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2000 Passat B5 wagon, 4-Motion, 2.8l ATQ
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Discussion Starter #1
After reading the helpful advice on this site, I decided to flush my cooling system [several times], then followed instructions on bleeding air out of the system while filling.

The good new is I get good heat at idle and at higher RPMs. I also do not hear any more gurgling noises from the dash. The hoses all get hot, to/from the heater core and to/from overflow tank (150F+ with Infrared thermometer). However, after warm-up and at idle, I do not see a stream of fluid going into the overflow [the tank is somewhat yellowed]; but when I shut the car off, I do see the fluid pushed in via the electric pump. Not sure what happens at higher RPMs. I see lots of condensation on the a few refrigerant hoses [I'm not running A/C].

I decided to monitor the coolant T via OBD (OBDeleven and VCDS-Lite tools), as it’s been made clear here the instrument T gauge has a large dead zone around 190F.

In my driveway on an overcast day (76F), if I leave the car at sustained idle after warmup, the coolant T registers in the 210 +/-6 F range. I was expecting much lower based on posts from other folks with VWs . If I go for a spin, the T may be a few degrees lower.

Of course, I wasn’t clever enough to measure the coolant T via OBD before I did the flush, so I don’t have a reference point.

Perhaps not by coincidence, I also now see a fault that was not there before: “Warm up catalyst, Bank 1 Efficiency below threshold”.

The water pump, T-stat, [but not the coolant temp sensor] were replaced at an independent shop 26K mi / 6 yrs ago.

B5 4-Motion wagon, 2.8l, ATQ
 

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Someone else will probably have a direct answer, but the radiator fan temperature switch, which is on the radiator outlet, turns the fan on low speed at 198-207°F, and high speed at 210-221°F, and that's at the radiator outlet, the lowest temperature in the system. The CTS is located essentially the hottest part of the system.

I'd test the CTS thusly: With VCDS-lite, I assume you can see measuring blocks.

If so, you can look at:
  • the ECM measuring block for engine temperature and
  • the Instrument Cluster measuring block for engine temperature.
Those two blocks' values are from the two independent sensors (thermistors) in the CTS. They should agree within a few degrees--if they don't, replace the CTS.
 

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Excellent advice given by FrescoGreen.

If all of that checks out, make sure your mechanical fan (driven by belt) is turning at same speed as engine. At that temp the viscous clutch should be 'locked' in fully.
Make sure your a/c condenser is free of debris. It is the first in line and will severely affect the radiators ability to remove the heat from the coolant. Check the radiator cooling fins also. Fine dust and smashed bugs will collect on the front of the radiator blocking air flow.
 
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I think it’s fine. I just did a similar check on my idling CC with AC running and the coolant temp was 215F, while the gage stayed pegged at 190.
 

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2000 Passat B5 wagon, 4-Motion, 2.8l ATQ
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Discussion Starter #5
measuring blocks ...
the ECM measuring block for engine temperature and
the Instrument Cluster measuring block for engine temperature.
The coolant T readings from the Instrument measuring block are consistently ~ 4° Celsius higher than the Engine measurements [e.g., Engine: 99 °C , Instrument: 103 °C]

The radiator fan seems to spin at the same rotational speed as the pulleys.

The Aux. Fan is controlled by Climatronic::
Economy mode: Fan off ; Non-economy mode: Fan on. I can’t tell that it’s spinning faster at higher T.

Photos of radiator fins attached - not very pretty:


vw-rad1.jpg
vw-rad2.jpg
 

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There is a fan switch in the lower radiator hose that controls the fan when it reaches the upper limit. It looks like the Coolant temp sensor at the back of the head, but it is simply an on/off switch. It cannot be scanned to determine whether it is working.

The belt driven fan always spins. What you need to verify is the airflow. When cold, the belt driven fan typically spins slower than the pulley it is attached to. The thermally-reactive fluid in the fan clutch assembly allows this. When the fan clutch gets hot from the air coming through the radiator, it becomes more viscous and locks the fan into sync with the pulley, thus allowing the fan to pull a higher CFM.

