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Discussion Starter · #1 ·
Hi all,
I have a 1999.5 Jetta GL with 2.0 AEG engine code. Recently, I have noticed that my car takes longer than usual to warm up. Also, when I use heater, the engine will cool down even if I am using the second position on the fan.
So, I figured that my thermostat is either stuck continuously in an open position or it is malfunctioning and it is not closing completely.
I have decided that I need a new thermostat and given that winter is already here, I could not postpone.
I started digging and the cheapest thermostat I could find for my 2.0 was well over $30.00. What was really interesting, it turned out that the TDI thermostat and the 1.8T thermostat have different prices and part numbers. However, I had the suspicion that they were all the same. Hence the experiment
and the DIY.
So the cheapest thermostat I could find at that time was from http://www.1stvwparts.com They have the 1.9TDI thermostat for $14.95 + whatever the price for the shipping was.
Later, I found a cheaper source for the 2.0 thermostat but I have already ordered.
One thing is for sure: the TDI thermostat IS THE CHEAPEST no matter where you find it.
DISCLAIMER:
This DIY is designed to serve merely as an illustration of the process of replacing a thermostat on a 2.0 AEG engine. I shall not be held responsible for any damage to you or your vehicle. Use this DIY at your own risk
With that said, lets begin:

PARTS AND TOOLS NEEDED:
NOTE:
The thermostats for 1.9 TDI, 2.0, and the 1.8T are rated for the same temperature: 87 degrees Centigrade. I strongly believe that they are interchangeable and can even fit certain AUDI engines as well!
1. 1.9 Thermostat. Part number: ALH (TDI) - 044 121 113 (Thanks Gary
)
OR 2.0 Thermostat Part number: AEG (2.0) - 050 121 113 C
Optionally, get the thermostat O-ring.
2. Thermostat Housings: AEG - 06A 121 121 C or ALH - 038 121 121 OPTIONAL
Notice the similarities in both the thermostat part numbers and the housing part numbers.
Note: You don't need to purchase a thermostat housing. I am giving you the info just in case
3. VW OEM G12 coolant. It is pink in color. Part number: G 012 A8FA4 (may be optional)
4. At least 2 empty 1 Gallon containers. I used 2 plastic milk gallons which I washed VERY meticulously.
5. 10 mm socket, 10 mm wrench, 10 inch socket extention, 3 inch socket extention, 3/8 drive wrench
6. Long neck funnel from Walmart for 99 cents.

PART 1 (preparing the car):
1. Lift the front of the car on jack stands.
2. Remove the middle splash shield:

3. Allow the engine to cool down
4. Carefully unscrew the coolant tank cap releasing any remaining pressure.
5. Remove the engine cover.
PART 2 (draining the radiator):
In order to avoid massive coolant spillage/loss you will need to drain the radiator. It is very simple task which takes little over 5 minutes. Please, be carefull, as the G12 coolant is highly toxic. Make sure it does not get in your eyes.
On the driver's side of the radiator, right next to the left splash shield, there is a small valve and a drain plug designed for draining the radiator.
Here is how the system looks like;

1. Have one empty gallon container ready along with the long neck funnel.
2. Carefully unscrew the valve counter clock wize. All you need to do is turn at about 30-45 degrees. Not even a half turn. You will hear/feel a click.
3. Slightly pull the valve knob away from the radiator about 2-4 mm. At this point coolant should be flowing from the drain plug.

4. As soon as the coolant starts flowing, capture it with the funnel and direct it to the empty gallon container. If you have a small hose that fits on the drain plug, you can use that instead of the funnel.
5. When the first container is almost full, push back on the valve towards the radiator, and screw it back on. Again, turn about 30-45 degrees. You should hear/feel similar click. The coolant should no longer be flowing.
6. Use another container and repeat until the radiator has been drained.
Note: You should expect to fill 1 gallon container and then about 1/3 of the second.

