M50 525iT Coolant Temp Gauge drops to blue, returns to normal...??

[rizal]

I have 163,000 miles on my '94 525iT, have owned it 6 years and 76,000 miles. It runs beautifully, never overheats, rarely needs coolant added (a cup every 4-5 months at most). New radiator and water pump in November '07 (prophylactically due to mileage) and since then I have this odd symptom, which consists solely of my coolant temp gauge needle dropping from mid-gauge (normal, at 12 o'clock) to the top of the blue range, sitting there severl minutes, then returning to 12 o'clock. Air bubble? More serious??
Help, please. Thanks very much.

[bad_manners_god]

I have 163,000 miles on my '94 525iT, have owned it 6 years and 76,000 miles. It runs beautifully, never overheats, rarely needs coolant added (a cup every 4-5 months at most). New radiator and water pump in November '07 (prophylactically due to mileage) and since then I have this odd symptom, which consists solely of my coolant temp gauge needle dropping from mid-gauge (normal, at 12 o'clock) to the top of the blue range, sitting there severl minutes, then returning to 12 o'clock. Air bubble? More serious??
Help, please. Thanks very much.

Sounds like a faulty temp sensor.

[Ross]

A failing thermostat could cause this symptom, or a faulty sensor/circuit.
Given the frailty of BMW cooling systems and the dire consequences of overheating I would change the thermostat first and observe the result.

[Paul in NZ]

A failing thermostat could cause this symptom, or a faulty sensor/circuit.
Given the frailty of BMW cooling systems and the dire consequences of overheating I would change the thermostat first and observe the result.
first thing to try is to unplug and plug in the connector to the gauge a few times.....
My gauge exhibited this behaviour and after i did this it was fine....surely the first thing to check

[Ross]

Agreed, Paul's idea is a better first tack.

[rizal]

Thanks, Paul. Where exactly is the connector located, and how do I access it? Thanks very much. Al

[Paul in NZ]

not 100 percent sure on the 525...on my 535 there are two leads one going to a sensor near the thermostat,and a brown plug and a blue plug into the main loom.One of those is the temp sensor for the gauge,the other the temp sensor for the ECU.Plug and unplug both...hopefully someone with a M50 will chime in and te you more specifically