A4 thermostat change How-To

Seth700s

Well-known member
Joined
Jan 26, 2010
Location
Manty, WI
TDI
2013 Jetta -purchased new
adamrc said:
I'm having the same issue here. My water temp usually stays around 172-176. I replaced the coolant temp sensor with the updated green one not too long ago and now I noticed that the upper radiator hose is hot while the bottom one is cold. Would this be due to a bad thermostat?
Your bottom hose is cold because the coolant thats in it is cold, and the t-stat hasn't opened yet. I don't know how cold it is by you. Here is northeastern Wisconsin its winter. I have my radiator blocked off and I drive 45 miles to work and my t-stat still hasn't opened. After I got the car I put in a coolant heater and replaced the t-stat. When you do this you want to let the car run to get out the air in the system and top of coolant at the right level. Anyway it took over 2 hours for the coolant to cycle meaning the t-stat opens and engine is at oper. temp. this is normally when the radiator fans kick in the (temp sensor tells them to).
I would swap out the t-stat in spring or summer, because the car wont take as long to warm up.

The t-stat's job is to stay closed till the motor gets to the temp the t-stat is rated for(eg:195*). After coolant(hot) flows through open t-stat to the radiator to the upper hose(not as hot) back in the engine taking the heat from the motor back to the radiator to cool off. No matter if the t-stat is open or closed your water pump is moving the coolant through smaller hoses. eg: To your heater core. This is why you have hot heat inside, if you never have hot heat then you got a t-stat problem (not always but most times).
 

dieselfuel

Veteran Member
Joined
Dec 28, 2008
Location
ohio
TDI
2003 Jetta TDI
O.K., I'm back to 41.91 tonight after filling up.

I didn't notice any difference in mpg concerning the turbo actuator adjustment, but the intake cleaning and the thermostat replacement made the difference.

Drove the car a lot harder than I normally would. I have no doubt I'll be back to 44 (hwy/cty) before long. Warmer weather on the way....!
 

Turbosprezarka

Veteran Member
Joined
Feb 1, 2006
Location
New England
TDI
2001 Jetta TDI (ALH) GLS, 5sp, Tornado Red, 301,593 miles, SOLD
Seth700s said:
Don't take this the wrong way. Do you understand how a thermostat works?
This was not taken the right way. I believe I do understand how a thermostat works. The thermostat will stay closed, unless the temp is high enough, where the wax motor will open it. The coolant flows into the lower radiator hose from the thermostat, that would mean in all cases with the thermostat closed, the lower radiator hose will feel cold, and there would be no flow through the radiator.

The question I was trying to ask that you had commented to is the following. I believe my thermostat is bad because VCDS indicates that I never get up to operating temperature. Suspecting the thermostat is partially leaking, I wanted to know if the lower radiator hose at the outlet of the thermostat should feel pretty hot, as does the upper radiator hose?

What I should have realized is that this will only depend on how much is leaking through the thermostat.
 

Seth700s

Well-known member
Joined
Jan 26, 2010
Location
Manty, WI
TDI
2013 Jetta -purchased new
Turbosprężarka- sorry if I took your post the wrong way and if I insulted your IQ, very sorry.

The "outlet of the thermostat" or thermostat housing, will feel hot because it is bolted to the engine block. The engine block is cast iron. You say the VCDS indicates your temp is around the 170's if I remember. What is your temp gauge on the dash at? Someone else posted that the gauge needle goes to 190, but has a range from 167 to 225 F. He claims- "The range is intended to keep the poor mindless driver from being concerned over normal variations in temperature." Anyway if your temp gauge goes to 190 on the dash, its in the range vw(and your car/ecm) thinks its okay. Meaning that is normal operating temp.

Have you read my post at the top of this page? It took a long time for the t-stat to open. That was done with a new 195f t-stat. I also said that I drive 45 mile to work with the radiator blocked off doing 73mph and the lower radiator hose is still cool. That tells me the t-stat closed. Its crazy. I don't have VCDS, so I don't know what the actually temperature is.

