I drive a '98 A6 2.8 quattro and I have an overheating problem. The temperature gauge goes a quarter over the middle line all of a sudden.

I was missing some coolant so I added up to the indicated marker. It still goes over.

Here's what I noticed so far:

When the engine is cold and I start it, the main radiator fan is turning at full speed. The small one next to it isn't. After a bit of driving when the temperature is past the half mark, the small fan is turning at full speed and the main one is turning but much more slowly.

Anyone knows what the issue might be and where I should start looking ?

Thanks!

 

What color is the coolant? Did you mix coolants? This is bad. If the coolant changes color you could have bad coolant. A drain and flush might be in order if you haven't done this in a while (like 80K miles). The two other obvious potentials are that your thermostat is bad, or, your water pump. If the water pump is the original part, the fins that move the fluid are made of plastic and have a tendency to wear and break(Newer pumps make the fins out of metal).

 

have you replaced the coolant temp sensor yet? sounds like about time , it may help