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Pelican Technical Article:
VW / Audi Heater Core Flush

Peter Bodensteiner

 

 
Time: 1 hour
Tab: $15
Talent:  
Tools:
Locking pliers, 6 feet of 3/4-inch hose, drain pan, garden hose with nozzle, Phillips screwdriver, towels, coolant
Applicable Models:
Audi A4 (1997-01)
Audi TT (2000-04)
VW Passat (1996-00)
VW Golf (2000-02)
VW Jetta (2000-02)
VW Beetle (1999-02)
Parts Required:
None
Hot Tip:
Do this project first if you suspect you need a new heater core, as it’s a lot easier than replacing the heater core.
Performance Gain:
Improves effectiveness of heater, reduces restrictions to coolant flow
Complementary Modification:
Change engine coolant, replace cabin air filter, replace wiper motor.
 
   

   

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     Over time, sediment and corrosion can decrease the effectiveness of the heater core in your Audi. The heater core is essentially a small radiator, and like the car’s primary radiator it is part of the car’s cooling system. Instead of dispersing heat into the air, it provides heat for the car’s climate control system. And, instead of being mounted out in front of the car like a radiator, it is typically buried somewhere beneath the cowl and dashboard, hidden behind various components.

     If you’ve ever heard of someone having to replace a heater core in a car, you’ve likely heard a horror story; it’s usually an extensive and frustrating job to gain access to it. This provides all the reason you need to perform the following project. If you heater core seems to be clogged up and in need of replacement, this procedure gives you an easy way to attempt to save it before you have to tear apart the car’s dashboard. More proactively, if you flush your heater core from time to time, it’s much less likely to become clogged and thus you’re less likely to ever have to perform (or pay for) a heater core replacement.

     Flushing the heater core isn’t completely unlike flushing your cooling system during a coolant change, except that it focuses exclusively on the heater core itself. You can use regular water from a garden hose, with the caveat that you shouldn’t subject the core to a great amount of water pressure, as this can damage it. A light flow is all you need.

     The heater core has two ports on the firewall—one in and one out. You can flow water through in one direction and then switch hoses to reverse the flow to flush out as much old coolant and debris as possible.

     Before you get started you should remove the plastic cowl cover from the engine bay to gain access to the two heater core hoses ports just to the side of the battery. I used a six-foot length of 3/4-inch heater hose, purchased at a local auto parts store. I sliced off a small length of hose to connect to my garden hose. The other end I ran down through the engine bay to a drain pan on the ground.

