A4 thermostat change How-To

frugality

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Sep 19, 2003
Location
Spring Lake, Michigan
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none, 2016 GTI
There doesn't appear to be a thermostat change how-to, so I took some pics as I changed mine. This was on my A4 Golf, which is probably identical to the Jetta, and probably pretty similar to a Beetle.

Coolant temperature issues:

If you are experiencing coolant temperature variation, it is likely that the coolant temperature sensor is the problem. They are more prone to going bad than are the thermostats. The needle will be low, and will bobble around a bit. I had this happen once, and replacing the coolant temperature sensor fixed the problem. A couple of years later, my coolant temperature was reading low, just within the next hashmark down from 190 degrees, so I thought my new coolant temperature sensor went bad already. I replaced it again and it didn't fix the problem. I replaced the thermostat, and now the gage pegs right to 190 and hold steady there, so it was indeed a bad thermostat this time. When the temperature sensor went bad, it bobbled and varied more. When the thermostat went bad, it was more consistent, staying in a position that was about one hashmark low.


Anyway, without further ado, on to the How-To:

Supplies:
New thermostat. (Part # 044 121 113)
New O-ring. (Part # 038 121 119 B)
New thermostat flange. (Part # 038 121 121)
50/50 mix of G12 (pink) coolant and distilled water.
Catch pan, rags, beer, etc.

Tools:
10mm socket for engine cover nuts.
T-25 and maybe T-30 torx drivers or bits for belly pan.
6mm socket for lower radiator hose clamp at flange.
5mm hex bit (allen bit) on a 6" ratchet extension for flange bolts.
Torque wrench.



* Remove engine cover.

* Jack up the front of your car, or put it on ramps (set your parking brake!), and remove belly pan. (T-25 torx screws, and maybe a T-30 or two...)

* Remove cap from coolant reservoir, to allow air in as the coolant drains.

* Open drain valve at lower left corner of radiator, on the lower radiator hose. It rotates maybe 30-45 degrees counterclockwise, then you pull the whole knob outward to open it up.


This is all the instructions the Bentley manual gives. I only got about 1/2-gallon of coolant out. I expected to get more than that, but in calling r90sKirk, that's about right. A bunch more drained out when I removed the thermostat, and this was impossible to catch, and also got grunge in it from running down the engine, so have some spare 50/50 mix of G12 (pink) coolant and distilled water handy.

I still have my side skirt attached, and it's in the way. I used a funnel to direct the coolant to a catch pan. A piece of rubber hose would do the trick, too, but I didn't have the right size handy.

* Remove clamp from the lower radiator hose at the thermostat flange. This is just underneath the fuel injection pump:


* Remove the 2 bolts holding the thermostat flange to the block, 5mm allen bit on about a 6-inch ratchet extension.

* Remove thermostat flange. The Bentley says to rotate the flange 90 degrees counterclockwise, but this is impossible given the diamond-shaped opening in the bracket that flange passes through. The only thing to do is to pull it straight back, which will damage the flange. (See picture at the end of this how-to.) You should now have this:


* I covered the alternator with aluminum foil to prevent coolant from spilling out on it.

* Remove O-ring. I used a small, curved pick. Coolant will start to leak out, unless someone has any recommendations for draining more coolant out of the block from some location other than the lower radiator drain. Based on how much coolant I had to add later to re-fill it, it seems I dumped another 1/2-gallon, or slightly more. If you only pull back a small bit of the O-ring, the coolant will run out in a more controlled manner, and not gush out. I was only able to catch about half of it in my oil change pan, because it dribbled everywhere down below.

* Remove thermostat. It should now look like this:


* Clean mating surface on the block.

* Install new O-ring and thermostat onto a new coolant flange. To aid installation, the thermostat holds onto 2 pins that are molded into the flange. Looking down the flange:

*** If you are re-using your old flange and have broken the pins/tabs, all is not lost. Refer to this excellent post by 'cattlerepairman':
http://forums.tdiclub.com/showthread.php?t=148452
Orientation of the thermostat in the block is not important. It doesn't have to be lined up with the tabs, they are just there as an assembly aid.


* Install thermostat/O-ring/flange assembly into the engine block.

* Reinstall bolts and torque to 15 Nm (11 ft.-lb.)

* Reattach lower radiator hose to new flange and tighten clamp.

* Double-check that your radiator drain valve is closed.

* Fill with new 50/50 coolant. I did this just through the expansion tank, and was just patient. Some have said that you can hook up a MityVac and draw the air out to help fill the system. Someone elsewhere showed a method of taking the upper radiator hose connection loose near the battery and pouring coolant into both hoses -- into the block and into the radiator. Fill the expansion tank so that it's at the proper level, and reinstall cap.

