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645ci coolant

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Hi guys I’ve just recently purchased a 2004 645ci from a friend but it’s been sitting for nearly a year… when he purchased the car 2 yrs ago he was told the temp gauge didn’t work and he never experienced any issues with the heating not working or over temp.


He dropped it off to me and gave it one last blast as you would and then the first drive I had of it the I noticed the Guage wasn’t working and it was obviously hot.

input the cabin heating on full and it was Luke warm for a start but then went cool, I thought the engine bay smelled a bit hot and the pipes at the top of the engine were hot however at no point did I hear the engine fan cut in ….

I understand these engines run hot but I would have thought it would have kicked in.I haven’t driven it since as I’ve been working but I am hoping to have a look at it today …

I have a couple of questions I hope you can help me with ..

1. What thermocouple/sensor controls the guage in the cluster and does this sensor also control the fan.

2. Is the thermostat a common failure? I think it’s relatively easy to change but how do I prevent any air lock issues when replacing? 

new to the 6 series but also have an e53 X5 which I’ve done a fair bit of work on.

thanks in advance for any answers and thanks for letting me join.

regards rob 


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Hi Rob

Welcome to the Forum

BMW use 2 sources for signals to the temperature gauge which uses an average from the 2

First is the Temperature sender in the water pump housing (and yes change the Thermostat while your there) the second temperature sender is in the Lower Radiator hose. The lower one also provides signal for the Fan, normal operating temp  should be between 75C and 115C (the needle in the middle of the gauge) I have an infra red thermometer and measure at the top hose

Running the V8's to hot can cause all sorts of issues non of them are cheap to fix, the fact that your heater is blowing cold would lead me to check the level and quickly low coolant is a likely cause as is an air lock. Remember your trans cooler and engine oil cooler are linked to the Rad. If topping up get the correct coolant and dont use just water.

Good Luck hope you catch it before any damage occurs


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Hi Dave thanks for the reply,


it’s dawned on me the dash temp gauge is oil temp so I need to locate that sensor! 

the fan visible behind the rad does turn,I saw it working after I warmed it up but I haven’t heard the electric fan cut in.

I have bought a replacement thermostat to fit but haven’t as yet i also found a video showing how to test the auxiliary electric water pump by holding the throttle down … mine appears to make noise but nothing come back into the header ….

also I can’t seem to get the water temp displayed on the dash using the secret menu hack it’s like the unlock code isnt accepted when I select option 7 it just goes back to displaying outside temp.


my vacuum pump is also a bit noisy ( sounds a bit like a wheel from a children’s bike stabiliser) other than that noise the engine sounds perfect and  the oil is clear of any sludge..


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Morning Rob

1st The temperature sender in the water pump housing is dual purpose it sends a signal to the DME which interprets that for both Oil Temp and Water Temp I would change it when changing the Thermostat 

If the Aux Water Pump isn't recirculating coolant, (1) is the Coolant level correct (2) You can hear the Aux Pump running but it doesn't seem to be moving coolant so check the Coolant valve on the nearside inner wing (not uncommon for these o fail) 

Is the noise the Vacuum pump (constant) or the Emissions air pump stops after a short while?

If all sensors are operating as they should there could be a problem with the cluster it is better to repair the original if you can there are a few companies who do it ECU Testing or Cartronix come to mind.

Good luck



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