The simple thinking I do: If you have hot heat blowing inside your car I would think your t-stat is working. If it was stuck open even a little bit your engine would never get to operating temp. You would also have no hot heat blowing in the car and your gauge would read lower (like when you first start your car on a cold day and crank the heat up but it only blows cold air). If the t-stat never opened your car would over-heat at best or blow a head gasket at the worst. For peace of mind you should just change the t-stat, it wouldn't hurt they are not a lifetime thing (in my opinion). This thread by the OP takes you step by step, and I must say that person did a great job.

Somtimes the vag-coms and the scan gauges makes you(or people in gen.) look too far into things. I'm the son of a auto mechanic. I have the basic knowledge of auto mechanics. So I'm not in anyway claiming to be a mechanic. That sould be in my signature
 
Last edited:

Turbosprezarka

Veteran Member
Joined
Feb 1, 2006
Location
New England
TDI
2001 Jetta TDI (ALH) GLS, 5sp, Tornado Red, 301,593 miles, SOLD
Yes you're right, VCDS does only read about 170, year round, and the dash gauge always shows 190. My concern is because VCDS used to show 190 a few years ago, and hasn't since. The heat also gets warm enough, but never really hot in the winter time. My dad has a 2009 TDI, and that heat gets real hot.

You're right, VCDS does make us look to far into things sometimes. And yes, I'm planning on just replacing the thermostat.
 

Seth700s

Well-known member
Joined
Jan 26, 2010
Location
Manty, WI
TDI
2013 Jetta -purchased new
Hey Turbo- are you going to spend the big money on a oem t-stat or go with the $9 stant?
 

Drnknmnky13

Veteran Member
Joined
Feb 13, 2010
Location
D.C.
TDI
2002 Golf 5spd, 2014 Passat DSG
Awesome writeup. My car doesn't get up to temp on the highway unless I floor it a lot. It does however get up to temp in stop and go traffic. Had the Tstat changed once with an autozone replacement. Just picked up an OEM ($32 from dealer). Have to go back and buy a flange (didn't know about that one) and I'll change it out as soon as the rain stops.

Thanks for all the info here guys. I would be lost without this forum.
 

Drnknmnky13

Veteran Member
Joined
Feb 13, 2010
Location
D.C.
TDI
2002 Golf 5spd, 2014 Passat DSG
Is this the part your calling the flange?
When I asked for a flange at the dealer today he wanted to give me the part that the temperature sensor sits in. He said the part coming off the tstat was just some tube.

Now he wouldn't be the first clueless parts guy I have dealt with... but wanted to make sure before purchasing.

Thanks.
 

Strider199

Veteran Member
Joined
Mar 15, 2010
Location
Canada
TDI
1999 Golf TDI New Body Style
Thanks for this How To. I live in Canada and when my thermostat stuck open I had zero heat on my way to work. This thread helped alot (I would have never thought to purchase a new flange). I changed mine a few months ago after visiting here and reading. Now after joining, I can say "Thanks".
 

adamrc

Veteran Member
Joined
Feb 26, 2010
Location
Tennessee
TDI
2006 VW Jetta TDI BRM - DSG
I changed out my thermostat last night with a new OE VW branded one. I was previously getting to about 174-176 degrees but now I'm getting to about 195-199 degrees according to my Scan Gauge. Does this seem a little high? I thought it should get no hotter than 195. I added about 1 gallon of coolant/water. Also the lower radiator hose seems to be cold as well as almost the thermostat is not opening at all. Could there be air in the system somewhere or even a bad thermostat?
 
Last edited:

Leo819

Veteran Member
Joined
Aug 13, 2003
Location
Limerick, Pa
TDI
Thomas C2 School Bus
Do not tighten without torque wrench. If you over tighten, you have the chance of pinching the o ring and ripping it. Which will spill coolant and you will be
SOL
 

Turbosprezarka

Veteran Member
Joined
Feb 1, 2006
Location
New England
TDI
2001 Jetta TDI (ALH) GLS, 5sp, Tornado Red, 301,593 miles, SOLD
Just wanted to update those interested or experiencing the same problem as me (see previous posts). I replaced my thermostat with an OEM and my temps are immeadiately back up from 170F to 190-195F! Very glad this is fixed.
 

runonbeer

Maintenance EnthusiastVendor
Joined
Apr 15, 2002
Location
Austin, TX/Chapel Hill, NC
TDI
'00 Golf 02M, '10 Golf 02E, '02 UTE 02M
I am now replacing T-stats along with the timing belt and water pump on higher mileage cars. With the WP out of the way, you're able push the Tstat out from behind using a small pry bar without snapping the little ears on the inside of the flange.