* Start car and check for leaks. Watch the expansion tank and add coolant if the coolant level drops.

* If everything seems O.K., reinstall belly pan and engine cover. Pat self on back....you have avoided the dealer once again. :)



Addendum #1:
This is what happens when you pull the old flange off. The original O-ring is pretty well stuck in the block, and holds the thermostat in the block. When you pull the flange, it breaks the 2 thermostat-holder pins.


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.
 
Last edited:

mchabot

Well-known member
Joined
May 11, 2006
Location
Liverpool, NY
TDI
2000 A4 Jetta Manual
Bump for a great procedure. Instructions were easy to follow. When I was done it took about a 1.25 gallons to get the cooling system filled back up. I know lots of people on here think thermostats don't fail but at 175,000 miles mine was definitely toast. The car wouldn't get up to temp in 50 degree weather. After a quick thermostat change and a test drive it heats right up. Just a heads up for everyone my local dealer had G12 for $24 a gallon. Thats actually a bit cheaper than some of the trusted vendors.
 

spoilsport

Top Post Dawg
Joined
Oct 3, 1999
Location
Houston TX
TDI
2000 Golf GLS Silver (Sold). 2005 Jetta TDI Wagon Tiptronic (daughter's)
Thanks frugality!

Very nice write up frugality.

My car can't get to operating temp even in Houston. Been going on too long and this will help greatly.

Thanks again! :)
 

egibbys

Veteran Member
Joined
Feb 20, 2008
Location
Salt Lake City, UT
TDI
1999 Jetta TDI GLS
I currently have the same issue and the OP did with the needle just barely under the 190 mark. My coolant temp sensor is new so I doubt it is bad. So I guess my thermostat possibly could use a replacement.

Anyway now to a semi related question. Would a bad thermostat effect fuel economy? I'm currently getting 33mpg with a manual transmission and I read on another thread that the thermostat could be an issue if nothing else is.

BTW Great write up.
 

DanG144

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Joined
Aug 2, 2007
Location
Chapin, South Carolina, USA
TDI
2005 A4 Jetta 5spd
Excellent write up.

You might try a heavy wire hook through the housing to the thermostat. Never pull on the housing, pull only on the wire hooked to the thermostat, and you will not break the tabs off.
 

paramedick

TDIClub Enthusiast, Vendor
Joined
Jul 29, 2001
Location
Versailles, Kentucky
TDI
2015 Audi Q5 TDI
If you have compressed air, you can get nearly a gallon of coolant out.

Remove the return hose from the coolant bottle. Pressurize it slightly with compressed air via blow tip. Coolant will come up into bottle, then drain into radiator. Repeat until very little coolant comes into bottle. Then put cap onto coolant bottle, and pressure the return nipple.

When it's time to refill the sytem, connect a mityvac to the return hose and pull vacuum. Refill bottle as needed. When you get a splash of coolant into the mityvac tubing disconnect it and reconnect return hose to bottle. It will take another bottleful of coolant after car is started to refill system.
 

DanG144

Top Post Dawg
Joined
Aug 2, 2007
Location
Chapin, South Carolina, USA
TDI
2005 A4 Jetta 5spd
I have had good luck with those techniques as well.

If you alternate the air injection between the vent line hose, and the vent line into the coolant reservoir, holding a finger over the other line, you can get even more.

This alternately pressurizes through the vent lines and through the reservoir's bottom line, and moves water around a fair amount.

But still, every time I pull the thermostat or the water pump (whichever I do first), more pee's out...

The only time it has not, is when working on an engine with the head off, I use a vacuum pump to suck the coolant out of the block with a small tube. I do this as I vacuum out the head bolt holes - I get the coolant at the same time.
 

Turbosprezarka

Veteran Member
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Feb 1, 2006
Location
New England
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2001 Jetta TDI (ALH) GLS, 5sp, Tornado Red, 200,000 miles
Great write up indeed. One question I was trying to understand from Bentley. When the thermostat is open, which was does the coolant flow? Out from the thermostat housing, or into the engine through the thermostat?

I have had the same problem for 2 years now. For starters, I had a temp sensor go, which resulted in fluctuations, replaced it, and it fixed it. Then 2 years ago I started noticing that instead of the coolant temp getting up to 190F and staying there, it only gets to about 170F, regardless of whether it is winter or the middle of summer. In fact I can't remember the last time the radiator fans have come on.

The only way I can get the temp to hit 190 is if I let it idle in the driveway for about 10 minutes after I have driven it.

