Coolant temp sensor and heat

[yb28]

Hi everyone,

So my temp gauge acts funny and go from middle and drop half way at around 100-140 km/h, I searched it up and asked someone that when the gauge drops, my heat should also be gone. However, my heater seems to come on whenever it wants and in some days (esp when its frigging -15 outside now) it wont even come on. I do have a flap motor code about the temp sensor motor and the ECS order will be here today. But I cannot tell if its the thermostat thats going or what since I cant test feel my heat.

I wonder if anyone else has something that's similar to my problem? Should I replace both thermostat or CTS (or are they the same thing? I'm talking about the one behind the alternator and apparently another one at the lower coolant hose)?

Please help out lol, its so goddamn cold..

[yb28]

bump

[SlickFix]

Clickity Clickit!

[yb28]

so youre just suggesting me to change the CTS first then thermostat ?

[walky_talky20]

Check coolant temp sensor reported value in the ECU and the CLUSTER. Try to do it when the gauge acts up (ie: data log it). If the values agree in both modules (ie: both dive cold when you accelerate or what-have-you), then it's probably the t-stat. Maybe. I'd probably still change the CTS first, though. Cause it's easier, and a lot cheaper.

[SlickFix]

If it were me, and I was driving without heat in the current sub 20 degree weather in Minnesota, I would replace both. It's funny how when it's really cold, you do things you normally would hesitate to do when it's 70 degrees out.

[yb28]

Check coolant temp sensor reported value in the ECU and the CLUSTER. Try to do it when the gauge acts up (ie: data log it). If the values agree in both modules (ie: both dive cold when you accelerate or what-have-you), then it's probably the t-stat. Maybe. I'd probably still change the CTS first, though. Cause it's easier, and a lot cheaper.

do you know which measuring block is that?

[yb28]

If it were me, and I was driving without heat in the current sub 20 degree weather in Minnesota, I would replace both. It's funny how when it's really cold, you do things you normally would hesitate to do when it's 70 degrees out.

lol so true..

[yb28]

bump

[walky_talky20]

I think its 003. Just kick through the first couple. It's right there. Same with cluster.

[virgoavant]

I noticed my temp gauge drops a little with the heater on full blast but I just figure thats the heater blower fan cooling the coolant down. Is that an issue or can I ignore that?

[yb28]

I noticed my temp gauge drops a little with the heater on full blast but I just figure thats the heater blower fan cooling the coolant down. Is that an issue or can I ignore that?

youre not the only one, dont think its a big deal, but dont think its normal either haha, search it up on here...i found some discussions..

my case however isnt as common so yeah..i ordered all 3 sensors so hopefully it'll fix the problem :|

[yb28]

so it seems like when its 10C and above my gauge dont do anything funky....

and heat comes on when its not subzero as well... anyone else has any input?

diagnosticator?

[old guy]

do you know which measuring block is that?

Monitor blocks 130, 131 and 132. Block 130 will give you the engine outlet temperature and the radiator outlet temperature. Block 131 will give you engine output and engine output specified. The specified will be around 100° c depending upon RPM. Block 132 will give you the radiator outlet temperature as well as the delta between the engine and radiator outlets and the thermostat heater duty cycle.

What you should see is that the block will heat up to around 100°c while the radiator outlet will remain steady. The delta will be significant. Once the engine outlet goes over 100°c you will see the thermostat duty cycle start to increase and the lower radiator outlet temperature will increase. The delta will decrease until you see the engine outlet drop below 100°c. The duty cycle will decrease and the thermostat will close.

If the engine outlet temperature never reaches 100°c and the duty cycle stays really low (3% to 5%) you need a new thermostat.

Edit: And I forgot to mention since I figured it was implied but if the engine outlet is below 100°c and the delta between the engine outlet and radiator outlet is low that would be another indication of a faulty t-stat.

[yb28]

Monitor blocks 130, 131 and 132. Block 130 will give you the engine outlet temperature and the radiator outlet temperature. Block 131 will give you engine output and engine output specified. The specified will be around 100° c depending upon RPM. Block 132 will give you the radiator outlet temperature as well as the delta between the engine and radiator outlets and the thermostat heater duty cycle.

What you should see is that the block will heat up to around 100°c while the radiator outlet will remain steady. The delta will be significant. Once the engine outlet goes over 100°c you will see the thermostat duty cycle start to increase and the lower radiator outlet temperature will increase. The delta will decrease until you see the engine outlet drop below 100°c. The duty cycle will decrease and the thermostat will close.

