I have recently noticed on my '99 A4 1.8T that the coolant gauge is showing inconsistent readings. On starts, of course the reading of the gauge is cold, but as the car warms up the gauge only moves to the first line...line and a half...of the gauge. Normal right? After only a mile or so the gauge will drop of to the cold line, the car is obviously warm enough to read a higher temperature on the gauge, but does not budge. If I stop and shut the car off for a while and then get back in the car to take off again the reading is centered up...gauge reading half way on the temperature gauge. After another mile the temperature drops down to the cold line again. The reading fluctuates on the temperature gauge during driving and never stays consistent.....any idea of what the problem is here?

Thanks,
Hobo

 

you need a new collant sensor, these things happen they are a known issue, it also helps to include year and engine type when posting questions like this, but im pretty sure that the prob here.

 

he did include the year and engine type,
http://www.audiworld.com/tech/eng47.shtml
heres a DIY its houldnt take too long since you got a 1.8t. just make sure you you get the right sensor. theres one that a square conector 4 pin and one that 4 pin curved sensor. if you need a the later one which should be green PM me as i have one sitting in my garage whichi bought but turned out to be the wrong sensor for my car.

 

Or this one: https://www.audiforums.com/m_57230/tm.htm

Be glad you have a 1.8T...my 2.8 got a bit frustrating. And it's smart to order 2 clips when you order the sensor, just in case you snap one.

 

I hate to be the bearer of bad news but the coolant temp sensor doesn't affect the gauge temp in the cluster in the least.
It just tells the ecm what temp the engine is, there is another sensor for the cluster whichalmost NEVERfails.
If you don't believe me go un-plug your cts sensor and see if it makes a s*** bit of difference on the gauge. It won't.
I would be looking at the thermostat, how many miles are on your car? Have you have the thermostat replaced before.

 

And peoplestop sending me PM's that im wrong about this, if you don't like the truth, call anaudi engineerabout it cause im gonna tell you to blow it out your a**.

 

Thats actually pretty interesting. Which sensor does send the temp to the cluster guage?

Its actually good to hear this b/c recently, I've been experiencing symptoms of a bad ECT sensor (hard starts when the engine is warm) while the cluster guage seems to be accurate. This makes me feel much better, and I'll be replacing my ECT soon.

 

The sensor that controls the cluster is mounted in different places, most commonly mounted on the radiator, or by the heater core. Either way i've never seen one fail yet.
There is a total of 3 or 4 sensors in the entire system actually.

 

Dang, you learn something new every day.




Now here's the thing...I'm not saying you're wrong, because you clearly know what you're talking about. However, up until this past Sunday, the gauge displaying my coolant temp would only work sporadically...it'd register cold 90 percent of the time, even after hours of driving. This past Sunday, I replaced the sensor discussed above. Since then, the temperature has risen to normal every time I've driven the car.

Coincidence? (seriously, i'm asking)

 

^^^^^Yes coincidence, its probably your thermostat sticking open.
The main reason i stand by that decision is not because of my knowledge, how i know this is i specifically asked Hienz, AKA Leader of Tech Line of Audi Of North America in Auburn Hills Michigan.
His main roll is to assist technicians when we don't know what the hell were diagnosing, and Hienz let me tell ya is one smart ****, plus he helped design the 12v 2.8L engine.