     If your cooling system is already filled with a 50/50 antifreeze to water mixture, keep in mind that flushing the heater core will replace that mixture with water, so your mixture will now contain more water than antifreeze. For that reason, if you are doing this procedure at the same time as an overall coolant change, you can flush the heater core after you drain the rest of the system, and before you fill it with new coolant.
Because I already had the front bumper off of the car, I decided to drain a little bit of coolant out of the system in order to minimize the spillage of coolant cause by removing the heater core hoses.
Figure 1
Because I already had the front bumper off of the car, I decided to drain a little bit of coolant out of the system in order to minimize the spillage of coolant cause by removing the heater core hoses. I’m not sure if it made much of a difference in this regard, so you can consider this step optional. I used a random hose to direct the coolant from the radiator petcock to the drain pan. For more information on this, see the Coolant Replacement article.
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Moving to the firewall, find the two heater core ports.
Figure 2
Moving to the firewall, find the two heater core ports. As seen in the previous Coolant Replacement article, these may be concealed by a plastic, accordion-textured cover that must be removed first. Use some locking pliers to squeeze the hose clamps and move them down the hose.
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Once you have removed the heater core hose, replace it with the length of hose that leads to your drain pan.
Figure 3
Once you have removed the heater core hose, replace it with the length of hose that leads to your drain pan. Note the blue towel I’ve placed below the hoses to absorb any fluid leaks.
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Here I have attached the shorter length of hose and the garden hose nozzle.
Figure 4
Here I have attached the shorter length of hose and the garden hose nozzle. Using a gentle stream of water, which you can regulate with the nozzle trigger, flush the core with water until the water running into your drain pan is clear and free of debris.
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Now, switch the hoses and reverse the flow through the heater core.
Figure 5
Now, switch the hoses and reverse the flow through the heater core. Again, use gentle water pressure; the last thing you want is for the water pressure to be high enough to damage the delicate heater core. Note the red towel, positioned to keep water off of the intake manifold and other underhood components that happen to be beneath the nozzle.
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Reattach the heater core hoses, but leave the left (passenger side) hose off slightly so that the hole on top is exposed to allow air to escape as the system is filled up.
Figure 6
Reattach the heater core hoses, but leave the left (passenger side) hose off slightly so that the hole on top is exposed to allow air to escape as the system is filled up. You can test whether the hole is open by passing a small screwdriver through it.
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Remove the three Phillips screws holding the coolant reservoir in place, raise it up, and place a towel or another object underneath it to elevate it about 4 inches.
Figure 7
Remove the three Phillips screws holding the coolant reservoir in place, raise it up, and place a towel or another object underneath it to elevate it about 4 inches. This ensures that the coolant reservoir is the high point in the system (along with the hole in the heater core hose) so air can escape as you add coolant to the system. Run the engine at 1500-2000 RPM while bleeding to help evacuate any air in the system. The heater core will be full of water, so very little additional coolant needed to be added in my case; your situation may vary. Again, for more information on this procedure, see the previous Coolant Replacement article.
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Comments and Suggestions:
Jake Comments: So I followed the heater hose and found nothing! So when the car was at normal temp and running I pulled the top heater core hose and next to nothing is coming out so that points me more to the waterpump cuz it's not flowing fast or strong enough, am I wrong?
November 23, 2014
  Followup from the Pelican Staff: You would be correct. No flow would be caused by the water pump. That is if there is no aux pump. Which it sounds like your vehicle does not have one. - Nick at Pelican Parts  
Jake Comments: Hello I have a 2004 Audi A4 1.8t just recently no heat was going through vents so I flushed the heater core and back flushed it, there was no debris and it flowed through nice both ways, put the hoses back on started her up let it run for 20 mins then bleed the antifreez! Still no heat so I touched the hoses and the IN hose was hot but the OUT was cold, so I guess my question is, is there a heater control valve or something it only blows hot air when I'm rackin up the rpms. Any answers
November 23, 2014
  Followup from the Pelican Staff: I think there is an auxiliary coolant pump. An electric one, small for the heater. Follow the heater hoses. Look for a small pump. - Nick at Pelican Parts  
jared Comments: What size of hose did you use to run water from the heater core to the drain pan and to run water into the heater core?
November 17, 2014
  Followup from the Pelican Staff: Garden hose size is fine. Just don;t let the pressure get over 4 psi. - Nick at Pelican Parts  
Ivo Comments: Hi,
I have a Passat 2000 May model, 1,9TDI ATJ engine. It warms up til normal temperatur90degree, but when I turn on heating the engine temp. drops down by 15-20 degree even when idling. Net thermostat and temp.sensor are bveen installed 6 months / 8000km ago.
Thank you in advance!
October 23, 2014
  Followup from the Pelican Staff: I would guess that the new thermostat is defective. - Nick at Pelican Parts  
zachriddle Comments: i have steam coming out of the same location on my 2000 A4. it happens right when the engine warms up to operating temperature. What do you think the probable cause could be? Water pump?
September 29, 2014
  Followup from the Pelican Staff: Is the engine overheating? Check the thermostat, then I would pressure test the cooling system. If the system will not hold pressure and there are no external leaks the head gasket may be faulty. - Nick at Pelican Parts  
Chris Comments: Looks nice, I am planning to undertake this project today. However, I have one question. I quote you: "Run the engine at 1500-2000 RPM while bleeding to help evacuate any air in the system". I did not find similar reference in the coolant replacement article, does this mean that heater core hose bleeding hole is exposed and reservoir is capped? Should I stop the engine as the coolant starts running out of the hole in the hose or won't it start running out as soon as I start the engine because water pump is circulating both air and fluid?
September 24, 2014
  Followup from the Pelican Staff: I would suggest following the bleeding procedure in the draining and filling article. If you have trouble, raise the RPM to heat the engine, sometimes this helps to open the thermostat. - Nick at Pelican Parts  
Herb Comments: What the best way to install a new heater core hose mine came loose when I was putting the new heater core in from the engine
1998 Audi a4
August 28, 2014
  Followup from the Pelican Staff: Unclip the plastic hose from the engine connection and loosen the clamp for the heater core side of the hose. Remove and replace it. - Nick at Pelican Parts  
Jim Comments: If you were worried about to much water pressure could you put the hose from the input side on the outlet side and use the engine water pressure to reverse flush the core?
July 30, 2014
  Followup from the Pelican Staff: I suppose that would work, if the hoses will reach. - Nick at Pelican Parts  
Bo Reed Comments: What about using CLR? How long would you lea bit in the core and would you delute it at all?
January 24, 2014
  Followup from the Pelican Staff: Personally, I would only flush with a cleaner designed for cooling systems. - Nick at Pelican Parts  
bill Comments: The pictures really helped me understand the written instruction. Thanks for taking the time to post these instructions. Very helpful
January 24, 2014
  Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts
 
