tylerhager

sorry for the slow response, the air which initially is hot on full blast goes cold at idle/stopping stays the same. Although, when the ac is on high is blows much harder then the heat is on high.

turbo510

So at low engine speeds, the heater core doesn't have enough flow, but does when the engine is running faster. The core could be partially clogged, or the water pump impeller could be defective. A clogged core is the usual culprit.

tylerhager

the waterpump was just replaced with the tbelt service. Whats the best way to flush the heater core? any possible chemicals to use?

mikefmcnally

I have the very same issue. I have a 2002 allroad 2.7. Had the timing belt and water pump/thermostat replaced. Before and after the job the car had this issue. When I drive the car the heat is hot. The second I stop at a red light the air remains blowing at the same force, but instantly goes cold.

As soon as I get the green light and romp on it, the heat is back. This is frustrating as hell. The overflow reservoir was replaced and the entire system checked for leaks and all checks out. The temp of the car remains mid range and the car never gets too hot. I can understand it being a heater core issue if the car gets heat. Usually it is blocked or it is not.
Also, there is no smell in the car or film on the windshield as if the heater core was bad. I think it was a poor design and position of the heater core and maybe there is an air pocket that causes the fluid to not get through the core when idling. Maybe the waterpump is not spinning quick enough at idle but when the rpms are above 2k it is pushing hard enough to get through the core. I am going to bring it in to get looked as soon.

tylerhager

Our problems are almost identical and the only thing i can think of for myself is a leak somewhere beings a throw a P0171 code, or a air pocket in the system that i cannot bleed out

weaselciuy

FLUSH the core, eventually replace coolang with new one 40%-60% water.
Dismantle parts that obstruct accest to most of the engine bay, check with a flashlight for broken hoses, vacuums.
Replace the thermostat and the water temp sensor.

See if the fan on the radiator stops if its blocked at start of engine, cold engine. Maybe its gone berserk cooling at max from start.

What temp does it display on the gauge inside the care? 90C?

Anyway, seems like a cloged ac radiator or bad thermostat. I had a bad thermostat. HEat was weak, and low blowing power.

Now i can take the winter HEAD ON !!!

turbo510

You've got to figure out if the heater core is clogged or not. Disconnect both heater hoses at the firewall fitting, then fit a hose on either fitting that's clean enough to blow into. You should easily be able with lung power to blow out coolant from the other fitting. If not, your problem is a partially-clogged core.

diesel dave

Have 01 Audi A4 1.8T that had same problem. First, even with new water pump there was no flow to heater core at idle. Second, heat core was flushed by a shop and there was flow but still seemed to be poor. The fix was I made a stand alone flushing unit wth small pump and 20L pail as holding tank. Used product called Kwik-Way Thoro Flush Radiator and Engine cleaner by Irontite and pumped it through the heater core for the recommended ten minutes as this stuff is very strong. See product on web for details. Read and follow product instructions completely. A lot of stuff came out of core. I installed a V6 VW coolant afterrun pump in the inlet heater core hose where there is a 90 degree bend and controlled it through a ground producung oil pressure sender and a relay. So it is a stand-alone system and only operates when the engine is running giving a positive coolant flow at all times. System now produces tons of heat at idle and beyond.

turbo510

Interesting solution, but did you try the heater after cleaning the core, but before adding your electric coolant pump? Even at idle the mechanical coolant pump should move hot coolant through the heater core.

diesel dave

After the new engine driven pump was installed the flow at idle had not improved and that's when I installed the 12V electric boast pump. With the electric pump the flow inproved and heat was marginaly better but still pathetic. Then I flushed the core as last ditch effort before installing new one as by this point I realized it must be at least a partially blocked core. The additional flow from 12V pump and a clean core solved the problem. From all the research I have done on the subject there are a lot of newer vehicles that have this low flow problem to heater not just VW and Audi. Most of the problems come from the heater core being the highest component in the system, trapped air in heater core and the fluid turning into a jello like substance in the presence of air which the low flow won't push out of the core thus causing blockage. By the way the original engine driven water pump was in good condition but installed new steel impellor version just to be safe.