Why does my car not give me heat at idle?
November 24, 2014 9:40 AM   Subscribe

My 2003 VW GTI VR6 warms up fine and gives heat while driving, but at idle it immediately blows cold air.

I bought this car at 25k miles around 2005; it's now at 120k miles. It runs and drives great. On cold days (like below freezing), it will warm up fine, in just a few minutes (1-2 miles), and it gives me nice heat while driving. My "commute" is 3.5 miles, 7 minutes, and it gets completely warm in that time.

The problem is that anytime I stop and the car idles, the heat goes away. The air coming out the vents goes cold almost immediately. The temperature gauge stays right in the middle like always. The car idles around 750 rpm according to the tach. If I give it a tiny bit of gas and bring the idle up a bit to 1200 rpm, the air becomes hot again. I can cycle the warm air on and off at a stoplight by doing this. The response is so fast, the engine is obviously not cooling off and warming back up. This is only mildly irritating while driving, but I miss the ability to start it and let it warm up inside to melt ice, etc., because even though the engine gets warm at idle, it doesn't blow any warm air.

Background information: The car is on its third water pump, due to them leaking but not outright failure; it has never overheated as far as I know. I've replaced the thermostat once myself (it is very hard to get to and there's no coolant drain, so this task really sucks), probably around 2009, and I don't remember if I had the dealer do it with the most recent water pump (around 2012). The only access to the coolant is at a clear reservior maybe 8" in diameter, and this shows the level is normal. The coolant looks and smells normal. You can't look in the radiator or access coolant anywhere else.

I have driven the car almost 10 years now, and interestingly it has done this some winters but not others. I wish I had kept a careful log of that and the repairs, but I haven't.

End of data, my speculation starts here: It seems like circulation through the heater core stops at idle, but starts again at slightly over idle.

What should I try, to fix this? Thank you, folks.
posted by fritley to Travel & Transportation (11 answers total)
Best answer: So clearly water circulation is an issue in volume. First thought are it's either

1: a weak water pump (despite the replacements) and the restriction required to push it through the heater core at idle is too much for it

2: A constricted core and a normal producing water pump can't get past the restriction (same as above, but opposite cause)

3: There is air in the system and it is only overcome by sufficient water volume.

I'd go through the factory recommended bleed method or just run it with the cap off until warm and try and find the highest point and try and bleed air out of it that way. If the system is 'full' but doesn't have enough water in it still, then chances are it's an air lock.
posted by Brockles at 10:08 AM on November 24, 2014 [1 favorite]

There may be a coolant loss into the cylinders that is slight over time and that you don't notice, by the way. So the coolant doesn't fall out of it in a noticeable way, but burns it off, and air is introduced into the system in some way which reduces the total water volume in the system. A teeny leak over time would be hard to notice and produce a small air lock that would cause issues with teh heater matrix.
posted by Brockles at 10:10 AM on November 24, 2014 [1 favorite]

Do you have air recirculation enabled?
posted by I-baLL at 10:48 AM on November 24, 2014

seconding brockles #3. I had the same issue in an 07 impala. There was an air bubble in the system, and purging it fixed the problem.
posted by cosmicbandito at 11:27 AM on November 24, 2014

Response by poster: Thanks, I forgot to say this is the BDF engine code, and I have a service manual and VCDS software+interface. The manual I have does not show a bleeding procedure, and some web searching hints it's supposed to be self-bleeding (highest point is the reservoir) -- not that that means the web is right, or that it's bleeding properly. I have never noticed any gurgling, for what that's worth.

I am about 90% sure it has done this on more than one water pump. It has happened off and on for years.

I will try running it with the cap off, and squeeze hoses and such, in the hopes I can get a burp out of it. A very slow coolant loss and failure to self-bleed seems quite plausible and would explain why it comes and goes, and why I haven't identified a clear pattern.

Thanks, Brockles, for sharing your expertise like always.
posted by fritley at 11:35 AM on November 24, 2014

VR6 VWs have two water pumps. One is electric. I think they might even serve partially separate loops.

The electric one is a Booster pump that sometimes runs after the car shuts off too. Maybe it doesn't always get enough current at idle? Weak battery? Electrical fault/issue somewhere?

I definitely think it's worth investigating that though. I almost bought a jetta vr6 a while back which was cheap because it was snafud with water pump issues the owner had gotten tired of, relating to that odd dual pump system.
posted by emptythought at 12:12 PM on November 24, 2014

I've done some poking around and trying to wade through the utter drivel on the various forums and I have found several people that have had internally clogged heater matrixes through coolant flushing/changing procedures being missed out on. So it's looking possible it is your heater core getting clogged up and just not being able to flow enough at idle speeds to make an impact on heating the matrix (the air cools the matrix faster than the reduced water flow can heat it).

Another possibility is if the heat distribution in that car is a vacuum system, in which case a vacuum hose may be leaking and the 'selector' for heat may be drifting back to cold until more vacuum is applied. I think it is a vacuum system on that car from some checking and I'd be inclined to look at and or replace all the vacuum hoses between the heater control valve and the manifold, mainly because it's likely to be pennies compared to a heater core so it's a good option to save some coin.... I don't know if the heater control valve defaults to hot or cold, though, which may be a way of discounting that possibility. If it defaults/springs to hot then it isn't a lack of vacuum.
posted by Brockles at 2:33 PM on November 24, 2014

VR6 VWs have two water pumps. One is electric. I think they might even serve partially separate loops.

My understanding is the second water pump is to circulate water when the engine is not running because the VR6 is prone to hot spots. So it maintains even cooling through the block and head to prevent warping. I don't think it does anything while the car is running, but it's been a LONG time since I looked at a VR6.
posted by Brockles at 2:37 PM on November 24, 2014

Response by poster: Thanks again - the manual says the filling procedure is to just keep filling the reservoir when it empties. There is no bleeding procedure I can find.

I put in a little extra coolant (it was OK, but under the "high" mark even though it was fully warmed up) and ran it with the lid off for quite a while. The fans never did turn on though, so it never got very hot - it's 35F here.

After running a long time, I got in and it was still blowing cold. I revved up to 2000, it started blowing hot, and then after that it still stayed hot at idle. I may have to wait until it gets cold again for a good test.

Sorry to hear you were wading through forums; I sure appreciate you. Mine (non-"Climatronic") has mechanical cable-operated flaps, thank goodness. The "Climatronic" are electric servos according to the service manual.

I hear the secondary circulating pump running after shutdown after A/C use in the summer. I don't think I hear it in the winter. The manual calls it "After-run coolant pump" so I don't think that can be part of this problem. But thank you, emptythought!

I have never had the coolant flushed (ahem) but I suppose it has been replaced with each new water pump.
posted by fritley at 4:13 PM on November 24, 2014

I experienced a similar problem with an older Corolla. It turned out that my heater had a blower fan that had died. When driving, the heat from the engine would blow through the vents due to more air flowing in from the front of the car, but when stopped, the heat stayed in the engine.

I got a new blower fan and it fixed the problem.
posted by tacodave at 4:38 PM on November 24, 2014

Response by poster: The weather's cold again, and I think it might be fixed - I had good strong heat at idle today. For posterity, all I did was fill the reservoir quite full (above the full line), then I ran it with the cap off until fully warm and circulating, then I put the cap back on.

Thanks again, Brockles.
posted by fritley at 12:07 PM on November 30, 2014

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