An E36 that overheats crawling in traffic but stays fine on the highway has an airflow problem, not a coolant-capacity problem. At speed, ram air cools the radiator whatever the fans are doing, so a weak fan only shows itself at idle and low speed. I see this constantly.
Two fans do two jobs:
- The engine-driven viscous clutch fan behind the radiator. It is the primary mover at low speed, and it freewheels once its clutch wears out.
- The auxiliary electric fan ahead of the condenser. It runs mainly for the A/C, but its temperature stages back up engine cooling when you are slow.
So for this exact symptom my prime suspects are a worn viscous fan clutch and a dead auxiliary fan. Check those two first.
Then I rule out the supporting causes: low coolant or an air pocket, a clogged radiator, a stuck thermostat, and the plastic-impeller water pump, which is failure-prone. If the pump is due, I fit a metal-impeller one.