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Temp Needle Acting Up


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#1 soulman123

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Posted 06 December 2012 - 10:17 PM

My temp needle us acting weird. If I drove for 15 mins the temp needle sits at 90 bang in the middle. But if I put the heater on it gradually goes to approx 25 % then if I turn it off goes gradually back up ?

I've never seen this on any car bar my last car a ll4 golf and it turned out to be a sticky thermostat and a temp sensor. The garage said start with the stat as its cheaper then that was ok problem was still there so they did the temo sensor.

I wonder if this is what's wrong with the focus.

the stat looks easy enough to get to on the 1.8 no idea where the temp sensor lives on this ?

(1.8 04 petrol)

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#2 Pitmonster

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Posted 06 December 2012 - 11:11 PM

The heater draws its heat from the water in your cooling system - somewhere in all that plumbing is a heat exchanger (which is activated by the temperature knob on your heater control panel).

So when you put the heater on, heat gets transferred from the water and into the cabin.

Therefore the water temp goes down, as does the temp gauge. And when you turn the heater off, this transfer of heat stops and your temperature dial goes back up

It's perfectly normal behaviour, and there's nothing wrong with your car.

In fact when a car starts to overheat, it's an old trick to put the heater on full whack because it helps to bring the water temp down.

#3 stef123

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Posted 07 December 2012 - 12:10 PM

A petrol engine should have no problem maintaining the water temperature when using the heater, diesels are a different story.

It sounds to me like your thermostat is opening slightly. I have never had an issue of the temp needle dropping when driving a petrol car, even when it is very cold outside.

What sort of speeds are you doing and what have you got the heater set to?

#4 artscot79

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Posted 07 December 2012 - 02:13 PM

i had mine in for a similar issue and was told that when its as cold as it is -1 the air going through the bonnet is around -5 the thermostat is closed and unlikely to open on short trips so the heat is simply coolant going through the engine and into the expansion tank since you are using the heater on hot with the fan above setting 2 the heat is being drawn from the engine therefore the coolant wont be as hot i had the thermostat and temp sensor checked and all were working perfectly he took a 2008 petrol out with me and the same thing happened as he says its a coolant sensor that shows the temp on the dial

#5 Pitmonster

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Posted 07 December 2012 - 04:02 PM

Unless your water temp gauge is in the red, it doesn't matter what it says.

It can happily go from 1/4 to 1/2 to 1/4 all day long, due to the effects of sitting in traffic, cooling fan turning on and off, and use of the heater.
Unless its in the red, it all makes no difference

Many new cars don't even have a water temp gauge at all - just a warning light if it gets too hot.

#6 soulman123

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Posted 08 December 2012 - 08:21 PM

A petrol engine should have no problem maintaining the water temperature when using the heater, diesels are a different story.

It sounds to me like your thermostat is opening slightly. I have never had an issue of the temp needle dropping when driving a petrol car, even when it is very cold outside.

What sort of speeds are you doing and what have you got the heater set to?


I'm on roads of 30/40/50 and the temp drops within 2 mins of putting the heat in speed 2.

#7 artscot79

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Posted 08 December 2012 - 10:07 PM

as said in this cold weather the coolant bypasses the rad so its from the engine to expansion tank directly then the pump draws the hot coolant from there back into the engine to maintain the temp as soon as the coolant goes through the rad the cold air will cool it quickly and the thermostat shuts again and the coolant goes back through the expansion tank the heater draws heat from the engine so if its on 2-4 and set to the hot section the engine struggles to keep the temp going the temp gauge is not engine temperature its coolant temp and with the method above the coolant will not stay hot easy way to test set the fan temp to cold or midway and watch what happens the engine will maintain the heat and the dial will sit higher

#8 stef123

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Posted 09 December 2012 - 12:30 AM

even in my diesel, i can easily maintain the gauge in the middle while driving at 60 with the heater set on high and the fan on full, like i say i any petrol car i have driven has never had an issue like that while driving - idling certainly.

a basic check you can do without removing anything is once the engine has warmed up slightly - enough to feel a bit heat in the pipes, check and see if both radiator hoses are warm. if they are you got a problem with the thermostat

#9 soulman123

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Posted 10 December 2012 - 01:01 PM

Slightly conflicting information hehe.

Out of interest I have been driving it about and hour 30 mins before I could stop to check the rad pipes both are equally hot. I'm guessing I should have done this on a cold start and kept feeling to see if the thermostat was opening. ill try that one again.

Also asked my usual garage one guy said its normal like some have said here, but the other mechanic said it sound like a partly stuck thermostat. He said its a sub ten pound part and 15 mins to change and easily accessible on the 1.8 engine.

Curiosity has me wondering who is right.

#10 stef123

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Posted 10 December 2012 - 02:17 PM

Slightly conflicting information hehe.

Out of interest I have been driving it about and hour 30 mins before I could stop to check the rad pipes both are equally hot. I'm guessing I should have done this on a cold start and kept feeling to see if the thermostat was opening. ill try that one again.

Also asked my usual garage one guy said its normal like some have said here, but the other mechanic said it sound like a partly stuck thermostat. He said its a sub ten pound part and 15 mins to change and easily accessible on the 1.8 engine.

Curiosity has me wondering who is right.


yup 30 mins of driving is way too long, let it run for 5-10 mins and then keep checking the hoses, only one of them should be warm until the thermostat opens.

#11 soulman123

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Posted 11 December 2012 - 12:44 AM

ok I had an 80 mile trip to do (motorway)
. I drove 5 mins and the temp was registering some heat. Checked both pipes and both were hot.

To further confuse things on the motorway with higher speeds mire cold aur blasting through, the temp needle didn't budge when on 4! then back at 40mph in second heat setting it drops.

the heat direction was set to aim at windows with the temp dropping.

if set to mid body/feet the needle does not move at any heater spwr mph speed.

Confusing lol.

#12 artscot79

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Posted 11 December 2012 - 02:25 AM

If both pipes were hot after just 5 mins of driving from cold then something is amiss in these temperatures it shouldnt be that hot that fast

#13 soulman123

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Posted 11 December 2012 - 11:50 AM

Im guessiong stef was right about the thermostat being stuck.

Its less that £10 from eurocarparts for a non genuine one which should do the trick.

I guess the process is drain the coolant from the rad drain doodah, 3 bolts on the housing, (is acess easy, easy to these bolts) remove old sensor and any seal and replace right way round as old one and bolt up, refill coolant with lid off and top up as nescessary ?

#14 artscot79

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Posted 12 December 2012 - 01:44 AM

From experience get a genuine one the non genuine dont tend to be that good

#15 cragger89

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Posted 12 December 2012 - 10:32 AM

It should always be in the middle (90degrees) unless there is a problem somewhere - thermostat seems most likely in this instance.

I am in the cold, cold south and once the engine has warmed up, the temperature needle never drops.

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