Coolant Temp is cold while driving, but warms while at idle

[BlazinB5]

After driving for 40 minutes, the coolant temp will show the car as being cold, but warms up when the car is at idle. I'm willing to guess that this would be a faulty coolant temp sensor?


Here's the coolant temp after driving for 40 minutes:


And after that 40 minutes of driving, I decided to let the car idle for 3 minutes and see what happens:

[Hamburger]

I had a short in the wiring for my radiator fan control module that caused the same issue, pretty random and didn't throw a code, might be something to check

[BlazinB5]

weirddd, my fans have been acting odd lately too. during the first start in the morning, i've noticed the fans louder than they normally are. i don't get a CEL either.

also, what is a "short in wiring"? i'm guessing that it has something to do with damage to the wire? and where is this radiator fan control module?

[old guy]

Check for codes. I suspect you have a P1296 as indicated by the fans running on start-up. Without any further information I would guess it's the thermostat. Do you have any method to monitor your temps while driving? VCDS or a Torque app?

[BlazinB5]

I actually do. I've got the torque app with the blue tooth dongle.

[old guy]

I actually do. I've got the torque app with the blue tooth dongle.

So what are they telling you as you drive?

[BlazinB5]

It's look like I have a P3081

[BlazinB5]

Based on that code, I believe I'm going to need a coolant temp sensor?

[SJorge3442]

Based on that code, I believe I'm going to need a coolant temp sensor?

http://forums.vwvortex.com/showthrea...estions-please

Its probably cause a sensor is toast somewhere. Open up the live time tracking in Torque while driving and see what your temps are reading.

[BlazinB5]

http://forums.vwvortex.com/showthrea...estions-please

Its probably cause a sensor is toast somewhere. Open up the live time tracking in Torque while driving and see what your temps are reading.

sweet, thanks for the link. it's only $12.95 for the updated green coolant sensor, so i may as well start with that first,


http://www.europaparts.com/coolant-t...59919501a.html

[SJorge3442]

sweet, thanks for the link. it's only $12.95 for the updated green coolant sensor, so i may as well start with that first,


http://www.europaparts.com/coolant-t...59919501a.html

There are multiple temp sensors. Especially on the 1.8. Go for a drive and check the live time stats and see what your coolant temp is. You could very well have a stuck thermostat too, which would explain the hot while idle and cold when driving IF your temperature gauge is reading the correct temps. Before you order anything, just check what torque says.

Also, order this clip when you buy the sensor. you WILL break it. http://www.europaparts.com/coolant-s...032121142.html oh and the o ring. http://www.europaparts.com/sealing-g...n90316802.html

[old guy]

You can replace the CTS but my suspicion is that the thermostat is faulty. If it was the CTS it would probably read low all the time. The temperature drop while steady state driving is consistent with a bad t-stat.

Good luck!

[Charles.waite]

You can replace the CTS but my suspicion is that the thermostat is faulty. If it was the CTS it would probably read low all the time. The temperature drop while steady state driving is consistent with a bad t-stat.

Good luck!

Came here to say this.

[oaybar007]

I'd try replacing your coolant temp sensors and go from there. If the problem persists, you've got a faulty thermostat.

[BlazinB5]

You can replace the CTS but my suspicion is that the thermostat is faulty. If it was the CTS it would probably read low all the time. The temperature drop while steady state driving is consistent with a bad t-stat.

Good luck!

yep, either the CTS or thermostat, but the good news is my car is due for a timing belt job, so it looks like i'll be replacing the thermostat anyhow! thanks for the info

I'd try replacing your coolant temp sensors and go from there. If the problem persists, you've got a faulty thermostat.

it looks like that's what i'm going to be doing

There are multiple temp sensors. Especially on the 1.8. Go for a drive and check the live time stats and see what your coolant temp is. You could very well have a stuck thermostat too, which would explain the hot while idle and cold when driving IF your temperature gauge is reading the correct temps. Before you order anything, just check what torque says.

