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Temp Gauge Goes down while driving.

Reaper1

Go Kart Newbie
Location
.
Who said the OP has a GTI?

Anyways, looks like you got your answer OP...more then likely a faulty coolant temp sensor is the culprit here and your situation was not normal like some have mentioned...

But isnt it weird that there are no faults related to these problems in the car's computer at all?
 

russiankid

Drag Race Newbie
Location
PA
Are you sure? The temps it provides seem reasonable and true.

It's never matched my scan data, pretty much just sits at 192. Could be they changed the programming for it at some point.

Agree with Wrath. Through scanning, mine sat at 194 while the scan went as high as 214 in summer traffic.


For OP, I don't think this is normal. I've never seen this happen on my MK7 even with temps well below 0F.
 

Wrath And Tears

Go Kart Champion
Location
Azusa, CA
Car(s)
17 Sport, 99 E36
But isnt it weird that there are no faults related to these problems in the car's computer at all?

It is strange, but not unheard of. It must be just with in the "acceptable" range so that it doesn't create a code. Most things have to get outside a certain range. For example BMW will pollute up to 1.5 times normal before setting a fault code. Strange stuff indeed.
 

Wrath And Tears

Go Kart Champion
Location
Azusa, CA
Car(s)
17 Sport, 99 E36
ThermostAt stuck open...


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Would be my guess as well.

Of course, I mostly work with fish and stuff like that, so...

Temp sensors are a common failure point for VAG, but a stuck open thermostat would cause similar issues. It would also cause a check engine light (after a few drives). This would be the first failed thermostat (versus just the thermostat/water pump leaking) I've read about for the gen 3, but you never know. You would notice your heat isn't as hot and it takes much longer to reach temps, if it ever does, with a stuck open thermostat. Erratic instrument cluster readings normally point to an issue with the sensor.
 

bdelaney4000

Ready to race!
Location
Chester, NH
Had the same issue on my MKV... dealer changed the thermostat, problem solved. I never ended up with a CEL from it, but after the new thermostat it never did it again. The car also seemed to warm up quicker after the repair too.
 

Tsi7

Ready to race!
Location
Ontario, Canada
By what the OP describes it sounds like a bad coolant temp sensor. A stuck thermostat would generally not be indicated on the coolant gauge unless it opens so early that the car is running extremely cold (or it's closed and the engine is overheating, but that would be pretty obvious)

As we know, the coolant gauge is a dummy gauge. In my mk4, my thermostat would open at around 78 to 81 degrees celcius... Well below the optimal 90 degree mark. There was zero indication anything was wrong, my heat worked, my coolant gauge showed 90 and I had no codes because the mk4 had no sensor on the thermostat and the coolant temp sensor was doing its job. I don't know how the mk7 is, but I imagine it's similar

If obdeleven is reading a normal coolant temp value (aka 200 degrees Fahrenheit) while the gauge is fluctuating then I would look at the sensor first before touching the thermostat... I would imagine if the thermostat is bad the temperature wouldn't sit at 200, it would probably be lower as thermostats tend to stick open

It's cheap to replace the coolant temperature sensor and I imagine it should be fairly easy to access and replace

OP it's your call... I'm not a mechanic but I'm going off previous experiences and coolent temp sensor sounds more likely then thermostat by what you described
 

Wrath And Tears

Go Kart Champion
Location
Azusa, CA
Car(s)
17 Sport, 99 E36
Most VW thermostats are set to start opening around 82C and fully open by 90C (the older ones would actually have a mark on them). Since the MK7 thermostat is build into the water pump, I'm not sure (haven't looked up) when it opens. I could buy a performance thermostat for my A4B5 that opens at 72C instead. For this reason, some companies like BMW have an electronically controlled thermostat (there's a heating element to open it early) when the car is being driven spiritedly, it will open the thermostat and keep it open, to run the engine cooler. Hot engines are better for emissions, but worse for performance.

There are actually multiple temp sensors. One in the thermostat / water pump, and one in the lower radiator hose. The one in the hose is what reports to the instrument cluster, as well as a secondary check for the ECU. The sensor in the thermostat / water pump is the main reference for the ECU.

If the car is just idling when you are checking it, even with an open thermostat the readings may look normal. Once you start driving the car, the additional airflow over the radiator will start to drop the temp pretty drastically.

Another way to check for a faulty sensor is to graph the coolant temp. Look for "fall outs" where the sensor all of a sudden starts reporting a vastly different number. Usually much lower than the temp actually is. Best way to manually verify temp is with a laser temp sensor (or whatever they are called).
 
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Jever

Ready to race!
Location
IL
Car(s)
2010 MK6 GTI
Hate to wake the dead thread, I needed to share and I'm hoping my future MK8 R doesn't have this issue.

My 2010 MK6 would have a fully "warmed" engine driving exactly 8 miles on mostly highway speed roads. I would then park and wait for my wife to get off work on occasion, sometimes I would sit for around 30+ minutes idling in winter weather (freezing & below) the water temp would slowly dip! I can't remember if the actual cabin heat would dip but if I recall correctly it wouldn't stay as warm in the cabin. I would always have the fans on low and I'm not one to keep the cabin like a sauna.

I strongly recall having exhaust smell inside the cabin when sitting for a long time as well. I would usually give up when the cabin got cooler or when I noticed exhaust smell I would go inside... I believe this was due to the exhaust being so close to the hatch opening and perhaps the seal wasn't very good.
 

mike-y

Go Kart Newbie
Location
Los Angeles
Car(s)
2017 GTI / 2016 R
one of my old cars did this, and it turns out it was a little low on coolant, and must have had a bit of an air pocket that was just large enough to not let the coolant system maintain constant pressure, but not enough to cause any other issue, except every time the thermostat opened up, temps dropped a lot more than they were supposed to. Then when the t-stat closed up again, temps rose back to normal. the temp needle would fluctuate up and down quite a bit.
 
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