First off, I realize i'm a newb to the Audi Forums, but i've been a user on BMW forums for years. I've tried doing a search for my exact symptoms, but haven't been able to find one through the various boards you guys have.

This is an 03 A4 w/ the 1.8T. He first started w/ a P code that referred to the coolant temp sensor. I've already double checked and in fact he has the green top sensor. The car has had a tune-up using the proper style plugs, timing belt was replaced before the Audi Recall/Inspection letter was sent, waterpump/fluids flushed (i'm guessing thermostat) and the ECU flash for cold start issues has been done. My problem is the P code is still around, it still shudders on cold mornings, and now the heater/temp guage aren't acting normal.

The biggest and most recent issue is what really threw me off. The 1st few times I drove the vehicle months ago the heater would work like normal, temp guage sat in the middle, but the CEL was on. As of late it's gotten colder here in Boise. Three times ago when I drove it, within the month, the temp guage only climbed to the 1/4 mark, but the heater blew nice and hot.

Of the last two times I drove it, the temp guage stayed at the 1/4 mark just like before, but then suddenly it flopped over the middle and the heater went ice cold. Once I returned home, nothing seemed out of normal other than how the heater stopped working. I drove it once more to return it to my friends house, and it did the same thing were it was 1/4 warm, heater nice and hot, then flopped again to the middle and went ice cold. I continued my drive to his home, staring at the temp gauge while waiting for the heater to warm up accordingly. It never did, and once I turned the vehicle off, the radiator overflow tank was boiling over, steam escaping and the top hose was burping itself.

I'm basically looking for ideas of elimination as to what I should start checking before I consider driving the vehicle again. I'm curious as to the type of coolant that was used from the shop that replaced the waterpump. I don't know if Audi's are a picky, but BMW's for the most part require a phosphate-free type of coolant, so I use Zerex G-05 coolant since it's compatible. Sorry for my ramblings, but I wanted to get it all out there to get as much help as possible

 

There are 2 coolant temp sensors. One is by the radiator and the other is in the coolant flange. Also you should use G12 coolant from the dealer only. At least thats what the manual says.

 

I didn't do the coolant change, so I have no clue what's in there now. This car is a Quattro, so the green top sensor I found is buried near the firewall in a bitch of a spot. Are there two green sensors, or is each sensor different? The thermostat is my 740iL is electronically controlled, so is the one in the Audi also electronic, otherwise it should have opened and functioned normally.

 

The a4 thermostat is electronic. There is a temp sensor in the hose at the bottom drivers side of the radiator.

 

Thanks man....what color is it supposed to be? I guess it doesn't matter, i'll just replace each one again and go from there. Once the car is back at my house, i'll update this thread w/ the results and the p code....i can't remember what it is now.

In the meantime, any other ideas or suggestions?

 

Are there any online versions of Bentley style books for these cars, or an online microfiche?

 

I believe both sensors started as black, but got replaced by green. I havent replaced the one in the front, but from what I see it looks like the same sensor.

There are manuals online, but i dont have one, or know who makes them. I think bentley may be one of them.

 

So, currently my issue is that the temp guage will sit at the 1/4 mark, heater works great, then suddenly it jumps over the halway mark and the heater goes ice cold. I continued to drive the car waiting for it to get warm, but to no avail. Once I got home, the car is boiling over. I thought maybe it was just a fluke, so the lasttime I drove it, it jumped to the middle again, still cold air, and overheating again.

It has the green top sensor on the back of the motor, and I have yet to check the other sensor supposedly located around the radiator itself. I've been reading various "overheating" threads, and some SWEAR it's this one on the back of the block, others say there are like 4 sensors total and that it could be any one of them. Please help. I'm at a friends now, and I wanna be able to get the car back to my house at least so I can have access to my tools.

 

Have you pulled the code from the CEL? That would be the first thing to check. Other than that I would think about replacing the thermostat. These aren't commonly done with the timing belt bc they are mounted on the drivers side of the engine away from the timing belt/ water pump area. It is a housing and thermostat combo, and you have to replace it all at once.

 

Also, when my coolant flange (the part on the back of the engine that the coolant sensor is mounted in) was leaking my heater would not work bc the system would not pressurize. I don't think this is your problem bc it shouldn't cause the temp gauge fluctuations, but I thought I would throw it out there.