Raina Comments: Would this work for an 1996 Audi a4 quatro ?
January 20, 2014
  Followup from the Pelican Staff: This article applies to:

Audi A4 (1997-01)
Audi TT (2000-04)
VW Passat (1996-00)
VW Golf (2000-02)
VW Jetta (2000-02)
VW Beetle (1999-02) - Nick at Pelican Parts
 
Pinny Comments: ok so what is your take on a 2000 vw passat heat core that was just replaced two years ago and just last fall was flushed out as noted above by dealership as no heat was being produced - and now they are saying the heat core needs to be replaced.....again
January 10, 2014
  Followup from the Pelican Staff: I would that debris has not entered it. Maybe from the water pump or other faulty cooling system components. - Nick at Pelican Parts  
hipnopputnous Comments: Very nice worked like a charm!
January 7, 2014
  Followup from the Pelican Staff: Thanks for the feedback. Glad we could help. - Nick at Pelican Parts  
CJ Comments: Thanks so much for the detailed instructions and the great pic's. Our 2003 Passat 1.8T LITERALLY had NO HEAT AT ALL! It's nice and toasty now and since I'm NOT a mechanic, I almost don't believe it!!! Thanks again.
January 4, 2014
  Followup from the Pelican Staff: Thanks for the feedback. Glad we could help. - Nick at Pelican Parts  
Frank Comments: Excellent instructions. I've been searching for instructions like these for two months. Our 2001 Passat had absolutely no heat. Now it's nice and warm just before winter. Thank you!!!
December 23, 2013
  Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts
 
josh Comments: can flush the heater core without doing a full coolent drain?
December 11, 2013
  Followup from the Pelican Staff: Yes, remove the hoses and run water through it. - Nick at Pelican Parts  
zippo Comments: Kudos. Too bad the factory does not hire people like you to put together maintenance instructions. Professional!
December 4, 2013
  Followup from the Pelican Staff: Thanks for the feedback. - Nick at Pelican Parts  
Mir11 Comments: Many Thanks for this complete step-by-step with pics.
Q: Step6 - If the hose hole squirts coolant, does that indicate that there's likely no air to be purged any more?
December 4, 2013
  Followup from the Pelican Staff: That's right it's not likely to have any more air, but sometimes more air shows up anyway, after some more driving.- Nick at Pelican Parts  
c180tom Comments: Great set of instructions! I used clear polyvinyl tubing instead of heater hose. I suspect the low-phosphate coolant is prone to plugging the heater core, especially with any air in the system.
Would you suggest an aluminum-compatible radiator cleaner over plain water?
November 22, 2013
  Followup from the Pelican Staff: I actually believe it is the impurities in the water that causes the corrosions that clogs the heater core. I've had some sucess with comercial grade plumbing cleaners unclogging the heater cores but you have to be careful to only leave it in a few hours or it will create a leak in the heater core. - Kerry at Pelican Parts  
C13 Comments: Awesome advise on raising the coolant reservoir!!! It made a huge difference, and I'm convinced that part of my heat problem was air in the system. Thanks.
November 16, 2013
  Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts
 

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  Applies to: 2000 Audi TT, 2001 Audi TT, 2002 Audi TT, 2003 Audi TT, 2004 Audi TT, 2001 A4 Quattro, 1996 Passat, 1997 Passat, 1998 A4 Quattro, 2002 Beetle, 1998 Passat, 1999 Passat, 2002 A4 Quattro, 2000 Passat, 2000 Golf, 1999 A4 Quattro, 2001 Golf, 2002 Golf, 2000 Jetta, 2001 Jetta, 1997 A4 Quattro, 2002 Jetta, 2000 A4 Quattro, 1999 Beetle, 2000 Beetle, 2001 Beetle
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