I could point you to bleed air out, test thermostat, test fans and temp sensor, but with these I will just say before wasting money on parts, have the compression tested on all cylinders and test for exhaust in cooling system to rule out a blown head gasket first as your over-heating and either its because of head gasket or can cause head gasket failure so before going down a long road to end up at head gasket repair as anything you do other than requires it to be cranked which leads to driving and continued over-heating to eventual head gasket failure if it is not failed already.
What have you checked? Is coolant full and how old is the coolant?
If head gasket is blown what do I do
The top of motor will need to come off, be pressure tested, checked for warp and if not have it re-surfaced and rebuilt if many miles are on it and re-installed with new gaskets and new head bolts. Also It is my opinion to find yes a warped head can be re-surfaced too and used but unless your fine with re-visiting this issue over and over find a used head checked to not be warped and use it or buy a new head. Head bolts stretch so always use new head bolts. A head once warped will continue to blow head gaskets. Let me in-form you that many cars sold used have former head gasket failures and the cheapest fix is to pull the head and put on new gaskets, many omit the correct way to fix this to just trade it back in or sell it off as good engine. Given enough running time a once warped head will blow its head gasket again even if it has been re-surfaced, A re-surfaced head is meant to pre-pair the surface for new gasket, it can not take a warp out, only melting it down and forming a new head will take the warp out, the warp is not just on the mating surface its all the way threw the metal from top to bottom or side to side. But remember what I already said above test for this with compression testing on all cylinders and test for exhaust in cooling system test first. You may just need a thermostat, a fan relay ect.
May need a water pump, just need to test, or get it to a shop.
most over heating issues are a bit more simple then a head gasket. if the car has been over heated continuesly you will how ever likely damage the head but the cars are designed to shut off before this happens. Have the water pump checked this is the likely culprit of the over heat. if this is a 2.8l engine and it keeps getting low fluid have the heating pipe checked at the back of the head for signs of leak. this is easy to pin point on a 2.8L engine as the leak will come down either the back of the engine on either the left of right and signs of coolant under the intake manifold will be present in pools of anti freeze. Other places for over heating is the cooling fans such as the clutch fan going bad or the electroinic fan stoping working which happens more often then people relize on the older audis. if the electric fan stops working the car can begin to over heat past 100degrees. an easy way to check for signs of leakage from the head would be filmy and milky oil and signs of the same type of resdue on the oil cap that is on the antifreeze cap. one last place to check is the antifreeze it self. if the anti freeze is older then 5 years its starts to lose its ability to keep the engine cooled at 10 years it will likely start to be more water then antifreeze. so if it has not been done in a while replace it and see if it helps. If you refilled the antifreeze with anything else other then the designated fluid for the car such as g12 antifreeze then you have likely caused it to gel and block the cooling system. a flush should beable to correct that sort of issue but it will require it done a number of times in a row at the same time to be sure it is all out of the system.
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