If the engine outlet temperature never reaches 100°c and the duty cycle stays really low (3% to 5%) you need a new thermostat.

Edit: And I forgot to mention since I figured it was implied but if the engine outlet is below 100°c and the delta between the engine outlet and radiator outlet is low that would be another indication of a faulty t-stat.

my heat comes on whenever it wants, usually above 10C when i DONT need it, but it has been blowing very minimal to no heat for a while now, the one heater core pipe is warmish and the other one isnt, probably a plugged heater core?

my temp gauge also drops randomly, i thought it was only at above 120 but now even at idling,

I logged it with vagcom today and my duty cycle is mostly at 3.9, and goes up sometimes if im accelerating.

thoughts? CTS, Thermostat and HC?

I will try to upload the log tonight and see if anyone want to analyze it ?

(apologize for copy and pasting the same thing on 2 different post, just want to find answers for this!)

EDIT: didnt read it carefully, i think i do have a bad thermostat because my engine never reaches 100 also a plug heater core...

[SlickFix]

oldguy doesn't ever post during the work day, usually only in the evenings, so it's going to be awhile before he chimes back in. In the meantime, did your duty cycle ever go above 5%? In block 130, did the engine outlet temp ever get to 100C? If not, then according to oldguy, you need a new thermostat.

Regarding you heater core, if one is warm and the other is cold, then yes, it's possible that your heater core is plugged. The problem here is that you could be fighting multiple issues right now. It wouldn't hurt anything to perform a flush on the heater core. If you do a google search for "b6 a4 heater core flush", you should find quite a few threads on the topic.

Did you already order the green CTS? That's the cheapest thing to replace first, and is good for preventative maintenance even if it's not the main culprit.

[yb28]

oldguy doesn't ever post during the work day, usually only in the evenings, so it's going to be awhile before he chimes back in. In the meantime, did your duty cycle ever go above 5%? In block 130, did the engine outlet temp ever get to 100C? If not, then according to oldguy, you need a new thermostat.

Regarding you heater core, if one is warm and the other is cold, then yes, it's possible that your heater core is plugged. The problem here is that you could be fighting multiple issues right now. It wouldn't hurt anything to perform a flush on the heater core. If you do a google search for "b6 a4 heater core flush", you should find quite a few threads on the topic.

Did you already order the green CTS? That's the cheapest thing to replace first, and is good for preventative maintenance even if it's not the main culprit.

hey slick!

yeah, as i said, eng temp never goes over 100, duty cycle mostly at 3.9 but does go over i think when im accelerating

yup will have it flush and yes i am fighting multiple issues, vagcom says my temp servo flap is fucked but the code comes on once in a while,

I hope i dont need to order a new heater core, but i already have a new tstat, the 4pin and 2pin CTS and servo motor sitting in the trunk :)

[SlickFix]

There ya go, get wrenching!

Btw, if you never want to worry about bleeding the cooling system ever again, do yourself a favor and invest in one of these. You'll be so glad you did.

[yb28]

There ya go, get wrenching!

Btw, if you never want to worry about bleeding the cooling system ever again, do yourself a favor and invest in one of these. You'll be so glad you did.

haha i have a mechanic to do all these for me for cheap, i dont have the tools or place right now to do it myself :( but thanks anyways!

[yb28]

There ya go, get wrenching!

Btw, if you never want to worry about bleeding the cooling system ever again, do yourself a favor and invest in one of these. You'll be so glad you did.

one more thing, isnt the centre of the gauge 90C ? why would it be 100C? (oh god i sound dumb now)

[yb28]

http://www.filedropper.com/log-01-115-130-134

http://www.filedropper.com/log-01-130-132-134

here are the logs!

Old guy or whoever, please take a look!

[old guy]

This is gonna take a bit of 'splaining so I will do my best to see if I can make a little sense out of what you are looking at with your logs.

First order of business is to give a basic understanding of how a mapped thermostat operates. The thermostat has a mechanical element that opens at a set temperature. The actuator in the thermostat has an incorporated heating element that the ECM can use to "trick" the thermostat into thinking the coolant is hotter than it really is. The ECM uses this feature to provide additional cooling when it feels it would be advantageous to the prevailing conditions.

