COLD AIR WHEN IDLING COULD BE...
1) Low on coolant - the most common reason for this heater problem. Make sure your coolant level is full.
If the level is low, it could be because the radiator cap is not sealing well or it could also be a cracked overflow container (hot coolant that bleeds out of the engine and drops to the street so you never see the puddle on your driveway).
2) An air-lock in the coolant system - it needs to be burped. Park your car facing-up the steepest hill you can find. Remove the radiator cap a (be careful - best to do when engine is cold) and rev engine to 2000 RPM for 60 seconds - hopefully the air will burp out the cap. Top it off with 50/50 antifreeze.
3) A disconnected or cracked vacuum line. I've heard about this one but never investigated.
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This happened to my Passat, and little by little the air kept getting colder and colder until there was no heat left....I just took it in to get looked at- The heating core is gone- the part itself not that expensive, but since they need to take apart the entire dash to replace, the total job is like $800, $700 of that is labor!
There are known problems with this vehicle's heater core not getting enough coolant flowing through it to blow hot air.
1) Even if the coolant looks good, there are problems with the heater
core plugging up. One solution is to use a heavy duty cleaner (CLR pipe
cleaner) to back flush the heater core. To know if the heater core is
plugged up, disconnect the 2 heater hoses at the heater core and
somehow connect a hose to one end of the core. Try to flow water
through, it doesn't have to be high pressure, and there should be a
steady stream. If not, try to get the cleaner directly into the heater
core and let it soak. Then back flush it and a lot of scale should be
seen coming out.
2) The other problem is that since the heater core is the highest point
in the cooling system air tends to get trapped in it. This should only
happen if the coolant goes low or someone has opened the system to
service it. There are a number of ways to bleed the system (vacuum
bleed, pressure bleed). One of the hoses should have a small hole in it
for bleeding air, pull that hose back just enough so the hole allows
coolant to flow out of it. Then accelerate the engine and get a good
flow coming out, when no more air bubbles are present, there should be
3)I've heard of bad water pumps with low flow causing issues as well as
an aux. water pump not fuctioning correctly leading to poor heat output
from the heater core. Someone should confirm this as I cannot get much
information on the aux water pump(electronic pump by the alternator
that is supposed to run after the car is shut off).
Ok, have your coolind system backflushed. Your heater core could be clogged. This could have been caused in using the wrong antifreeze system or mixing the green with the vw g-12 antifreeze. This could ran you about $110 to 150 dollars at a radiator shop...stay away from the Jidffy lube or oil doctor's . This could be your cheaper alternative to solution to your problem..good luck..