Cooling System Questions

I have a couple questions about the cooling system on my 2000 TDI Jetta.

1--Has anyone replaced the thermostat? I think that mine may be sticking and I would like to replace it. Are the thermostat and gasket items that are readily available at local parts store (Pep Boys, NAPA, Advance Auto, Auto Zone, etc.) or are these "dealer items"?

2--Where is the coolant temperature sensor located? Is that a difficult part change? Are these "dealer parts" or available elsewhere? (Just interested in this because this seems to be a problem with TDI's by reading this BBS.)

Thanks!

See Cooling System 101.



1 - Connector
2 - Coolant hose, upper
3 - To heater core
4 - From heater core
5 - To connector in coolant hose, upper
6 - Engine Coolant Temperature (ECT) Sensor (G62) Blue in color
7 - Bracket
8 - O-ring
9 - Connector (Always replace)
10 - Bolt: 10 Nm (7 ft lb)
11 - Coolant line
12 - To radiator, upper part
13 - O-ring (Always replace)
14 - To expansion tank, lower part
15 - Bolt - 15 Nm (11 ft lb)
16 - From radiator, lower part
17 - Coolant thermostat
18 - Oil cooler
19 - Coolant or Water-pump
20 - Cooler For EGR
21 - To expansion tank, upper

First, thanks for the replies. The information has been good so far.

Here is why I think that my problem is the thermostat. When I get in the car and start it up, the temperature will gradually build up to 190 F (approximately 90 C). It will run at 190 F for a little while. After driving for a while with the temperature at 190 F, the temperature guage increases a little bit and then slowly falls to the cold zone. It stays reading cold until I stop the car and it sits. After the car sits, the temperature guage will read above the cold zone and do the same thing as described above.

I have had on one occasion have the temperature guage start dropping, reach the cold zone, and then start to slowly rise again and then slowly drop into the cold zone.

Since the temperature guage gradually increases and then gradually decreases into the cold zone, I feel that it is the thermostat.

Reading the posts on the temperature sensor leads me to believe that movement in the temperature guage is sudden/quick and it goes up and down quickly. Mine moves slowly. Also, I understand that coolant sensor problems cause performance problems. I haven't noticed any adverse performance or rough engine running.

Additional information and ideas are appreciated.

Garret
2000 Jetta TDI

Sure sounds like a sticking thermostat to me...

How is the heater output during this fluctuation? Does it put out heat and then cool off as the gauge goes down? If so, then the thermostat may be at fault. If not, then the sensor may be at fault.

The other possibility is the cooling fans on the radiator. If they're turning on when they shouldn't be, and staying on too long, it could be their sensor or relay that is bad. Next time the temp starts to go down, (pull over if you have to) roll down the window and determine if you can hear the fans running. The symptoms described actually point more to this being the area of the fault.

What evidence do you have that the thermostat is sticking? I know it can happen (I've had it happen on other vehicles), but I have yet to hear of it EVER happening on a TDI! After mine warms up, the gauge parks itself at 90 degrees C and stays there no matter what. (Except if it's really cold, like -20 C outside, then it drops below that if I'm stuck in traffic, because the engine doesn't generate much heat.)

I don't think the thermostat itself is anything special, but getting to it is a royal pain in the tail on this engine, it's down at the bottom near the water pump. Make sure that's the real problem before you try to change it.

Coolant temp sensor is simple, it's on the coolant hose that comes out of the cylinder head facing forward towards the left of the car, just unclips and pulls out (some coolant will drain out when you do this). But why do you suspect that there is anything wrong with it? If your glow plugs are not glowing, and the temperature is below freezing and the car has been sitting outside all night, then maybe it's an issue. Yes, coolant temp sensor is a dealer-only item.

Don't change things unless you're sure they are the source of the problem...

The heater still puts out heat, but it does cool down a little.

I didn't think about the radiator fans. I'll have to check those out.

Like I said earlier, before all of this started my TDI ran consistantly at 190 degrees F no matter what the temperature outside.