The coolant temperature is measured at two points. The rear CTS measures the coolant temperature as it exits the block. The lower radiator hose CTS measures the temperature coming out of the radiator after it has been cooled. As long as the thermostat is closed the temperature at the lower CTS will be low. Considerably lower than the temperature leaving the block at the rear CTS. When the thermostat opens you will see a rise in temperature at the lower CTS as the thermostat begins to allow the coolant to begin circulating through the radiator and into the block. When the thermostat is closed the coolant completely bypasses the radiator and flows back through the thermostat housing via the lower coolant hard pipe and the last coolant drop line from the upper hard pipe return line into the thermostat housing. When the coolant gets hot enough the thermostat opens up and the radiator circulation begins. All this is controlled by the mechanical portion of the thermostat. Looking at your logs I can see that your mechanical portion is controlling the exit temperature to around 93°c. Anything above that and the ECM starts to increase the duty cycle.

Now the fun part begins. As long as there is no load on the engine the ECM is content to leave the thermostat alone and let it do its thing. When this is occurring you will see a duty cycle of 3-4%. Yours is set at 3.9%. Whenever you increase the load on the engine the ECM recognizes that an increased load will result in increased block temperatures so to compensate the ECM increases the duty cycle to the thermostat. This "tricks" the thermostat into thinking the coolant is hotter than it really is and consequently the thermostat begins to open. This is quickly evidenced by the increase in temperature at the lower CTS as the coolant now begins to flow through the radiator into the block. Now you will see the lower CTS temperature increase and the upper CTS temperature decrease. As long as the increased engine load is present the ECM will keep the thermostat open. Once the load decreases the ECM will leave the thermostat alone and let it do its thing.

With all that in mind you have to look at your engine load, your block exit temperature and your radiator exit temperature to see if your thermostat is operating correctly. Yours appears to be reacting pretty much as expected with possibly a few incidences of partially sticking in the open position. I'll give you an example:

In your first log take a look at the data between lines 2640 and 2760. What you will see is that the heater duty cycle is essentially off. That means the thermostat should be controlling the exit temperature to around 93°c. But what you actually see is the block temperature dropping and the lower CTS increasing. The only way this can occur is if the thermostat is open and at the recorded temperature it should not be open without a little help from the ECM duty cycle. This doesn't come back into check until around line 2760 where the block temperature gets back to where it should be considering the engine load.

Granted, it isn't a dramatic case of a sticking thermostat but it just happens to be what you recorded in this particular logging session. With all the parts you are getting ready to replace I believe you will take care of your problem. I hope my explanation makes sense.

Good luck!

[yb28]

wow , thank you so much!!!!!

yeah I dont think it's stuck permanently either, however would you say that also caused me to have heat problems? Or the other explanation is that the tstat is faulty and the position at duty cycle 3.9% isn't really at 3.9%, my friend doesnt believe that my HC is plugged because then i would've had coolant spilling and other things.

and you mean when the 'few incidences' where it sticks open is when the temperature drops correct? also should the engine temp be 100+ and now below then?

[old guy]

The incident I was referring to was between lines 2640 and 2760 where the outlet temperature was dropping yet the block temperature wasn't hot enough to cause the thermostat to open and there was no duty cycle calling for it to open. The only way for that to occur would be for the thermostat either sticking open or just extremely slow to react. If you log block 131 you will see the engine output as well as the engine specified output from the ECM. This will tell you if your system is performing as specified by the ECM. My specified output is 100°c. If I observe the in/out/specified temps what I see is that as soon as the output temp goes above 100°c the duty cycle increases until the temp reduces back to the set point.

Based on what I see with your logs I suspect if you observe block 131 you are going to find that the specified output from your ECM is going to be 92-93°c.

As to your friends comments. A clogged heater core will not cause coolant spillage or anything else other than no heat in the cabin.

[yb28]

yeah, only engine oil was above 100C but my engine never passed 100, peak was 96 i believe...

I have heat at above 10C, so I think it is the tstat then or else I wouldnt have any heat at any temperature right?

[SlickFix]

This is gonna take a bit of 'splaining so I will do my best to see if I can make a little sense out of what you are looking at with your logs.

First order of business is to give a basic understanding of how a mapped thermostat operates. The thermostat has a mechanical element that opens at a set temperature. The actuator in the thermostat has an incorporated heating element that the ECM can use to "trick" the thermostat into thinking the coolant is hotter than it really is. The ECM uses this feature to provide additional cooling when it feels it would be advantageous to the prevailing conditions.

