Probable Bad Coolant Temp Sensor 1.8T

[krjone01]

2003 A4 B6 1.8T about 158,000 on the odometer. About 72k on the current engine, replaced under 1.8T extended warranty.

Its been giving some fits trying to start from a cold engine in the morning now that the overnight temperature are starting to drop in the Fall.
Its immediately trying to drop the engine RPM down into idle range on start, about 760rpm, instead of running at a higher RPM for a few minutes to let the engine warm up a bit.
From everything I have researched so far, this would indicate that the computer thinks I am starting from a warm engine, so a probable failure of the coolant temperature sensor.
However, I am not getting any new unexpected fault codes.

Hooked up VCDS light tonight to get some readings. These are the Cold reading after that car has been setting in the driveway for 48 hours.

Measuring Blocks - Cold Engine

Group 134:
Oil Temp: 28c
Ambient Temp: 19c
Air Intake Temp: 23c
Engine Outlet Temp: 18c

Group 135:
Radiator Outlet Temp: 22c

Group 001:
Engine Coolant Temp: 59c

So I let the engine warm up to operating temperature, basically where the temperature needle points straight up, about midpoint on the gauge scale.

Measuring Blocks - Hot Engine

Group 134:
Oil Temp: 58c
Ambient Temp: 19c
Air Intake Temp: 31c
Engine Outlet Temp: 81c

Group 135:
Radiator Outlet Temp: 22c

Group 001:
Engine Coolant Temp: 87c

So the coolant temp sensor shows about 40c over ambient when cold. That doesn't seem right, but then it does show a higher temperature as the engine warms up so its not dead. Is this a normal reading, or has it failed and needs replacing ?

Also what is the Radiator Outlet Temp in Group 135 measuring ? The reading did not change between a cold and hot engine so I fear I have another dead sensor somewhere.

[TobiqueSlang]

87c is about operating temp(middle of a gage) by what you're showing I thing your 2 pin sensor on the bottom the rad might be bad if it's always reading 22c. the rad outlet should always be the coolest point in the system but 22c would be about ambient temperature.

[old guy]

Cold start idle variations: Clicky click®

Mapped cooling system: Read post 14 and 22 in this thread: Clicky click®

[krjone01]

Cold start idle variations: Clicky click®

Mapped cooling system: Read post 14 and 22 in this thread: Clicky click®

Thanks, that is some good info there.

I will double check the radiator output value this evening. I did all those readings in the driveway and its possible that the thermostat never opened up. The drive home today should get some coolant flowing through the radiator.

Still uncertain if 40c over the ambient for the block exit temperature sensor for an ice cold engine is OK, or if that indicates a sensor malfunction.

[old guy]

Still uncertain if 40c over the ambient for the block exit temperature sensor for an ice cold engine is OK, or if that indicates a sensor malfunction.

Not OK. I would start with replacing the sensor.

[krjone01]

Can anyone verify it this statement is correct.

The coolant temperature sensor contains a simple four-pin thermistor to relay the temperature readings to the ECU and the coolant temp gauge. Two of the pins send information to the ECU while the other two pins send their information to the coolant temperature gauge. It is possible to have only two failed pins in the sensor that cause only the ECU or gauge to read incorrectly.

Its from this page. https://blog.fcpeuro.com/how-to-repl...a6-golf-passat

This would be consistent with what I am seeing in the Measuring Points. I have "Engine Outlet Temperature" - 130 Groups and "Engine Coolant Temperature" - 001 group and they are not tracking the same. I am presuming that the 001 point is what is shown on the gauge and the 130 values are what the engine computer uses.

I have also verified that the radiator outlet sensor is working. The engine has to get a lot hotter than just idling in the the driveway before the thermostat gets opened up and I get flow through there. Might indicate I need a new thermostat in the future, but I doubt that has anything to do with my cold startup issues.

[EuroxS4]

Post a complete scan of the ECM so I can tell you whether you need a ecm update for your issue or not.

[old guy]

Rear coolant temperature sensor is indeed a four pin. One signal is used by the ECM and the other signal is sent directly to the display gauge (not connected to the ECM). The display gauge is not linear. It reads straight up at ~170°f and does not go past straight up until ~ 230°f.