A couple of thoughts:

First, there are two separate coolant temperature sensors. One sends a signal to the dash guage, and the other sends the signal to the computer. You want to be sure that the coolant is actually cooling down, and it's not just a faulty sensor. However, if the you are getting less heat from the heater when this happens, then yeah, your thermostat is dead. Perhaps someone here has a testing procedure for a coolant temp sensor?

Also, when replacing the thermostat, find a GOOD one. I've had the best luck with Stant thermostats. I do not recommend buying one from Autozone. I've had one of their cheap t-stats die in less than 4 months in the old A2 Jetta. As a result, I had to redo this whole messy job over again. So I don't recommend the chain store self labeled thermostats that are mostly made in China. Instead, call around to "real" auto parts stores and ask what company makes the thermostat. If they can tell you, you're on the right track.

Impex has your thermostat, but they show the price as $35.83. That's odd 'cause I bought mine from the dealership for $19.35 - admittedly for a B4, but I don't know why the A4 'stat would be nearly twice the price. Maybe the Impex number O-501-211-113-C, isn't the correct number for an A4 TDI thermostat?

IMO, I'd buy OEM. A $20 part doesn't break the bank and a failure is a PITA in "best case".

HTH,
Patrick

The input from everyone has been great and helpful. A couple more questions though.

1--On the diagram posted, why does it state to always change part #9 (connector @ thermostat)? On other vehicles where I have changed the thermostat, this is a reusable part. Is this a plastic part or what?

2--Someone stated that there were two seperate coolant sensors. I thought that the coolant sensor (ECU) on the diagram above sent the signal to the computer and the dash guage. If it does not, where is the guage sensor?

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by ladysmithgwl:
1--On the diagram posted, why does it state to always change part #9 (connector @ thermostat)? On other vehicles where I have changed the thermostat, this is a reusable part. Is this a plastic part or what?
<hr></blockquote>

That be plastic. While it is reusable, it definitely is a limited life span part. When I bought my B4, it had a persistent coolant odor that I couldn't locate the source of. The water pump was weeping a little so I replaced it. The next week, the coolant flange(same purpose as the upper no. 9 in the diagram) cracked along its entire length(even tho' I hadn't touched it) - I had found my leak Once again, it won't break the bank - the B4 coolant flange was $15 at the dealer.

I would make the decision based on how much trouble that connector is to get to and whether you're doing the work yourself. If it's easy to reach and you're doing it yourself - I'd say wait 'til it breaks. Like mine, it'll probably give lots of advance notice. If the answer to either the above is "no", or if you tend to stress-out over possible problems like this, then go ahead and replace it.

HTH,
Patrick

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by ladysmithgwl:

2--Someone stated that there were two seperate coolant sensors. I thought that the coolant sensor (ECU) on the diagram above sent the signal to the computer and the dash guage. If it does not, where is the guage sensor?<hr></blockquote>

There are two sensors in the same unit #6. 4 wires are in the connnector to it. One pair sending the temp signal to the ECM and the other pair sending the signal to the instrument cluster.

Update!

After all of the good input and some further investigation, I think I know what the problem is with the "falling" temperature guage.

Finally when the temperature guage was reading cool, I had a chance to look under the hood. I discovered that the fans were not running and that the upper radiator hose was hot and the lower one was cool. So, that information pretty much "kills" the ideas of a bad thermostat or a fan problem. Also, yesterday my wife stated that the temperature guage started going up and down. It didn't go up and down quickly, but it was fluxuating.

With all of this new information, it seems that the coolant sensor is going bad (maybe that should be has gone bad). Any thoughts of agreement or disagreement?

Also, the coolant sensor doesn't look too hard to change. Is that a correct assumption? Any "tricks of the trade" to changing these? Because, I'm planning to do it myself.

Finally, can anyone supply me with the part number for the coolant sensor (2000 Jetta)?

Drain the coolant first. There is a petcock in the radiator.

Temp gauge reading low, upper hose was hot and lower was cool.. doesnt' that still sound like a bad 'stat?

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> Temp gauge reading low, upper hose was hot and lower was cool.. doesnt' that still sound like a bad 'stat? <hr></blockquote>

I guess that this could be yes and no.