I really hope that a new thermostat will fix this problem.
 

dieselfuel

Veteran Member
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Dec 28, 2008
Location
ohio
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2003 Jetta TDI
Turbosprężarka,

At the time you noticed your coolant temp was only 170F, did you also notice a big hit in fuel mileage? Like a 20% or more hit?

My coolant is only at 160F and I'm down to 33 mpg. Used to get 44.

I have a feeling the thermostat is my problem. (Everything else has been eliminated, except injectors/nozzles.)

df
 

Turbosprezarka

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Location
New England
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2001 Jetta TDI (ALH) GLS, 5sp, Tornado Red, 200,000 miles
Dieselfuel,
Since Jan 2009 I was getting around 51-52 tank after tank, but it was all highway. I was witnessing low temps with Vag-Com, but at that time they were around 177F during the summer.

Since around September I started noticing my mileage drop off, and I wasn't really sure why. It has gotten much worse since the cold weather started and winter went into full gear. I have gone down to 45-46 on average, and the last time I checked, was when my coolant temp was around 170F.
 

ryanmiller

Veteran Member
Joined
Oct 7, 2009
Location
Portland OR
TDI
2003 Mojave Beige Golf
for a properly operating TDI in 35º weather should the temp needle be at 190 when fully warmed up?

mine dosn't get above 170 ever in lower temps

but it heats up the cabin quickly and maintains 170 well
 

DanG144

Top Post Dawg
Joined
Aug 2, 2007
Location
Chapin, South Carolina, USA
TDI
2005 A4 Jetta 5spd
Yes, it should get to 190.

It will actually read 190 over a temperature range of about 167 to 225 F. This was the way it was set up by VW. The computer actually controls the needle position

So if you are reading 170 on the gauge, then it is actually only about 150 F on VCDS or a scangauge.
 

Seth700s

Well-known member
Joined
Jan 26, 2010
Location
Manty, WI
TDI
2013 Jetta -purchased new
Turbosprężarka said:
Great write up indeed. One question I was trying to understand from Bentley. When the thermostat is open, which was does the coolant flow? Out from the thermostat housing, or into the engine through the thermostat?

coolant flows through the engine, through open t-stat, through radiator, up through the upper hose, back through the engine

but I'm just a newbie
 

Seth700s

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Jan 26, 2010
Location
Manty, WI
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2013 Jetta -purchased new
frugality said:
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.
If you broke the tabs why not save the $40-50.
 

rocketeer928

Veteran Member
Joined
Jan 24, 2008
Location
A New Englander currently in northeastern Illinois
TDI
2003 Jetta GLS TDI 5-speed
My Scan Gauge II typically reads 172°F, except when it's really cold. It's obviously lower in colder weather. Below, you can see I'm averaging around 45 mpg, which I think is lower than it should be, since I typically don't have a lead foot. Yes, the idiot gauge reads 190°F. So, should I suspect that my thermostat needs changing?
 

Overfuelin5spd

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Feb 11, 2010
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Smokeville, Wisconsin
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03 5spd
It will actually read 190 over a temperature range of about 167 to 225 F. This was the way it was set up by VW. The computer actually controls the needle position


of course its controlled by the computer, but its not measuring the temp in the main radiator loop, its measureing block temp, and I highly doubt that VW would let there be a that large of a margin for error in their gauge or make a gauge that is not accurate
why even put a gauge on the dash then?? just put a needle with a pointer that goes from cold to hot and no where in between

that doesnt make sense to me, but Ive never worked on any cars before im just a newbie
 

DanG144

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Location
Chapin, South Carolina, USA
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2005 A4 Jetta 5spd
I changed mine because it would never get over 182F, even in the summer down the interstate, dragging a trailer.

The trick is to make the determination that your thermostat is the cause, especially in the winter. Even running your heater can keep it from getting up to thermostat temperature.

I drove mine for a week one winter with no radiator (completely blocked off). I could easily keep the engine under 200F just by using the cabin heater, and since this was winter in South Carolina the afternoon temperatures were over 50F. I did have to keep the fan on 3 at full heat (with the windows open to keep ME from overheating) when it was 50 F ambient and I was driving down the interstate at 70 mph for long periods.

I also tried 3 thermostats before I got one that sealed tightly and worked at the temperatures I wanted. These are not truly precision instruments.
 

Seth700s

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Location
Manty, WI
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2013 Jetta -purchased new
rocketeer928 said:
I'm aware of this. However, since I am on the low end of this scale at 172°F, according to the ECU and my Scan Gauge II, should I consider changing the thermostat?
is the thermostat even opening? by this I mean is the lower hose hot? compare feel the upper hose it should be hot. how is the heat on the hottest setting? if it was stuck open your gauge wouldn't read 190 and you wouldn't get good heat. in my car its way too hot to keep on the hottest setting w/fan at 1 on a trip 20 miles or more. last saturday I put in a new 195f t-stat, because I was adding a coolant heater. it took over 2 hours before the t-stat opened. I drive 45 miles to work. I pop the hood, and the lower hose is cold. car reads 190. so I block off the radiator, and guess what hose is still not warm.
 