PART 3 (Electrical Connections):
Note: You need to have your 4-digit security code for your radio. Once you disconnect the battery, you will need this code to reactivate the radio and the A/C controls depending on the model.
1. Disconnect the positive terminal on the battery
2. Remove the rubber nut-cap as shown below:

3. Remove the nut below it and free the big cable
4. Unplug the little plastic switch show below (this will free some space for your tools):

You can see that the thermostat is almost behind the alternator. YOU DO NOT NEED TO REMOVE THE ALTERNATOR TO REPLACE THE THERMOSTAT
PART 4 (Removing coolant hose):
In order to make things easier and to free some space (which you need) you have to unplug the big hose that goes from the thermostat housing to the radiator:

1. Use pliers to squeese and release the clamp. At the same time carefully wiggle the hose out of the thermostat housing.
2. Be carefull not to break the thermostat housing as it is plastic may crack.
3. Once you unplug the hose, direct the hose upwards so that it does not drip coolant.
PART 5 (Removing Thermostat housing):
This is the tricky and the most time consuming task. There are two bolts that hold the thermostat in place. They are shown below:

One is on top and it is very easy to reach. The other is at the bottom of the thermostat housing. The space is very limited. I could not take a picture of it as I could not get the right angle with my camera.
1. Use the 10 mm wrench to unscrew and remove the top bolt:


2. Build yourself this set-up:

This should allow you to unscrew the lower bolt. Note that the elbow joint is sitting between the alternator and the big wire which you freed in step 2 of Part 3. The space is tight so be patient.
3. Here is how the thermostat housing looks like when you remove it:

4. Pry off the thermostat O-ring with a screw driver:

5. Now the thermostat is freed and you can remove it. Small amounts (about 1 cup) of coolant will spill. Nothing to be afraid of.
2.0 and TDI thermostat comparison:
The first thing you will notice is that the 2.0 thermostat is shorter than the TDI thermostat. This is no problem. The TDI thermostat fits perfectly fine in the engine block. This is the only noticable difference. Also, the TDI thermostat has an extra spring. The pictures below explain it all:


NOTE: the diameter is the same. It just looks on the picture like that the 2.0 thermostat is smaller in diameter. It is not!

PART 6 (Installing TDI thermostat):
1. Put the 1.9 TDI thermotat in its "bed". It should fit perfectly fine.
2. Install the new O-ring. The O-ring actually keeps the thermostat in place.
Here is how the O-ring looks like:

Both the thermostat and the Oring are installed here:

3. Reinstall the thermostat housing, starting from the top bolt.
4. Tighten the bolts. I don't have the correct torque specs, and even if I did, I don't see a way to fit a torque wrench inside! Use common sense.
PART 7 (Putting things back together):
1. Once you have tightened the two bolts, plug the big radiator hose on the thermostat housing. Again, be carefull not to crack the plastic housing. Make sure that the clamp is properly seated.
2. Plug the little alternator switch
3. Attach the big alternator cable and screw the nut back in.
4. Put the rubber nut cap on the nut.
5. Make sure that the little radiator valve is closed!
6. Fill the coolant back in the expansion tank allowing it to sink in. You should be able to fill it back in.
7. Check all electrical connections again!
8. Connect the positive battery terminal
9. Start the engine and watch the coolant level. Add if needed!
If all is OK, then you are done.

Have a beer and enjoy your work.

======================
First impressions:
Everything works just as expected. The car heated up very quickly and once it reached normal operating temperature, it kept it constant. I drove for a while with the heater on max and the fan on 4. No problems whatsoever!
I hope this DIY will find you just in time if you need to tackle this job yourself.
Comments and suggestions appreciated!