The coolant temperature is measured at two points. The rear CTS measures the coolant temperature as it exits the block. The lower radiator hose CTS measures the temperature coming out of the radiator after it has been cooled. As long as the thermostat is closed the temperature at the lower CTS will be low. Considerably lower than the temperature leaving the block at the rear CTS. When the thermostat opens you will see a rise in temperature at the lower CTS as the thermostat begins to allow the coolant to begin circulating through the radiator and into the block. When the thermostat is closed the coolant completely bypasses the radiator and flows back through the thermostat housing via the lower coolant hard pipe the last coolant drop line from the upper hard pipe return line into the thermostat housing. When the coolant gets hot enough the thermostat opens up and the radiator circulation begins. All this is controlled by the mechanical portion of the thermostat. Looking at your logs I can see that your mechanical portion is controlling the exit temperature to around 93°c. Anything above that and the ECM starts to increase the duty cycle.

Now the fun part begins. As long as there is no load on the engine the ECM is content to leave the thermostat alone and let it do its thing. When this is occurring you will see a duty cycle of 3-4%. Yours is set at 3.9%. Whenever you increase the load on the engine the ECM recognizes that an increased load will result in increased block temperatures so to compensate the ECM increases the duty cycle to the thermostat. This "tricks" the thermostat into thinking the coolant is hotter than it really is and consequently the thermostat begins to open. This is quickly evidenced by the increase in temperature at the lower CTS as the coolant now begins to flow through the radiator into the block. Now you will see the lower CTS temperature increase and the upper CTS temperature decrease. As long as the increased engine load is present the ECM will keep the thermostat open. Once the load decreases the ECM will leave the thermostat alone and let it do its thing.

With all that in mind you have to look at your engine load, your block exit temperature and your radiator exit temperature to see if your thermostat is operating correctly. Yours appears to be reacting pretty much as expected with possibly a few incidences of partially sticking in the open position. I'll give you an example:

In your first log take a look at the data between lines 2640 and 2760. What you will see is that the heater duty cycle is essentially off. That means the thermostat should be controlling the exit temperature to around 93°c. But what you actually see is the block temperature dropping and the lower CTS increasing. The only way this can occur is if the thermostat is open and at the recorded temperature it should not be open without a little help from the ECM duty cycle. This doesn't come back into check until around line 2760 where the block temperature gets back to where it should be considering the engine load.

Granted, it isn't a dramatic case of a sticking thermostat but it just happens to be what you recorded in this particular logging session. With all the parts you are getting ready to replace I believe you will take care of your problem. I hope my explanation makes sense.

Good luck!

That's all very interesting and highly educational, old guy. Any idea why the 3.0 doesn't have the bottom CTS? I wonder if the 3.0 cooling system acts in a similar manner, or is completely different.

[old guy]

That's all very interesting and highly educational, old guy. Any idea why the 3.0 doesn't have the bottom CTS? I wonder if the 3.0 cooling system acts in a similar manner, or is completely different.

The 3.0 doesn't have a map controlled thermostat so it simply opens or closes based on the coolant temperature. No interaction with the ECM.

[yb28]

The 3.0 doesn't have a map controlled thermostat so it simply opens or closes based on the coolant temperature. No interaction with the ECM.

old guy, if i have heat at other above 10C or at any temp..that means my heater core isnt stuck correct?

[old guy]

old guy, if i have heat at other above 10C or at any temp..that means my heater core isnt stuck correct?

I'm not quite sure what you are asking me. Let me put it this way. Once your block is up to temperature you should be able to get full heat out of your heater core regardless of the outside temperature. When everything is working properly the coolant is always circulating through the heater core.

[yb28]

I'm not quite sure what you are asking me. Let me put it this way. Once your block is up to temperature you should be able to get full heat out of your heater core regardless of the outside temperature. When everything is working properly the coolant is always circulating through the heater core.

just want to see if my heater core is plugged or not, what I'm asking is, I do get heat when the ambient temp is at 10 degrees celsius and above; occasionally, I will get some heat when its subzero degrees but rarely. I'm just trying to see if I need to order a new heater core, or if it's just the tstat that is making my hvac all crazy haha

[SlickFix]

I don't think we really know at this point. Why don't you try and burp the cooling system again and see if you can get some air out of the system. If you're 100% sure that you've got all the air out of the system, and with the engine at operating temp, the inlet hose to the heater core is hot, but the outlet hose is cold...then your heater core is likely plugged. If both hoses are hot, then maybe you have a blend door sticking or something.