Seth700s

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Manty, WI
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2013 Jetta -purchased new
DanG144 said:
I also tried 3 thermostats before I got one that sealed tightly and worked at the temperatures I wanted. These are not truly precision instruments.
wow thats a waste of time!
try this: fill a pot with water, place on stove top, heat untill t-stat pops open, have a digital thermometer? use it to find out what the temp the water is when the t-stat pops. a teacher I had did this with every new t-stat he ever installed. I've tried this and it pops very close to whatever the t-stat should open at. eg. 175 deg f.

and if you buy a new gasket it should seal if install the right way. gasket less then $3.
 

DanG144

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Location
Chapin, South Carolina, USA
TDI
2005 A4 Jetta 5spd
Seth700s said:
wow thats a waste of time!
try this: fill a pot with water, place on stove top, heat untill t-stat pops open, have a digital thermometer? use it to find out what the temp the water is when the t-stat pops. a teacher I had did this with every new t-stat he ever installed. I've tried this and it pops very close to whatever the t-stat should open at. eg. 175 deg f.

and if you buy a new gasket it should seal if install the right way. gasket less then $3.
Thanks for the tip, Seth. I first did that in 1966; of course I did not have a digital thermometer.

I found that they tended to operate differently in a pan of water and in the car with differential pressure on them. One did not seal tightly after initially opening, ever again. Though it looked fine in the pan. One popped earlier each time over several days until it steadied out below 180.

Being stubborn, I tried again.
 

adamrc

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Feb 26, 2010
Location
Tennessee
TDI
2006 VW Jetta TDI BRM - DSG
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?
 

dieselfuel

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Dec 28, 2008
Location
ohio
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2003 Jetta TDI
I replaced the thermostat in my 03 today.

The old one was only reaching 160F.

I have not check the coolant temp of the new one yet, but the heater is blowing considerablty hotter than before with the original thermostat.

I'm expecting my mpg to go back up closer to 44 (w/old thermo., I was at 33).

I also had the intake cleaned and what a HUGE difference there is in power. I don't remember my Jetta have this much power when it was new. (I now have 118,000 miles)

df
 

egibbys

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Joined
Feb 20, 2008
Location
Salt Lake City, UT
TDI
1999 Jetta TDI GLS
dieselfuel said:
I replaced the thermostat in my 03 today.

The old one was only reaching 160F.

I have not check the coolant temp of the new one yet, but the heater is blowing considerablty hotter than before with the original thermostat.

I'm expecting my mpg to go back up closer to 44 (w/old thermo., I was at 33).

df
Keep us updated when you fill up.
 

dieselfuel

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Dec 28, 2008
Location
ohio
TDI
2003 Jetta TDI
Today I went for a drive, got off the highway, with the engine running, I took out my meat thermometer and took a read from the coolant tank: 186F!

Before, with the original thermostat, I was getting 160F through the coolant tank.

df
 

Turbosprezarka

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Feb 1, 2006
Location
New England
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2001 Jetta TDI (ALH) GLS, 5sp, Tornado Red, 200,000 miles
I think I'm going to change my thermostat as well. Is there anyone out there, who after getting the engine up to the max temp (160, 170F), has felt the coolant hose right off the thermostat and noticed that it has been cold?

And who has then replaced the thermostat and now observes higher coolant temps at full operating conditions?

I stated my vehicle will only get up to 170. But, the coolant hose right off the thermostat stays cold. I thought was interesting, since that if the thermostat or its seal was leaking, that this hose would feel semi warm. And I guess it does, but it is too difficult to determine if the temperature is caused by the ambient air temp under the hood or the small coolant leak inside. I describe it as being "cold" as it is much cooler than the upper coolant hose in all cases.

T
 

Seth700s

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Joined
Jan 26, 2010
Location
Manty, WI
TDI
2013 Jetta -purchased new
Turbosprężarka said:
I stated my vehicle will only get up to 170. But, the coolant hose right off the thermostat stays cold. I thought was interesting, since that if the thermostat or its seal was leaking, that this hose would feel semi warm. And I guess it does, but it is too difficult to determine if the temperature is caused by the ambient air temp under the hood or the small coolant leak inside. I describe it as being "cold" as it is much cooler than the upper coolant hose in all cases.

T
Don't take this the wrong way. Do you understand how a thermostat works?
 
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