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Discussion Starter · #1 ·
Hi everyone, I've just bought a 740i, the car runs nicely but when driving sometimes the temperature gauge will rise rapidly from half way right to the top, there are no signs of overheating and after a while will come back to normal.
The possibility of air lock was mentioned by a friend but reading through other posts I notice that there can be problems with the instrument cluster too.
The temperature gauge sender appears to have been replaced very recently and I notice that it has 3 wires going to it (I'm a bit old school, only used to 1 or 2). So I wonder if I could test.
Sorry for long post but tried to include as much relevant info as I can, also, if I've made any mistake in etiquette such as wrong thread please move and correct me.
 

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What is the month listed as on your production label? can be found in the engine bay.

Also the temperature sender also serves the ECU on your car
 

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Discussion Starter · #3 ·
Its November 1997. I'll have to change the car details, documents say 98 but that's registration date.
 

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Temp switch is shown as number 9 here http://www.realoem.com/bmw/showparts.do?model=GF82&mospid=47517&btnr=11_1535&hg=11&fg=35.

The gauges on e38s are designed to sit right in the centre of the gauge, if they do shoot up suddenly then they are over heating and while steam may not be boiling out you really shouldn't keep driving it like this.

How long before the car starts over heating?

I would also try bleeding the cooling system as well.

How to bleed the cooling system.
1.Jack the front of the vehicle up onto stands as high as possible,
2.Turn the ignition on,engine off,heater on fan setting 1 and temperature fully hot,
3.Remove the rad cap/expansion cap,remove the bleed screw from the rad and IF you have them the bleed screw(s) from the thermostat housing/top hose, return pipe or EGR thermostat.
4.Fill the radiator up and keep pouring till there are no more air bubbles coming out from the bleeds, when good then install bleeds and tighten down(don't snap them),ensure the rubber O-rings/seals are still on the screws/on the bleed holes.
5.Suck some water out so it's not full to the brim, replace the cap and then start the car. Run her up and check for heat from the vents when warm and check if over heating still persists.
6.If good then check the water level again in the morning when cool.
 

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Discussion Starter · #5 ·
Thank you for the advice Ronnie. Today it went right to the top when I first started it from cold, then when I drove off it came back down to the centre again, I've noticed some splitting/cracking of the outer casings of the wire leading to the sender and the wire is a very tight stretch.
 

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That does sound more like a electrical issue then as it went straight up from cold, if it keeps doing this then try removing the sensor plug and see if it plays up, if it shoots up still then goes down then you have a short somewhere
 
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