My engine temperature wanders up and down a lot

['99.5manual]

Hello forum. I just got this new 1999.5 Audi A4 and I absolutely have fallen in love with it. There are a few little issues though. My question for the forum is: my coolant temp isn't stable. It kind of wanders very slowly up and down between one bar and the 50% mark. It does this no matter what speed I'm going, for instance just putting around town through neighborhoods or going down the freeway. Temperature not stable. What does that sound like to you guys?

[walky_talky20]

- The coolant temp sensor can absolutely cause this. It is actually 2 sensors in one: one sensor for the gauge, the other for the ECU. They can fail separately.
- The thermostat can also be to blame. They are designed to fail where the engine will run colder than it should (fail-safe).
- It *can* be the instrument cluster gauge itself. That is pretty rare on the B5, though.

I'd say without any further details or observations, it is 50/50 between the sensor and the thermostat. They are both known to fail after some time, and neither piece is very expensive. So it isn't really wasted money to pick the easier one and try that first.

There are some related tests you could do with VCDS or similar scan tool to gain a little more insight:
- Compare ECU Temp reading to Cluster temp reading (they should always agree within a small margin)
- Do a "needle sweep" of the cluster (output test mode) to make sure the temp needle itself behaves properly.

[A4Qwattro]

Does your HVAC air temp fluctuate when blowing? If you haven't noticed that, then it's most probably the coolant temp sensor is just faulty. An alternative could be air in the system but normally if there is air, the temp should bounce between middle and high, not middle and low.

['99.5manual]

- The coolant temp sensor can absolutely cause this. It is actually 2 sensors in one: one sensor for the gauge, the other for the ECU. They can fail separately.
- The thermostat can also be to blame. They are designed to fail where the engine will run colder than it should (fail-safe).
- It *can* be the instrument cluster gauge itself. That is pretty rare on the B5, though.

I'd say without any further details or observations, it is 50/50 between the sensor and the thermostat. They are both known to fail after some time, and neither piece is very expensive. So it isn't really wasted money to pick the easier one and try that first.

There are some related tests you could do with VCDS or similar scan tool to gain a little more insight:
- Compare ECU Temp reading to Cluster temp reading (they should always agree within a small margin)
- Do a "needle sweep" of the cluster (output test mode) to make sure the temp needle itself behaves properly.

Wow thanks guys your responses are fantastic and thorough. I've got the Chilton repair manual coming in the mail, so that will tell me how to change the thermostat and sensor, and I even have the VCDS USB interface device ordered too, through ross-tech.com. Reading through various things online it looks like I really should invest in this interface device as it looks like it shows volumes of insight and real-time data to help me fix my car on into the future.

I will post back on this forum thread when I have progress to share. Thanks!

['99.5manual]

Does your HVAC air temp fluctuate when blowing? If you haven't noticed that, then it's most probably the coolant temp sensor is just faulty. An alternative could be air in the system but normally if there is air, the temp should bounce between middle and high, not middle and low.

Hard to say at the moment, it's summer so it's hard to tell if the warm/ambient air temp fluctuates, maybe I'll stick a food thermometer in the vent and check it. My AC doesn't blow cold at all yet, but that's going to be another upcoming forum post ;) I've got my VCDS interface coming already...