We have an audi a4 2.6cc , the heater only blows cold air we have had a new heater matrix a new water pump and thermostat and a new radiator now it blow luke warm air but not hot,please any ideas

While the problem might be a leaky head gasket or the degassing system blocked, the cause is probably an almost blocked heater matrix...

The thermostat would not stop the heaters from getting hot . Is flush out the heater matrix make sure water flows nice and free . But I would be looking at the water pump and at the same time flush main radiator and heater matrix at the same time. The thermostat stuck shut will cause it to overheat but not stop the heat out of the blowers.

Maybe you have a problem with the matrix radiator.

The heater core is probably clogged, or the valve that lets radiator fluid flow through the core is not working.

1st you need to make sure no air in your system open you radaiator cap and your heater to max and see if air bubles or circulation is going on fill up accordingly otherwise you have blockage in your system or water pump if no circulation ////make sure no bubles all bubles should be gone////look for coolant circulation in your radiator make sure your thermostat is not defective or placed in the right position.

the thermostat may be stuck open, not allowing the the water in the radiator to warm up, or you may have a bad exchange on the dash that won't allow you to turn it from cold to hot.the resevoir is called an overflow resevoir.when the coolant heats up it expands and flows into there, when it cools back down it uses the coolant from the resevoir.

make sure heater core is hooked up some times when the heater core goes some people just plumb it back ti itsself

Sounds like the valve to the heater matrix is jammed shut, the actuator has burnt out or a blocked heater matrix. Trace the smaller diameter pipe from the thermostat housing and see if you can manually operate the valve.

I have read on an audi forums sitethat sometimes air gets stuck in heater core and can cause the problems you are experiencing. You need to get the coolant bleed. Easiest way would be to go get it flushed by one of those flushing machines. Hope this helps. I found solution at audiforums.com