98' Audi A4 1.8T Quattro - 5spd. manual

Just picked up my dream car last month its a B5 A4 1.8T as stated above. I love the car, its clean but has no maintenance history. Has a few miles on it +140K, but this one was obviously cared for so I jumped on it. The timing belt job does haunt me but I know it has been replaced given the mileage, other than that I am in love with my new B5. Its the nicest car i have ever owned.

Now for my problem...

Recently I noticed that when coming to a stop with the clutch in the idle will surge low between 6000-1100 RPMS. It surges about 2-3 times when coming to a stop or when you throttle out of gear. I have been doing my reading and I am leaning towards a coolant temp sensor (ECT). Recently I noticed that my coolant gauge was reading really low when I first got the car and now the gauge doesn't work at all. Yesterday when I started the car the cooling fan kicked on after it sat all night in 30 degree Chicago winter weather. My coolant temp. sensor is blue with a blue connector, and I plan on going to buy a replacement tomm. I just don't know the part number, I know there are a few different units out there, I just want to get the right part. I already put in some new NGK Iridium IX BKR6EIX's for the hell of it and I cleaned the Idle temp sensor located after the throttle body cleaned with MAF cleaner (it was dirty and very oily). I just changed the oil with Mobil 1 Synthetic 0W-40. And today I pulled the plug to the ECT and the surge went away and the idle was smooth, I left it unplugged and when I went to start the car it was really tough to turn over, until I plugged the sensor back in and the goofy surge is now back... What gives? I hope my problem is as easy as changing the ECT Sensor.

Let me hear what you guys have to say.

 

-Changed the coolant temp sensor (CTS) for a new one from the dealer. Gauge now reads perfect. Took me 2 minutes to change it out.

-My idle surge was because I recently disconnected the battery and needed a Throttle Body Alignmet (TBA). Idle is now perfect. did the myself too!

 

How is a, "Throttle Body Alignment" performed?

 

I searched google under "A4 Throttle Body Alignment" Basically you will get a lot of people saying you need the VAG-COM to perform it. But I read that you turn the key "ON" and then press the accelerator for 3 seconds, then turn the key to off and take yoyr foot off the accelerator, but do not remove it from ignition - wait 2 minutes and start the car. Seemed to work perfect for me.

Off topic, I see you have a P11 G20! Are you on G20.net? I had two P10's before I got my Audi. Unfortunately my baby (96 G20 5spd.) was rear ended and totalled in November, reason I picked up my A4. I miss my G soo much, I know those cars inside and out.

Here is a pic of my P10 from last summer:

 

I am a member of G20.net. The G20 is my wifes car, but I recently purchased another B5 A4 for her, so no more G20.