On the other hand, replacing the heater core with a new OEM one as preventative maintenance is not a bad idea, if you can afford it. Peace of mind is worth something.

[yb28]

I don't think we really know at this point. Why don't you try and burp the cooling system again and see if you can get some air out of the system. If you're 100% sure that you've got all the air out of the system, and with the engine at operating temp, the inlet hose to the heater core is hot, but the outlet hose is cold...then your heater core is likely plugged. If both hoses are hot, then maybe you have a blend door sticking or something.

On the other hand, replacing the heater core with a new OEM one as preventative maintenance is not a bad idea, if you can afford it. Peace of mind is worth something.

yeah...just a pia to change the HC :|

[jacobsen]

Where are the blend doors located in the b6? In the dash? Do they simply restrict flow into the core?

[old guy]

Where are the blend doors located in the b6? In the dash? Do they simply restrict flow into the core?

If you remove the glovebox you can see the blend door actuators. As you cycle through the heater/AC/defrost/upper/middle/lower vent positions you can figure out what does what.

Coolant is always circulating through the heater core. There are no adjustable flow restrictions. Temperature control is achieved by the blend door positions.

[jacobsen]

They just mix air for hte climate control ah.

I am prepared to order all 3 parts of the cooling system... Sensors and thermostat... Any one in MN have vcds?

[yb28]

little update, changed out 2 CTS' and tstat, temp gauge hasnt dropped anymore, warm up faster. I will vagcom it again once I have chance, i had full heat yesterday for about...30 minutes and then its all ice cold again (except for defroster) I wonder if its the climate control blend motor is fucked...i had a code before but it went away?? OG any thoughts?

[old guy]

little update, changed out 2 CTS' and tstat, temp gauge hasnt dropped anymore, warm up faster. I will vagcom it again once I have chance, i had full heat yesterday for about...30 minutes and then its all ice cold again (except for defroster) I wonder if its the climate control blend motor is fucked...i had a code before but it went away?? OG any thoughts?

With the car at idle feel your hoses going into and out of the heater core. If the bottom one is hot and the top one is cold you probably have an air pocket in the heater core. If both hoses are hot but still no heat it would be a blend door problem.

[yb28]

hey old guy, another question..

so heat comes out on the defroster vents only, well occasionaly the middle vents have heat too but rare, dont think the foot well has heat either.. but defroster vents always have heat..

I've gotten a code on the left temp flap motor before and already swapped that flapper motor out....

any inputs?

going to flush heater core this weekend since one of my pipes cold..but then how do i get heat on the defrosting vents?!

[old guy]

hey old guy, another question..

so heat comes out on the defroster vents only, well occasionaly the middle vents have heat too but rare, dont think the foot well has heat either.. but defroster vents always have heat..

I've gotten a code on the left temp flap motor before and already swapped that flapper motor out....

any inputs?

going to flush heater core this weekend since one of my pipes cold..but then how do i get heat on the defrosting vents?!

Those two events should be mutually exclusive. You can't have heat from the defrost vents without having heat in the heater core and you can't have heat in the heater core without circulation. Exactly how much heat are you getting through the defrost setting?

See if you can find someone with VCDS and run the servo motor reset procedure in the basic settings mode. It could be that you have a "confused" servo motor that needs to be realigned.

[WB650]

Fighting a similar issue...So far in the past few weeks I've had the following replaced:

Coolant Hose from Turbo (Failed dumping all coolant)
Thermostat (was leaking bad for a while)
J-Plug (was leaking and for some reason shop didn't replace with the T-Stat, they did do it "for free" though)
Water Pump (supposedly failed)
CTS (more below)

Picked it up the other day after the J-Plug was replaced, fan was running on high for 5 minutes at cold start and the tech said "IDK why its doing that, just started today", get to work then come back out an hour later and I'm getting severe misfires (which I had only had intermittently on cold starts but car was obviously still warm now) and I got codes for random misfires and Engine Temperature Too Low. Go to Google and find that the fan running on high and possibly misfires are from the CTS so I told the shop to replace that ASAP and they did it for free. So far so good wit the car since Thursday night.

I did notice this morning though I didn't have heat the first few miles of city driving until I got up to freeway speed. I've been having this issue since the beginning of the other cooling system work, but it seemed to be fixed after the WP was replaced. Coolant level is normal (finally). I'll have to see what happens after work. Also, I'm in the Bay Area so it isn't like its freezing here LOL.

How many CTS are there? I'm guessing the one they did was the easiest one at the top for the coolant flange?