The lower radiator hose is really cool and when you squeeze it, it doesn't feel like there is any coolant circulating through it.

If it was warm instead of being so cool, I would say yes. To me it is way to cool or the radiator is super efficient.

Question, I just Got my 1996 Passat TDI, Love it,

the owners manual says coolant never has to be changed. Is this the case, and why not, is it some type of synthetic?? If you drain it do you just replace the same fluid and without a flush??

Thanks,

Steve

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by ladysmithgwl:
If it was warm instead of being so cool, I would say yes. To me it is way to cool or the radiator is super efficient.<hr></blockquote>

If the thermostat were closed, wouldn't both hoses be cool?

I have no useful technical information to add to this thread but I wanted you to know that when my 99.5 Golf was brand new the temperature gauge fluctuated slightly over the 90 deg C mark on two occasions. Both times were on the interstate and both times it would not drop back down until the car was turned off and restarted. I didn't have to let the car cool down for the gauge to return to it's normal position. I attributed this to a erroneous reading from the sensor. I took no corrective action and I have not had it reoccur in over 2½ years.

[ December 26, 2001: Message edited by: FowVay ]</p>

ladysmithgwl:

For future reference, the two radiator cooling fans are programed to operate at the following times:

Radiator Fan Thermo-Switch Operating Temperatures

* Stage I - Low Speed

Switch on...............197* - 206*F (92* - 97*C)
Switch off..............183* - 195*F (84* - 91*C)

* Stage II - High Speed

Switch on...............210* - 221*F (99* - 105*C)
Switch off..............195* - 208*F (91* - 98*C
____________

Naturally, running the A/C will also activate the cooling fans as well.

Moreover, the (ALH) TDI diesel engine uses a 195*F thermostat that actually starts to open at approximately 185*F (85*C) and is fully open by approximately 221*F (105*C), with the central spring-loaded butterfly valve traveling about 0.28" or 7mm from closed to fully opened.

Note: See also Bentley Publisher's 2000 Jetta / Golf Repair Manual - Section 19.

I've had to drain the coolant several times to work on my gauges and now the timing belt. The coolant is supposed to last for a long time, and it does appear good when I drain it into a bottle. Have seen no deposits in the glow plug manifold I took off either. I did buy a gallon to make up some I spilled (50/50 w/distilled water) and the genuine VW stuff is darned expensive - like $20.

PS: Mine is a 2000 model so it might be different than the '96.

[ December 26, 2001: Message edited by: VelvetFoot ]</p>

Thanks for all of the cooling system information. It has been very helpful.

Checked with the dealer this morning and they have the coolant sensors in stock for $36.85. Since the temp. guage is "up & down" that is where I'm going to start.

I won't get a chance to change the part until the weekend, but I'll post to let everyone know if it corrects the problem.

Probably won't help, but that price seems high to me.

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Probably won't help, but that price seems high to me. <hr></blockquote>

What prices have others been paying for this part?

I saw on a post that someone paid $50.00 for the part.

Do you have a part number?

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> Do you have a part number? <hr></blockquote>

Dealer gave me a part number, but it seems to be bogus.

Not to bring up a dead issue or "beat a dead horse", but I did tell everyone that I would post with what happened on my car.

Since my temp guage started going up and down, I decided that the coolant sensor was the problem, not the thermostat. I went to the dealer to get the temp sensor and they sold me part #078-919-501-B (blue sensor) for $22.00. Well, that was the wrong part. I needed part #059-919-501-A (green sensor) for my car.

I changed the sensor myself and have put over 300 miles on my car and no more "up and down" temperature guage.

Well, that is the update to my original post. BTW--wouldn't you know that the correct sensor for my car was $36.85. Figures that it would be the one that costs more.

Beats $200+ for a MAF...

Lady,

Are you saying that the blue and green sensors look the same? Do they have the same plug configuration? Someone on another thread was telling me that the green and blue are not the same. I replaced my black one with a green one and am still having problems.

Just trying to figure this sensor thing out.
Rob