A4 B8 Sporadic Heating Issue

[Michael-B8]

Hello Everyone 👋

I'm hoping I can get some answers/help here , I've not long bought an A4 B8 2.0Tfsi 211hp , it's been having an odd issue with heating , every now and then the heating completely stops working after a couple start ups or if I stop somewhere for like half hour or so after a journey (not every time) at that point no matter what you do the heating doesn't come on , Once it's left to fully cool down so let's say 3-4 hours at least it all goes back to normal , when the heating works it's fantastic , originally thought it's the Heat Exchanger but it just doesn't behave that way to me because it's got no consistency sometime it just doesn't show the issue at all and if it were to be at least partially blocked it wouldn't work as well as it does  , I feel like the issue begins as soon as the coolant temp drops slightly (I presume when creating tiny vacuum after it cools slightly ) , I tried many things but I can never replicate it myself , I've put it under a pressure tester to see if there's any visible leaks but I could not find anything even though the needle does move ever so slightly after like 10-20 min , the carpets are all dry and I've seen right under the heater box and I didn't see any residue from coolant , I'm hoping someone has come across this issue before as I can't seem to find a cause 

 

Thanks in advance 

[cliffcoggin]

Michael.

Can you reverse your tester so that it applies negative pressure instead of positive? If that pulls bubbles of air from the cooling system into the reservoir it shows it to be air locked.

[Michael-B8]

5 minutes ago, cliffcoggin said:

Michael.

Can you reverse your tester so that it applies negative pressure instead of positive? If that pulls bubbles of air from the cooling system into the reservoir it shows it to be air locked.

I wouldn't be able to do it with that tool I don't think , is there a different way to apply negative pressure ? I've never tried doing that before so not sure how to go about it , I've bled it on the core outlet where the hose has a small hole to release air but no air was coming out , there is one other thing I thought was odd ,in the expansion tank where the return pipes go should there be constant flow like on BMW where it's always got a stream? Because I've noticed mine only has a small amount of flow