Edit: oops, missed Andreaspassats' explanation. oh well.
 

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Well, I've seen worse.
The a/c condenser looks to be pretty normal for the age of the car. All of those fins can straightened out. Using a tweezers, you can carefully straighten the fins. Tedious work but it will improve your cooling efficiency.
The radiator looks as though it's fairly corroded. I might be wrong but that's the way the picture portrays it to be.
One of my cars had a radiator that was corroded pretty bad. I purchased a new Behr radiator, I was surprised at how much more affective it was at removing the heat.
As far as the temp gauge goes, I think you're fine. According to your measurements, you're still not at the boiling point and as long as you have the proper mix of coolant your good all the way up to the 225 - 228 degree range.
Unless you have coolant residue anywhere on the radiator that would suggest a slow leak, keeping what you have should be just fine. If you're like me though ( I tend to have car parts OCD), replacing the radiator really isn't that big of a deal. Brand new Behr radiators are going for around $150, even Genuine Audi/VW sell for less than $200 now
 

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The belt driven fan always spins. What you need to verify is the airflow. When cold, the belt driven fan typically spins slower than the pulley it is attached to. The thermally-reactive fluid in the fan clutch assembly allows this. When the fan clutch gets hot from the air coming through the radiator, it becomes more viscous and locks the fan into sync with the pulley, thus allowing the fan to pull a higher CFM.

Edit: oops, missed Andreaspassats' explanation. oh well.
You just did a better job of explaining it. :p
 

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They do make radiator combs for the fins. I would also rinse it out with a hose. The condensation on the a/c lines indicate the a/c was running, which is common on the Climatronic unless it's in Econ mode. As stated earlier, for the engine driven fan, it's about ariflow. I've seen fans moving at hot temps that had some airflow, but you could easily stop them with a gloved had.
 

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Discussion Starter #11 (Edited)
The radiator looks as though it's fairly corroded. I might be wrong but that's the way the picture portrays it to be.
Here is a closeup of the lower portion, which I assume gets the most damage.

Using tweezers to straighten the fins seems like a monumental task - I tried with a straight blade spudger [in a different area than shown], which is faster - but still seems it will be an unending task.
vw-rad3.jpg
 

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Discussion Starter #13
I would also rinse it out with a hose.
I'll give the hose a try, after hitting it from the inside with a leaf blower. Never realized all those little pebbles get stuck between the fins.
 

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Using tweezers to straighten the fins seems like a monumental task - I tried with a straight blade spudger [in a different area than shown], which is faster - but still seems it will be an unending task.
View attachment 99662
This is a good way to kill time while in quarantine. ;)
 

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Those 'fins' are damaged only at the front, but leave plenty of area around them for air to get through. After the first 1/8", the metal is no doubt fine the rest of the way. I also don't think that 210F is abnormal. The temp sensor is mounted on the piping that collects coolant from the cylinder heads, before it returns to the pump, so you can expect coolant there to be hotter than what's in the radiator. And at idle, the pump is running slowly, so the coolant in the heads takes longer to pass through.

The viscous fan should have definite drag when stopped right after running after a long idle. If not, that clutch should be replaced. I had to do that on my B5 A4 2.8.
 

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The viscous fan should have definite drag when stopped right after running after a long idle. If not, that clutch should be replaced. I had to do that on my B5 A4 2.8.
What year is your B5 A4 2.8? My 2005 A4 1.8T, although technically a B6, has two variable-speed electric fans, no engine-driven fan.
 

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My B5 was the early '96 version with the wide (better looking) euro licence plate area in the trunk lid. It was the 12-valve engine too, SOHC. One electric and one mechanically-driven fan.
 

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Discussion Starter #19

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That looks to me like its operating correctly.
 
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