Also, order this clip when you buy the sensor. you WILL break it. http://www.europaparts.com/coolant-s...032121142.html oh and the o ring. http://www.europaparts.com/sealing-g...n90316802.html

well, since I am due for a timing belt, it looks like i'll be replacing the thermostat anyhow - talk about perfect timing

[Charles.waite]

I would only trust OEM or Behr brand Thermostats FYI. There have been a rash of defective thermostats on here recently, I would only buy legitimate ones.

[BlazinB5]

I would only trust OEM or Behr brand Thermostats FYI. There have been a rash of defective thermostats on here recently, I would only buy legitimate ones.

yup, Behr is the brand of thermostat that comes with the europa timing belt kit, so i should be good.

http://www.europaparts.com/timing-be...early-vin.html

i'm going to order that kit with every option except for the accessory belt tensioner assembly. and this would be the thermostat assembly that comes with the europa kit: http://www.europaparts.com/thermosta...111k-behr.html

[Let it snow]

It is a good kit. FYI. It does not come with the hardpipe to coolant flange o-ring or j-plug /o ring. Don't know why they are not included. Have fun.

[Let it snow]

Sorry hardpipe to thermostat o-ring.

[Charles.waite]

Sorry hardpipe to thermostat o-ring.

You don't need that unless yours is toast though.

Also if you haven't ordered the j-plug DO get it. It's definitely something that should be replaced.

[BlazinB5]

It is a good kit. FYI. It does not come with the hardpipe to coolant flange o-ring or j-plug /o ring. Don't know why they are not included. Have fun.

i'll probably be drunk by the time i finish it all

You don't need that unless yours is toast though.

Also if you haven't ordered the j-plug DO get it. It's definitely something that should be replaced.

will do, and thanks for the heads up. the more i can get done while i'm in there, the better.

[SJorge3442]

You don't need that unless yours is toast though.

Also if you haven't ordered the j-plug DO get it. It's definitely something that should be replaced.

Not sure how the acc belt tensioner is on the 1.8, but I reccomend all 3.0 guys to do it. i didnt and then I was putting my car in service+ two months later to replace it.....

[FromS60toB61.8t]

Haven't read earlier posts but it's either thermostat or coolant temp sensor.

I had the same problem with a brand new non oem temp sensor.

Just replaced temp sensor with oem and an oem thermostat and all issues gone.

Don't know if it's been mentioned or not but thermostat is replaceable without having to take anything else out besides coolant tank. Atleast it was for me, I have sai/evap delete tho so I have less vac lines.

[BlazinB5]

Not sure how the acc belt tensioner is on the 1.8, but I reccomend all 3.0 guys to do it. i didnt and then I was putting my car in service+ two months later to replace it.....

i was actually wondering about that too. ECS doesn't have an optional kit with that item, but europa does, and i would really hate to have to go back in there for the damn thing.. still not sure if i'll need it though.

Haven't read earlier posts but it's either thermostat or coolant temp sensor.

I had the same problem with a brand new non oem temp sensor.

Just replaced temp sensor with oem and an oem thermostat and all issues gone.

Don't know if it's been mentioned or not but thermostat is replaceable without having to take anything else out besides coolant tank. Atleast it was for me, I have sai/evap delete tho so I have less vac lines.

yep, i'll be ordering up a green coolant temp sensor along with my timing belt kit, and a magnetic oil drain plug for the hell of it

[Jflow23]

I got mine from rock auto...came with the audi symbol scratched off. Win!

[g huns]

I had my rear flange off last weekend. Thought I had the air bubbles all out, but yesterday I heard it make gurgling sound after I shut it off. Burped it again, seemed all good. Have heat, gauge pointed straight up. This morning I fired up the Torque app for the drive in to work. Temps were right around 200F, gauge pointed straight up. At one point I glanced at my phone and the temp was 250F. I watched it for a second and it dropped right back to 200. A few minutes later, back to 250 and then within a couple of seconds, back to 200. It also threw a P3081.