I don't know the brand off hand but the aftermarket Tstats that I get will engage in the ears.

and yes, torque wrench is important.
 

johnmarkoe

Member
Joined
Apr 29, 2009
Location
palm bay florida
TDI
jetta 2001 tdi
johnmarkoe vbmenu_register("postmenu_2935842", true);
Newbie

Join Date: Apr 2009
Location: palm bay florida


vag-com
please help. i see the ECU but not ECM should i ?
I was driving one day and all of a sudden it soundid like a spark plug wire came off ha ha . any way i replaced all injectors with karma pp357. got vag-com from ross-tech, looks like i see all controllers but their is a lot missing. vag com dose not show any codes
 

steelmb

Veteran Member
Joined
Apr 12, 2009
Location
MB
TDI
2003 Jetta Wagon
johnmarkoe said:
johnmarkoe vbmenu_register("postmenu_2935842", true);
Newbie

Join Date: Apr 2009
Location: palm bay florida


vag-com
please help. i see the ECU but not ECM should i ?
I was driving one day and all of a sudden it soundid like a spark plug wire came off ha ha . any way i replaced all injectors with karma pp357. got vag-com from ross-tech, looks like i see all controllers but their is a lot missing. vag com dose not show any codes
What the heck has this got to do with this thread? Ah it's a newbie hijack.
 

Coasty

Well-known member
Joined
Nov 12, 2008
Location
Bolling AFB, DC
TDI
2002 Golf Manual 265k miles
I changed out my thermostat last night with a new OE VW branded one. I was previously getting to about 174-176 degrees but now I'm getting to about 195-199 degrees according to my Scan Gauge. Does this seem a little high? I thought it should get no hotter than 195. I added about 1 gallon of coolant/water. Also the lower radiator hose seems to be cold as well as almost the thermostat is not opening at all. Could there be air in the system somewhere or even a bad thermostat?
Adamrc, I just replaced my tstat due to an overheating condition now I seem to be having the same problem you are having... bottom hose is cold, but engine still overheats a little. I did have hot heat blowing before the tstat change, now I have cold air and the bottom hose is cold. Trapped air maybe? How did you fix your problem?
 

DanG144

Top Post Dawg
Joined
Aug 2, 2007
Location
Chapin, South Carolina, USA
TDI
2005 A4 Jetta 5spd
I often use a mityvac on the vent tube going to the coolant ball reservoir. I pull a vacuum on it until I get an ounce or so of fluid. This seems to preclude the big void in the system.

I hope that is all your problem is, and not a failed water pump.

The only alternative I know is to let the engine run until the thermostat finally opens, and be ready with a lot of coolant.
 

adamrc

Veteran Member
Joined
Feb 26, 2010
Location
Tennessee
TDI
2006 VW Jetta TDI BRM - DSG
Adamrc, I just replaced my tstat due to an overheating condition now I seem to be having the same problem you are having... bottom hose is cold, but engine still overheats a little. I did have hot heat blowing before the tstat change, now I have cold air and the bottom hose is cold. Trapped air maybe? How did you fix your problem?
Turns out nothing was wrong. It was a lot cooler outside when I posted that. Now that it's been in the mid 90s here, the thermostat fully opens and both the top and lower hoses are hot. The coolant temp stays around 192 degrees in the summer.
 

Coasty

Well-known member
Joined
Nov 12, 2008
Location
Bolling AFB, DC
TDI
2002 Golf Manual 265k miles
I ended up having a faulty water pump. It failed just after my T-stat did... somewhat unusual, the impeller separated from the drive shaft.
 

