Gazza82's 1998 Alfa Romeo 156 TSpark 2.0

156 TS Temperature Issues

Once warming up temperature gauge rises to 90'ish then drops suddenly .. this is the thermostat opening and filling the engine with cold coolant frpm the radiator ... then it will rise again and should sit around the 90 mark. The 156 temperature gauge is notoriously inaccurate so check the real temp using a diagnostic.
If it stays low, 70'ish, the thermostat is stuck open meaning the coolant never gets up to temp. This isn't good for the engine, and will use more fuel.
If it goes way over 90 and fans are cutting in properly, radiator is probably goosed ...
If low fan doesn't cut in, it's normally the resistor mounted just below the top hose. To test, connect the two wires together and fans will then run in a high speed but at correct point.
Another way to test the fans is to pull the temperature sender connection off the sensor and with the ignition on, the fans should run ... this is because the ECU is now confused and sort of goes into safe mode. You can of course test the fan relays using a diagnostic too.

Fitting a new thermostat

Click here and scroll down the how-to's page