Modified by vasillalov at 3:25 AM 12-26-2004
 

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Discussion Starter · #2 ·
Re: DIY: 1.9 TDI Thermostat in 2.0 AEG engine. Proper draining of r ... (vasillalov)

late night bump for those that missed it this afternoon
 

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Re: DIY: 1.9 TDI Thermostat in 2.0 AEG engine. Proper draining of r ... (vasillalov)

dobra rabota pak. http://****************.com/smile/emthup.gif
 

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Discussion Starter · #5 ·
Re: DIY: 1.9 TDI Thermostat in 2.0 AEG engine. Proper draining of r ... (izzo)

This needs a bump for those who missed it yesterday!
 

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Re: (BarMunky)

Another great write up Vasil, will definitely come in handy for me. http://****************.com/smile/emthup.gif



Modified by V_Dubster718 at 7:11 PM 12-14-2004
 

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Discussion Starter · #8 ·
Re: (BarMunky)

Quote, originally posted by BarMunky »
I would be worried that the TDI thermostat operates at a different temp...good writup though.

Nope,
all thermostats are rated for 87 degrees centrigrade.
 

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Discussion Starter · #9 ·
Re: (vasillalov)

ttt
 

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Discussion Starter · #11 ·
Re: (vasillalov)

bump for those that missed it couple of days ago!
 

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Discussion Starter · #12 ·
Re: (vasillalov)

this needs a bump for those that missed it last couple of weeks.
 

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Re: (vasillalov)

Quote, originally posted by vasillalov »
this needs a bump for those that missed it last couple of weeks.

before i order parts... i just want to make sure the TDI thermostat is working fine?
 

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Re: DIY: 1.9 TDI Thermostat in 2.0 AEG engine. Proper draining of r ... (vasillalov)

This needs to be made a sticky in the 2.0 DIY
 

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Re: DIY: 1.9 TDI Thermostat in 2.0 AEG engine. Proper draining of r ... (NINj4)

Just my $0.02
If you're disconnecting the battery, its safer to disconnect the NEGATIVE terminal. Since the chassis is ground, it'd be pretty easy to touch the wrong thing while taking off the positive lead... and end up hurting yourself.
 

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Re: DIY: 1.9 TDI Thermostat in 2.0 AEG engine. Proper draining of r ... (digitalhippie)

Quote, originally posted by digitalhippie »
Just my $0.02
If you're disconnecting the battery, its safer to disconnect the NEGATIVE terminal. Since the chassis is ground, it'd be pretty easy to touch the wrong thing while taking off the positive lead... and end up hurting yourself.

http://****************.com/smile/emthup.gif ALWAYS take off the negative lead first!!!
Gary
 

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Re: DIY: 1.9 TDI Thermostat in 2.0 AEG engine. Proper draining of r ... (VgRt6)

the 2.0 thermostat and o-ring cost me $7 at AutoZone, it's not that expensive anymore...
also, the parts guy i deal with at my dealership told me it's very very rare that the thermostat will go and that people usually think its the thermostat when it's actually the water pump...
so i got the complete timing belt kit from ECS Tuning and I'm just gonna replace everything today without a Bentley manual or a decent DIY... hopefully God will be on my side...
 

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Re: DIY: 1.9 TDI Thermostat in 2.0 AEG engine. Proper draining of r ... (vasillalov)

I'm gonna guess that the difference is the TDI 'stat opens slower, due to less waste heat produced by the diesel and longer warmup times.
One way to test the theory would be to put them side-by-side in pan of water and turn on the heat, but I ain't gonna do it.
 

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Re: DIY: 1.9 TDI Thermostat in 2.0 AEG engine. Proper draining of r ... (tadc)

Did mine today , mine was opening and closing at random. At hwy speed it was running cold. It stays at 190 now. I used the standard 2.0 thermostat from germanautoparts.com ,its made in Germany. BTW, my car has 104k , 99.5 Jetta 2.0 Rich......
 

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Re: DIY: 1.9 TDI Thermostat in 2.0 AEG engine. Proper draining of r ... (rfiglia)

Quote, originally posted by vasillov »
Note: You should expect to fill 1 gallon container and then about 1/3 of the second.

are you sure, I don't think you would be able to hold the funnel with that pressure.
 
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