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Discussion Starter · #1 ·
For the last week or so, the temp gauge would climb much faster than normal. Within a couple minutes after starting the engine, it would already be up to 190. It would never overheat.
I just replaced the coolant temp sensor and have the same problem.
VCDS shows a temp of 92 Celsius (198) under the engine measuring block, but reads 99 Celsius (210) under the instruments measuring block.

Before I replace the sensor in the lower radiator hose, what else could be causing this?
The Google says it could be the thermostat or the radiator fan. I'm hoping it's neither of those.

Does anyone have a VCDS method to test if the fan or t-stat is faulty?
Or am I barking up the wrong tree?
 

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Did you try bleeding the system? Up to operating temp in 4 minutes was normal for my V6 4Mo, but the starting temp here is warmer.
 

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If the thermostat doesn't open, it would overheat quickly.
It wouldn't take long to reach 230 deg F, and would continue rising.

The part in the lower radiator hose is the fan thermal switch, and it controls the fan speeds 1 & 2.
You have not provided any info in this thread that would indicate a problem with the fan.
 

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Discussion Starter · #5 ·
If the thermostat doesn't open, it would overheat quickly.
It wouldn't take long to reach 230 deg F, and would continue rising.

The part in the lower radiator hose is the fan thermal switch, and it controls the fan speeds 1 & 2.
You have not provided any info in this thread that would indicate a problem with the fan.
Oh, it is heating up quickly. Within 2 minutes both heater core hoses are almost untouchable.I haven't driven it a long enough distance/time for it to overheat, although I still have the "cold/OK/overheating" instrument cluster temp gauge, so w/o using VCDS I don't know the actual temp.

I was expecting the fan to turn on while it was this hot, but it didn't. That is why I also bought a new fan thermal switch.

The reason I settled on the thermostat being bad is that, while the heater core hoses were hot, none of the other hoses were. Not the upper/lower radiator hoses nor either of thermostat housing hoses.

I have read somewhere that, if I unplug that fan thermal switch, the fan will run by default. I'm gonna try that just to see if that is true before I change that switch.

After all that, if it still overheats, I'm gonna tackle the thermostat.

....unless the water pump is (also) going bad.....

Both thermostat and water pump were replaced less than 20,000 miles/32,000 KM ago.
 

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You have not provided any evidence of overheating.
I suggest you monitor the temp with VCDS and run it much longer longer, if the temp reaches 230 deg. F, switch off.
Report results.
 

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If the hoses to and from the radiator are not warm/hot, then the thermal switch will not do anything. It only kicks the fan on if the radiator outlet temp is high enough to warrant it.

That said, I can only think of a blockage or thermostat not opening as the cause for coolant not flowing. The only other outlet in the block for flow is the heater core.
 
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Your symptoms do indicate a blockage, but that is normal when started cold.
The thermostat is closed when cold (causing blockage), it doesn't start to open until the coolant at the back of the thermostat reaches about 200 deg. F.
You are switching it off before the thermostat opens.
 
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