What owners see first
The repair pages describe overheating as a warning in the cluster, often with reduced power, an electric fan running at full speed and a coolant temperature that climbs in traffic. A low coolant warning, a sweet smell and pink or blue crust around hoses or seams point at a leak rather than a pump. This guide is about the overheating symptom itself; for where leaks come from on each engine, see the BMW Coolant Leak: Common Causes by Engine.
Electric water pump and thermostat
On the N52, N54 and N55, the electric water pump is the repair owners describe for an overheating warning with the fan at full speed, and the pump can set fault codes in the 2E81 to 2E85 range. Commenters advise replacing the thermostat at the same time because it is the same drain and access, and one commenter suspects some quotes double-count labor for the pump and thermostat, so ask. See Electric Water Pump + Thermostat.
If only the thermostat has failed, the signs are different: the engine is slow to warm up, the temperature gauge reads low, cabin heat is weak, or a thermostat fault code such as 2E81 appears. Our data has two real quotes for a standalone thermostat, and says that if the pump is original and high-mileage, doing both at once saves labor. See Thermostat Replacement.
N20, B46, B48 and B58 pumps
The N20 uses an electric pump, and the B46 and B48 use a mechanical pump with a bracket. Owners describe an overheating warning, a leak at the front of the engine and the electric fan running hard. A BMW tech recommends the full pump and bracket assembly on B48 cars because the bracket gasket can leak, while another says to replace it only if cracked. See Water Pump (N20 / B46 / B48).
On the B58, owners describe coolant loss, a smell or crust near the front of the engine and an overheating warning. Ask up front whether your build date needs the pump support, because pump scope is what nearly doubled one quote in the source thread. A dealer tech cites BMW customer-pay time of 2.5 hours for the pump only and 4.9 hours with the support. See B58 Water Pump (Coolant Pump).
Radiator and expansion tank
On the E90 family, a coolant leak at the radiator side tanks brings a low coolant warning and overheating. Manual and automatic cars take different radiators and N51 SULEV cars need a special, pricier one, so order by VIN. One owner got an N51 radiator replaced free under the SULEV emissions warranty, and E90post owners call the swap an easy DIY, with the cooling system bled afterwards. See Radiator Replacement (E9x).
On the F30 and F32, the expansion tank, coolant flange and turbo coolant return line are the leak points, and overheating happens in severe cases. A cracked tank vent hose causes the same sudden leak and is a much cheaper fix, so check it first. See F30 Coolant Expansion Tank & Coolant Lines.
When it might be a head gasket
Coolant loss with no visible leak, white exhaust smoke, bubbles in the coolant tank and overheating are the head gasket signs on the repair page. Oil leaks are often mistaken for a head gasket problem, so the page says to confirm with a leak-down or combustion-gas test before approving the job. Early E90 N52 cars used aluminum head bolts that can break and mimic a head-gasket leak. See Head Gasket Replacement and the BMW White Smoke From the Exhaust: Causes and Costs.
Before you pay for the big job
The pattern across these pages is to match the symptom to the cheapest likely cause first. A low coolant warning with visible crust is a leak hunt, a full-speed fan with a rising gauge points at the pump or thermostat, and coolant loss with no leak points inside the engine. Once any cooling part is replaced, the system has to be refilled and bled, so many owners do a coolant service at the same time (Coolant Flush / Coolant Service, BMW Coolant Change: Long-Life Coolant, Flush Service and Bleeding).