No heat at idle after some soft of coolant flush/drain/change/treatment? Here is fix
If you have tried everything, bled the system 2-3 times using the bleed valve located at the bottom of the engine, and are extremely frustrated and want to replace the heater core.. dont do it, its just a finnicky heater core air pocket.
The heater core generally only gets pressure when the car is revved up, because it is a passive system (i think), if there is any sort of air pocket, it wont leave through the hose for some reason, and needs to be bled properly.
There are two coolant hoses on the firewall directly next to the battery for the heater core.
The right hose(outlet) has a bleed hole on it. The hose needs to be pulled part way out, until the bleed hole is exposed.. you can see the tiny hole in the pic, ignore the numbers i borrowed this pic
Once the hose is pulled out to the point of the little hole being exposed to coolant, rev the engine up to 3-4k for 3-5 minutes. There will be coolant loss, and YOU ARE NOT DONE at the first sign of coolant through the little hole.. Keep going. For a while. And have someone push the hose in while the engine is still revved up after 3-5 min. (be careful here, as it can be HOTTT).
Re: the factory procedure calls for an extension tube to be threaded into the reservoir opening....
Yeah i know.. but my way will work.. I just see a hand full of people running around with no heat at idle, and cant figure out why. Plus, there is no definitive guide when i searched.. so this is enough for someone to get heat at idle!
Do you think that an air pocket could also prevent coolant flow to the feed hose for the heater core? I am working on a 101,000 mile 2001 A6 Avant 2.8l V6 30V ATQ that has no flow to or from the heater core. With both heater supply and return hoses disconnected from the the heater core pipes in the engine compartment there is no coolant flow whatsoever in the heater core feed hose with the engine running at idle and warmed up. This A6 just had a timing belt, water pump and thermostat replacement. With air pressure applied to the heater core and supply and feed hoses in both directions there is flow and no indication of a blockage and the G12+ coolant is very clean. The engine does not overheat at idle and the I/P gauge is reading normal at straight up midrange. There is some return coolant flow visible in the small diameter hose leading to the coolant reservoir. This is my friend's shop that deals primarily with BMWs so their Audi repair experience is limited.
Thank you. We did the same thing . Works great now. With the C5 A6 the heater core is very low behind the I/P
so the coolant has a hard time to flow to the core at idle speeds making for large air pockets.