Expert: Sean Halliday - 2/10/2010

Question
QUESTION: Hi. My BMW recently started to experience a strange issue. After the car warms up to halfway on the temperature gauge the needle starts to "bounce". It will bounce up to 3/4 and back down again. After more driving it will go to 3/4 and then start bouncing into the red zone and back down to 3/4. When the needle bounces it is rapid. Could it be a thermostat? Or could it be one of the sensors I see on top of the thermostat housing (there is a blue one and a brown one)? I checked to see if one of the sensor wires are loose and they are on tight. When I turn the car off and then turn the key (only to the position where the radio will play) I notice the needle still bounces. It only bounces when the car is hot, not when it is cold. Hopefully it is something simple. Thanks.

ANSWER: Possibly a short in the wire from the sender or a faulty gauge on the dash.I would change the sender on the engine and if the same,try a different gauge.

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QUESTION: Thanks Sean, I truly appreciate the quick response. Would that sender be one of the 2 sensors on the thermostat housing?

Answer
Yes it is and it is usually the blue one.Check with the parts store for the correct one as some are different colors due to different makers.An easy way to test the gauge is earth the wire from the sensor with key on and if the gauge goes all the way to hot,it is ok.This is with key on engine off.