[XRING600]

I have been going through this 1999 Passat, AEB engine, and got concerned that my temp gauge never reached normal operating temp. 190 degrees if I am correct. So I took a reading with VCDS and found that the top temperature I got was 60 degrees Celsius which is around 140 F. My temp dash temp gauge hardly ever gets past the 1st 3 white lines. So is this a sign of a bad temperature sensor and if so which one? I have found one just in front of the battery but it looks like another is implanted in the radiator, drivers side bottom corner.

 

[cuppie]

You can't say that it's a faulty temp sensor (or its wiring), unless you can verify (read: with a contact temperature probe) that the engine is actually at the temperature that it should be.

That said... Have you replaced the thermostat yet?

 

[XRING600]

Cuppie I am in the trouble shooting stages so I have not replaced the thermostat yet. Your suggestion about the probe is a good idea and I will track one down tomorrow.

Would you happen to know what the 2 different temps shown on the VCDS readouts pertain to? I would think that the larger number is the sensor but I cannot guess what the lower number might be.

 

[Steve in Chicago]

My guess is the lower temp is from the IAT in the intake manifold. Temperature of the intake air as it enters the manifold.

The temp sensor you found in the lower radiator is what triggers the aux electric fan to come on. It does not talk to the engine computer.
The temp sensor you found at the back of the head sends temp data to the ECM and also temp data to the cluster for the gauge.
That one should be achieving about 90ºC when fully warm.

That sensor is actually two sensors in one part. You can use this to cross-check them. Open the engine computer controller and note the coolant temp reading. Then close that and open the instrumentation controller (gauge cluster) and find the reading being sent to the temp gauge. The two temperatures should be fairly close to each other. If one is way off from the other, the sensor is bad. If they are both indicating around 60ºC when fully hot, you probably have a thermostat that is sticking open.

 

[XRING600]

Thank you Steve for the instruction. The sensor was bad. I pulled it and found it was the original with 223,000 miles on it. Made April 99. However it did not solve the temp issue and I believe I also have a stuck open thermostat. The car only rises to 150 F or so.

Another problem I have is finding G13 coolant in the town where I live.

 

[Steve in Chicago]

You can order it online from places like ECS and other Euro automotive parts suppliers. Some people have run other types but I forget which ones. The OEM formula is sulfate free so I imagine that's the type you'd be looking for. If you do run an alternate type, make sure to flush every bit of the old type out of every part of the system. Mixing types can sometimes cause a chemical reaction and form globs and goop that clog up the heater core.

 

[XRING600]

Without trying to generalize things would low operating temperature cause some of the Readiness failures I am seeing in VCDS?

 

[Urlik]

I believe low coolant temps can cause those issues.

 

[Steve in Chicago]

When you clear codes, you also reset all the readiness monitors to null. After you've driven the car a bit, the various systems have an opportunity to come on-line and test themselves and log a readiness state. I'm guessing you haven't driven the car much since clearing codes. Your AEB has no EGR, AC monitor or SAIP so that's why those three have come up as satisfied. Bad O2 sensors or bad O2 sensor heaters should pop specific trouble codes in few drive cycles.

 

[XRING600]

Thank you Steve. Great explanation.

 

[XRING600]

So I got to thinking about the thermostat. If the temp outside is 40 degrees would the car ever reach max operating temp? I drove it today and some of the time it did level out at 190 degrees. Most of the time it was lower, maybe 150 or so.

And I did recheck the readiness and all showed passed so Steve you were correct about giving the items a chance to poll and post the results via VCDS.

 

[cuppie]

Thermostat's dead. Needs new.
40*F isn't that cold. Even at single-digit outside temps, the engine should still hit (and stay at) full operating temp.