Thermostat or coolant sensor? Temperature gauge fluctuates from normal to cold

[Satummoo]

When I drive, sometimes my temperature gauge will work and sometimes not. My car will warm up to the middle and then all of a sudden drop all the way to cold and sometimes stay their for awhile or go back to the middle. Do I need a new thermostat or is it a coolant sensor issue?

[2.7taudi]

When I drive, sometimes my temperature gauge will work and sometimes not. My car will warm up to the middle and then all of a sudden drop all the way to cold and sometimes stay their for awhile or go back to the middle. Do I need a new thermostat or is it a coolant sensor issue?

This happened to mine and it went away after I did the timing belt, water pump, thermostat.

But if money is an issue start with the coolant sensor. They are very cheap and easily accessible.


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[Seerlah]

Both can be replaced for $35 shipped. Just replace both of them. That's what I did when mine my gauge was acting up (not as bad as yours). I first did the CTS and that did not fix the issue. Replaced the thermostat next, and issue was fixed.

Thermostat with o-ring: $20 shipped from MJM
CTS with o-ring: $15 shiiped from MJM

[R3V3NANT]

I must be reading this wrong. You're talking about a whole thermostat assembly for $20? I only see them at MJM for $130, which is around what I paid through another site.

[Seerlah]

My mistake. I never knew your guys thermostat was like that. I have the same 06A block. And figured you had a provision on the block for the t-stat with o-ring, and the flange simply bolts to the block. This is how it is on my block (same 06A).

Just a flange held on by 2 5mm hex bolts, and the t-stat sits underneath that.

[Charles.waite]

Nope. We got screwed. For the b6 and b7 you have to replace the whole housing. It's possible you could Dremel it open and replace the tstat itself and then jb weld it shut but I would never trust that.

OP 99% sure your issue is the thermostat. Replace that and it'll be resolved. I would tke a look at the coolant temp sensors though and if they look old and cruddy I would replace them too.

[Satummoo]

thanks, kind of annoying when the PO just had the timing belt service done 2k miles before I bought the car and didn't change the thermostat, agh!

[daihashi]

I must be reading this wrong. You're talking about a whole thermostat assembly for $20? I only see them at MJM for $130, which is around what I paid through another site.

I think I paid $95 for mine over at ECStuning. Behr brand.

[alexwonggjy1]

Mine was acting weird: basically cannot show any temperature no matter how long I drive

Replaced the coolant temperature sensor and it works.

Your case, I think, is relevant to the thermostat .... not the sensor

[PNW Avant]

Nope. We got screwed. For the b6 and b7 you have to replace the whole housing. It's possible you could Dremel it open and replace the tstat itself and then jb weld it shut but I would never trust that.

OP 99% sure your issue is the thermostat. Replace that and it'll be resolved. I would tke a look at the coolant temp sensors though and if they look old and cruddy I would replace them too.

Where are both the sensors? I'm replacing the T-stat (+J plug) and rear flange shortly so I think this would be a good time to do the sensors, as well.

[R3V3NANT]

The top sensor is on the rear coolant flange, just behind the head under a mess of hoses and whatnot. The other one is located at the bottom of the car, driver's side, just off the radiator. It's removed to drain the coolant.

And for the love of all that is holy, do NOT buy an Ebay offbrand thermostat, no matter how good the deal looks or the feedback is. For only $20-$30 more, buy a Behr thermostat that you won't have to replace in a month. I have first hand experience on this.

[PNW Avant]

The top sensor is on the rear coolant flange, just behind the head under a mess of hoses and whatnot. The other one is located at the bottom of the car, driver's side, just off the radiator. It's removed to drain the coolant.

And for the love of all that is holy, do NOT buy an Ebay offbrand thermostat, no matter how good the deal looks or the feedback is. For only $20-$30 more, buy a Behr thermostat that you won't have to replace in a month. I have first hand experience on this.

Thanks for the tips on the sensors.

I already purchased the T-stat; got it from genuine audi parts. I put no less than OEM or equivilent (and only in some cases) on my car.

[Charles.waite]

Thanks for the tips on the sensors.

I already purchased the T-stat; got it from genuine audi parts. I put no less than OEM or equivilent (and only in some cases) on my car.

Behr is OEM. I bought a behr one for my b7 and it was identical, just had the rings filed off, haha. But yes I agree, I only go OEM quality or better wherever possible. I hate doing things twice.

[waldo1324]

i replaced the top and bottom sensors and my warm ups are still janky.

i concur on the thermostat- and just pull the mani, its right there. may as well do j plug too.

[Charles.waite]

It's actually a relatively straightforward DIY. Just need a few Allen sockets and a bit of patience.

Definitely do the j-plug too. Ticking time bomb those things are...

[waldo1324]

Ticking time bomb those things are...

thanks Yoda Waite

[travrach]

thanks, kind of annoying when the PO just had the timing belt service done 2k miles before I bought the car and didn't change the thermostat, agh!

Just another idea..seeing how T-Belt and I'm guessin water pump was done not long ago, maybe worth check for air pocket!

[Satummoo]

Just another idea..seeing how T-Belt and I'm guessin water pump was done not long ago, maybe worth check for air pocket!

How would I go about doing that?

[R3V3NANT]

Man, I know OG had some great information on bleeding the system, which you'll have to do to relieve any leaks and check that possible cause off your list. I can't find it for the life of me, though.

There's a bleeder screw (an allen head) on a hard coolant pipe over the manifold that you can unscrew partially to relieve pressure. I would take the screw off the coolant reservoir and hold it upwards, so the coolant hoses that go to the radiator are pointing down. You can squeeze the radiator hose to see if any air bubbles come out of the pipe.

Additionally, you can take off the rain tray and loosen the hose that goes to the heater core - the left one, I believe - and do the same squeezing procedure to force coolant out of there. You only want to pull that hose back until the little hole in it is off the hard pipe.

I'm pretty sure you can do this cold i.e., without having to warm the car up. Hopefully someone else can chime in and correct anything I missed.

[travrach]

Yea he got most of it, . best way is as was mentioned to pull back heater core hose under plactic cowl by battery, you will see a small hole in one of the rubber hoses. You want to pull clamp back, then pull rubber hose back till the small hole is not on pipe any longer, you will only have like 1/4 inch of rubber still on heater core. Start car, basically as coolant starts to flow the air will purge out of hole, as you see a steady stream of coolant put the hose back on. Other things to help are lifting the coolant tank up in air some to raise its level above hoses. Generally it takes some time to get all the air out of these system. Hope this helps some.

[Charles.waite]

Yea I replaced my flange almost 1 months ago and I'm still finding air in the system.