Hello,

I recently got this 02 A4 1.8 quattro the coolant guage goes from cold to a little under half and flickers when doing so then goes to cold. If i turn the car on and off i can get it to bounce around under the half mark. I already installed two temp guages at the thermostat housing one from napa and one from audi and still the same the connector looks good. does any one know what the other temp sender on the lower radiator hose is for audi said it controlls a aux water pump but IDK.....?

 

No such thing as an aux water pump. And, there is no temp sensor in the thermostat. What the 1.8t has is 2 temp sensors, one in the coolant flange at the back of the engine ("green sensor") and one as you describe at the bottom of the radiator. My understanding is the green one gives the signal to your gage and a combination of the green and black temps to open and close the thermostat. So, you erratic temp gage should be remedied by the green sensor at the back of the engine. Is that the one you changed?

 

Yeah i replaced the green one on the back of the engine twice one from napa and one from the audi dealer but i havent done any thing with the black one on the lower radiator hose. I thought that the flange on the back of the engine was the thermostat housing?

 

No, the thermostat is located down by the alternator bolted to the side of the block.

 

If you replaced the "green" sensor and behavior is the same, then that usually points to the thermostat itself.

 

So i replaced all the cooland temp sensors the black one, and the green one and now the guage dosent work at all. The guage will work if you drive the car and get it to operating temp and then shut the engine off the guage will rise up to just under the half mark. someone said the thermostat but it dosent make sence to me. any help is appreciated thanks

 

sorry the guage only rises up after you shut the engine off and re start it with the vehicle at operating temp.

 

My audi is a 2003 a4 2.0 alt motor , the housing where the thermostat sits in perished one of the fittings that the water hose connects to , i replaced the part and subsequently find that my temp gauge does not work most of the time , sometimes it sits at 90 but for the most part sits at zero , also it steams all the coolant out when i sit in traffic , i found that it had also popped the coolant bottle so i replaced that and the cap.I filled it up again to the min line and revved the engine between 1500 and 2000rpm to make sure no air bubbles , but everytime i sit in bumper to bumper traffic i just get steam pouring out the bonnet and if i switch the car off then it just dumps the coolant out big puddle under the car , yet i just cannot see where it is leaking , and if i fill it up again it doesnt leak , only when i sit in traffic and travelling in first or second gear does the problem occur and i get no warnings of overheating , not even coolant light comes on , it only comes on after ive lost all the coolant , has anybody ever had this problem , phoned audi themselves even their technician was stumped as to what it could be , had the motor redone 6months ago and have had weird problems ever since , the first was the cam sprocket sheared in half chewed the timing belt and then they had to do the whole job over again and now this coolant problem , if i leave car ideling in driveway and rev it the fans come on when they supposed to and it doesnt leak , problem only happens in traffic driving in first and second gear

 

Did you replace the thermostat or just the fitting?

 

replace the thermostat! the green sensor basically works the "choke" the other sensor is the Gauge but the fact the gauge moves when the car is hot but not running suggests the thermostat is stuck open. or possibly a bad wire harness someplace