I have never replaced the black temp sensor in the lower hose, but the green one in the rear flange is a year old, though not an Audi part.

If the black sensor controls the gauge, which points straight up once the car is warmed up, does the Torque app read the green one that seems to be all over the place?

Now that I think about it, the fan seemed to come on really soon after starting it this morning. Well before the car was warmed up. If the black sensor sensor goes bad and causes the fan to run when it shouldn't, will the gauge still work correctly?

[SJorge3442]

I had my rear flange off last weekend. Thought I had the air bubbles all out, but yesterday I heard it make gurgling sound after I shut it off. Burped it again, seemed all good. Have heat, gauge pointed straight up. This morning I fired up the Torque app for the drive in to work. Temps were right around 200F, gauge pointed straight up. At one point I glanced at my phone and the temp was 250F. I watched it for a second and it dropped right back to 200. A few minutes later, back to 250 and then within a couple of seconds, back to 200. It also threw a P3081.

I have never replaced the black temp sensor in the lower hose, but the green one in the rear flange is a year old, though not an Audi part.

If the black sensor controls the gauge, which points straight up once the car is warmed up, does the Torque app read the green one that seems to be all over the place?

Now that I think about it, the fan seemed to come on really soon after starting it this morning. Well before the car was warmed up. If the black sensor sensor goes bad and causes the fan to run when it shouldn't, will the gauge still work correctly?

250!?! I would say either air in the system or bad sensor.

[old guy]

I had my rear flange off last weekend. Thought I had the air bubbles all out, but yesterday I heard it make gurgling sound after I shut it off. Burped it again, seemed all good. Have heat, gauge pointed straight up. This morning I fired up the Torque app for the drive in to work. Temps were right around 200F, gauge pointed straight up. At one point I glanced at my phone and the temp was 250F. I watched it for a second and it dropped right back to 200. A few minutes later, back to 250 and then within a couple of seconds, back to 200. It also threw a P3081.

I have never replaced the black temp sensor in the lower hose, but the green one in the rear flange is a year old, though not an Audi part.

If the black sensor controls the gauge, which points straight up once the car is warmed up, does the Torque app read the green one that seems to be all over the place?

Now that I think about it, the fan seemed to come on really soon after starting it this morning. Well before the car was warmed up. If the black sensor sensor goes bad and causes the fan to run when it shouldn't, will the gauge still work correctly?

The temperature gauge gets its reading from the green 4-pin sensor. The green sensor sends one signal to the gauge and a separate signal to the ECM. The sensor in the lower radiator hose sends a signal to the fan control module as well as to the ECM for cooling system performance monitoring. If the ECM receives conflicting information the default is to run the fans continuously.

[g huns]

The temperature gauge gets its reading from the green 4-pin sensor. The green sensor sends one signal to the gauge and a separate signal to the ECM. The sensor in the lower radiator hose sends a signal to the fan control module as well as to the ECM for cooling system performance monitoring. If the ECM receives conflicting information the default is to run the fans continuously.

Got it. Can I see what values each sensor is reading with Vag-Com?

[ECS Tuning-Audi]

Definitely seems like symptoms of a stuck open thermostat. It makes sense at idle that the temp jumps up to normal since the coolant flow is pretty slow and there's not much air circulating around the radiator (if your fans aren't kicking on) then one you start driving the water pump increases flow and there's more air flowing around the radiator.

We have complete thermostat replacement kits available below.

Click HERE to order or for more information


Jason

[old guy]

Got it. Can I see what values each sensor is reading with Vag-Com?

Clicky click

[g huns]

Clicky click

Thanks. My Vag-Com skillset so far extends to making my windows go down with the remote and having the doors lock automatically.

[BlazinB5]

for anyone interested, it was the thermostat housing this entire time. the piece inside of the housing (the thermostat) was jigging around and was loose, so it was clearly busted. new thermostat housing in, and fresh coolant, and now the coolant temp is back to normal - AND no fans going crazy upon startup.