it does not overheat anymore. So if you have a 180 degree thermostat in there, try changing it back to stock. I've got a theory for why it was overheating but if you don't have a 180, it doesn't matter really ...