Coolant temperature?

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Deadcentre
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Coolant temperature?

Post by Deadcentre » Mon Aug 03, 2015 4:37 pm

Hi Guys,

Quick one. 2005 Passat tdi 130 AWX estate. According to the climatronic menu my coolant temperature tops out a very consistent 78 degrees Celsius irrespective of anything I do. I do not have any of the symptoms of a sender unit failure and am reasonably sure it's a lazy thermostat. The temp gauge sits still bang at the 12 o'clock position. The car is ten years old now with 90k miles on it so I don't begrudge her things like this.

Question is... Is 78 degrees normal / enough (gut feeling says no)? What should the normal temp be? Is there any harm running at what I think is a cooler than normal temp? Is it worth me replacing the thermostat or will any minor savings in fuel never be recovered when compared to the cost etc? If an increase in temp will be more mechanically sympathetic then I'll change it.

Regards

Steve

PS love this forum and have used your knowledge so many times. Keep up the good work!

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Will UK
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Re: Coolant temperature?

Post by Will UK » Mon Aug 03, 2015 5:54 pm

replace thermostat. should be 90c, not 78. mpg usually drops when the engine not reaching 90c

Deadcentre
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Re: Coolant temperature?

Post by Deadcentre » Mon Aug 03, 2015 6:43 pm

Thanks. Will look into it.

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Will UK
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Re: Coolant temperature?

Post by Will UK » Mon Aug 03, 2015 7:51 pm

buy only genuine. do the job once. you will need o ring as well and coolant to top up.

Deadcentre
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Re: Coolant temperature?

Post by Deadcentre » Tue Aug 04, 2015 10:22 am

Thanks

maclee
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Re: Coolant temperature?

Post by maclee » Tue Aug 04, 2015 11:08 am

If you still have the original coolant temp sensor I would replace that first, (using a genuine part).

It could be that the coolant is at 90C, but the sensor is misreading (hence the 78C in your display).

Changing the sensor is a lot quicker and easier than changing the thermostat.

I am not for one moment saying the thermostat is not suspect, just making a suggestion that might save you some time and trouble.

Deadcentre
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Re: Coolant temperature?

Post by Deadcentre » Tue Aug 04, 2015 4:20 pm

I see what you mean.

neil123
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Re: Coolant temperature?

Post by neil123 » Mon Aug 17, 2015 9:10 pm

I had the same issue with my AWX tdi. I replaced both the coolant temp sensor and thermostat at the same time. Only use genuine parts tho.
Problem solved.

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toonmania
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Re: Coolant temperature?

Post by toonmania » Tue Aug 18, 2015 6:11 am

Yes I had similar issues on my engine replaced both CTS and thermostat took around 30 mins max and it's been perfect since :-bd
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ms51ves3
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Re: Coolant temperature?

Post by ms51ves3 » Tue Aug 18, 2015 9:45 am

Will UK wrote:replace thermostat. should be 90c, not 78. mpg usually drops when the engine not reaching 90c
I don't think you'll notice any difference in MPG. My cars sits at low 70s on the Climatronic and I can get 68MPG on a run.

Save your money and spend it on diesel.
Last edited by ms51ves3 on Tue Aug 18, 2015 7:09 pm, edited 1 time in total.

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harveypj
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Re: Coolant temperature?

Post by harveypj » Tue Aug 18, 2015 10:35 am

As above,
Changed my thermostat when mine was running around 78degs. It runs at 90 now but no real difference on MPG.
Ambient air temp and humidity seem to be able to make a huge difference at times.
I think as long as your above 70 degs probably not worth worrying about. This is probably why the dash binical needle reads 90, VW factoring in thermostat performance tolerance.
Pete
B5.5 PD130 AVF Sport est

maclee
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Re: Coolant temperature?

Post by maclee » Tue Aug 18, 2015 11:01 am

The sensing elements for the engine management system and the temp gauge are actually two different devices, both built into the same actual 'sensor' unit, (hence the 'sensor' that you change has 4 pins, 2 for each of the 2 sensing elements therein).

A situation can easily occur where one of these sensing elements drifts out of tolerance while the other is working correctly. This may be why there are these differences in values between the displays. It could be that the coolant temp is actually 78deg with the instrument sensor over reading and showing 90, or that the coolant temp is 90deg with the engine management under reading at 78deg.

If you change the sensor and both readings are than pretty well the same it will prove which of the sensing elements was at fault, and if both then read low, 78deg, it suggests that a new thermostat is required.

It is tempting to leave it, but I would not do so, and here is why.

Everything may be working fine now, BUT if the actual coolant temp is low, (usually because of a faulty thermostat), you will notice it in winter when the lack of heat in the coolant adversely affects the operation of the heater.

Personally, I would change the sensor (with both sensing elements) first, and if the coolant temp remains low I would go for the thermostat.

Who knows?, having done all that you may find you still have this difference, (for some other reason), but at least you will have peace of mind and know that these two components are new. What you really do not want is to find that you have an ineffective heater in winter, and have to do this job in the cold, (not that summer is being very wonderful at the moment!).

jimbof
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Re: Coolant temperature?

Post by jimbof » Tue Aug 18, 2015 12:44 pm

maclee wrote:Everything may be working fine now, BUT if the actual coolant temp is low, (usually because of a faulty thermostat), you will notice it in winter when the lack of heat in the coolant adversely affects the operation of the heater.
My temp has been low for a while (around the 78 mark as per the OP). Changed the CTS as this was definitely faulty (VCDS). I had been debating the merits of doing the thermostat, but your explanation clinches it for me, and actually makes me realise that my car has been like this for a long time - my heater performance in winter has always been awful. Thanks! :)

Deadcentre
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Re: Coolant temperature?

Post by Deadcentre » Thu Aug 20, 2015 7:22 am

Thanks Guys.

Will do them both I think. Good advice.

Steve

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