PDJetta

Top Post Dawg
Joined
Nov 6, 2003
Location
Northern Virginia
TDI
'04 Jetta GLS TDI Pumpe Duce Platinum Grey w/ Leather
"Addendum #2:
This is a comparison between the OE thermostat ($66 at my local dealer) and an aftermarket thermostat from one of our trusted vendors ($19). The aftermarket thermostat will not attach to the flange. The pointy end is not big enough to catch on the 2 pins. I recommend going with the original VW thermostat. The aftermarket one did come with a new O-ring. The VW one did not, and has to be purchased separately.

The 2 pins of the flange are supposed to engage within the 'arch' on the top of the thermostat. There is no provision for this on the aftermarket thermostat, however. Also, as you can see there are some other differences in construction between the two thermostats. "

Great write up. Thanks!

Whaler used to be the OEM thermostat, at least it was for the '85 Jetta TD, I had. Wonder who makes the OEM one on the left.

--Nate
 

dieseler

Active member
Joined
Dec 22, 2002
Location
MO, USA
TDI
A4 2003 Black
Excellent write up. I have been experiencing temp variations on the dash gauge up and down under normal operation. The gauge would get up to the normal 190, and then fall all the way to zero, and then return to 190. The gauge never went above 190, and I was not losing coolant.

Reading through this forum, I determined that the issue *must* be the coolant temperature gauge. Replaced that today with the new green plug and the problem persists. Same behavior as before.

So now, am I correct to assume that the problem must be the thermostat? I do have a vag-com, and am feeling stupid for not checking that first. Thought I would get some thoughts, then take another crack at this. Should I use the vag com to check for actual temp variation prior to ordering a tstat? Anything else that this could be?
 
Last edited:

DanG144

Top Post Dawg
Joined
Aug 2, 2007
Location
Chapin, South Carolina, USA
TDI
2005 A4 Jetta 5spd
A question, did you replace the temperature sensor (green plug that sticks into the coolant loop) or did you replace the gauge (take your dash apart?)

You can check with the VCDS and look at the temperature sensors for the engine module in measuring block 7, and the gauge in the instrument module (cant remember the group) to compare them during the problem.

If it is doing the change very rapidly, then it is most likely a wiring issue, or connection issue.
Thermostats rarely work normalyl then fail open rapidly causing a temperature drop, then come back up to normal. So I suspect wiring. But anything is possible.
If you look at both sensor elements,and they show the same thing, then it is probably the thermostat.

It could be the display gauge itself, but that is not as common as the other failures.

Dan
Excellent write up. I have been experiencing temp variations on the dash gauge up and down under normal operation. The gauge would get up to the normal 190, and then fall all the way to zero, and then return to 190. The gauge never went above 190, and I was not losing coolant.

Reading through this forum, I determined that the issue *must* be the coolant temperature gauge. Replaced that today with the new green plug and the problem persists. Same behavior as before.

So now, am I correct to assume that the problem must be the thermostat? I do have a vag-com, and am feeling stupid for not checking that first. Thought I would get some thoughts, then take another crack at this. Should I use the vag com to check for actual temp variation prior to ordering a tstat? Anything else that this could be?
 

dieseler

Active member
Joined
Dec 22, 2002
Location
MO, USA
TDI
A4 2003 Black
Thanks for the reply Dan- I replaced the sensor in the coolant loop, green plug. I did not replace the gauge in the dash.

I will check using VCDS for variations between the 2 blocks. I also didn't think this looked at all like a stat problem, which is why I went right to the coolant temp sensor.

Does anyone know which is the other block that I should compare to #7?

The change is happening very quickly according to the dash gauge.

So, to confirm, if it is a wiring problem, or a problem with the dash gauge itself, you would think the temps would be different from each other when I compare the 2 VCDS measurements? But, if they are the same, you would think it is the thermostat?
 

DanG144

Top Post Dawg
Joined
Aug 2, 2007
Location
Chapin, South Carolina, USA
TDI
2005 A4 Jetta 5spd
That is correct. Sorry I have no references to the right measuring block number in the INSTRUMENT controller, but if you open the INSTRUMENT controller and just page through the blocks you will find it clearly labeled.
 

dieseler

Active member
Joined
Dec 22, 2002
Location
MO, USA
TDI
A4 2003 Black
Thanks Dan- is there a way to view the 2 measurements at the same time? Been a while since I used my VCDS.
 
Top