Car's heater doesnt work when sitting still

[Aharami]

Recently, I got a bunch of work done on my car by a local mechanic (nice guy - family friend of a good friend of mine. So Im inclined to believe that he didnt rip me off). Prior to this, my car, a VW GTI VR6 (w/ 133k miles on it) was overheating when standing in traffic. Turned out that the radiator fans werent working. So here is a list of stuff I had done by the mechanic:

-replace both radiator fans
-replace fan control module (did this one myself)
-replace thermostat
-flush and fill coolant
-replace both timing chains + guides + tensioner
-replace serpentine belt
-replace clutch with stock sachs clutch
-replace flywheel with lighter flywheel (got clutch+flywheel combo a while ago - so had those laying around)

all parts I purchased myself from various online vendors
ever since work was done on my car (Oct '07) I noticed a 1-2 mpg drop. But I shrugged that off to winter blend gas.
Recently I've been noticing that the heat doesnt work when the car is stopped. I feel air coming out of the vents, just not hot air - so fans are working. But if I rev the engine and hold the revs for a second or two, hot air comes out.

Also it seems that the initial heat up phase from when the car is turned on to when i finally feel warm/hot air coming out of the vents has increased significantly. It used to be such that I'd feel hot air coming out as soon as the engine reached normal temps. But now it takes much longer than that. Since I'm having all these problems with the heat, I thought I'd try and see if the AC works. AC isnt working either.

What could be some possible reasons why it is acting up like this?

 

[cprince]

check the engine coolant. There might not be enough coolant circulating when the engine is idle. Or the water pump is bad. You might have a leak in your AC system.

 

[Aharami]

coolant levels are normal. water pump is a possibility, but I hope it's not cuz that's a pain to replace on these cars

 

[Aharami]

how does the heating system work? I initially thought that my radiator fans arent working again since heat works when the car is moving. But the fact that heat comes out when engine is revved at idle threw that reasoning out of the window.

 

[Zenmervolt]

Check the heater control valves. These may be vacuum operated and if you have a slight vacuum leak in the lines going to the control valves, or a defective valve itself, you can have exactly the issue you describe because the system may not be letting coolant flow through the heater core.

ZV

 

[cprince]

In most cars, the heating system works by circulating engine coolant into a hollow coil called the heater core located under the dashboard. The heat of the engine is used to warm the cabin. There is a valve that controls whether or not engine coolant is allowed into the heater core based upon user's setting. This valve could be bad. Another scenario is that the thermostat is bad or installed incorrectly; thus, allowing engine coolant to flow into the radiator and cool down too much. Check your temperature gauge.

 

[Aharami]

thanks for the tips guys. I'm not really car savvy enough to diagnose this by myself. But all your pointers will help when I call up the mechanic again

 

[Mxylplyx]

If the car is warmed to normal operating temp, and you still get no heat, I would lean towards a bad heater control valve, or a clogged heater core. Since you said it takes alot longer for the engine to heat up, it sounds like your thermostat might be stuck open. When you rev your engine, do you see your temp gauge rise to normal, and then drop back down shortly after letting off the gas? Another thing to consider, if you are up north and it's blistering cold, a tune up on your cooling system might have made it so your car wont get up to temp. Many people have to block airflow through their radiator to allow their cars to heat up to normal temp. A faulty cooling system prior to the work you had done might have made this unnecessary.

Just noticed you said this only happens when the car is stopped. Thats odd, as I would figure it would be the other way around. Water pump or thermostat is the only thing I could think of.

 

[Aharami]

my temp gauge is always at "normal" (well except right after a cold-start). But from what I gather, VW's temp gauge is very inaccurate. It will only start going up from the "normal" position when the car is about to overheat.

On a related note, I was stuck in rush hour traffic for quite a while today and noticed my temp gauge creeping up. Went back down to normal as soon as I started moving again. My cooling system still has problems

 

[T2urtle]

Coolant temp sensor!!!! one of the cheap $20 fixes.

also air pockets seems to me the more likely issues.

also was everything OEM or autozone stuff.

i got a coolant sensor and T-stat from autozone on parent's camry. It runs a little hotter and the coolant gauge goes a little higher compared to OEM

 

[LordMorpheus]

Originally posted by: Zenmervolt
Check the heater control valves. These may be vacuum operated and if you have a slight vacuum leak in the lines going to the control valves, or a defective valve itself, you can have exactly the issue you describe because the system may not be letting coolant flow through the heater core.

ZV

possible, but the engine vacuum is at the highest at idle. Any throttle at all drops the vacuu, to very little. If the case was "heater ONLY works when sitting still" I'd agree.

 

[DougK62]

Sounds like classic thermostat problem.

 

[Zenmervolt]

Originally posted by: LordMorpheus

Originally posted by: Zenmervolt
Check the heater control valves. These may be vacuum operated and if you have a slight vacuum leak in the lines going to the control valves, or a defective valve itself, you can have exactly the issue you describe because the system may not be letting coolant flow through the heater core.

ZV

possible, but the engine vacuum is at the highest at idle. Any throttle at all drops the vacuu, to very little. If the case was "heater ONLY works when sitting still" I'd agree.

That's assuming that the valves use vacuum to open. Some use vacuum to close. They may also use vacuum in various ways to adjust the position of the internal flapper doors that direct airflow over the heater core.

ZV

 

[Zenmervolt]

Originally posted by: DougK62
Sounds like classic thermostat problem.

No it doesn't. The coolant is reaching operating temperature in a normal amount of time. He's saying that he doesn't get heat until long after the coolant has already warmed up.

I just read that the A/C isn't working either. To the OP: If you can verify that the compressor is cycling, then the fact that the A/C is also not performing in the cabin does strongly point to a problem with the vacuum lines that control the flapper doors in the HVAC system, or with the electronics that control the flapper doors if they are not vacuum operated.

ZV

 

[ViviTheMage]

ask your mechanic that did it for ya?
or vortex...they seem to know a lot

did you swap the temp sensor with the new green one?

 

[marvdmartian]

OP, I don't want you to think that I think you're dumb, or don't know the difference........but did you put in the correct antifreeze??

I only ask because about 5 years ago a buddy of mine did a coolant flush on an old Ford Ranger he had, and had no idea that there was a difference between the orange and the green coolants, and that it was a bad idea to mix them. He filled his coolant system (which was supposed to take the green coolant) with the new orange coolant, and thought he was good to go. He even drove it to and from work for a couple days without a problem.

Then one morning he starts overheating on the way to work, and he's thinking WTF?? He waited for the coolant to cool down, popped the cap on his radiator, and his system is still full (minus a little bit from the boil over). Now he's really confused! He started the engine, and the first thing he noticed is that his coolant is pretty much just sitting there, no movement to speak of through the radiator, so he figures out it's the water pump. He nurses the truck home, grabs his other vehicle, and goes to work, picking up a new water pump on the way home, so he can change it out.

Imagine his surprise when he pulled off the old water pump, and the pump impeller was pretty much GONE. Yep, mixing those two coolants pretty much acted like an acid on his water pump impeller, and ate it up. Lucky for him, the guy at the parts store sold him the green coolant with his new water pump, and advised him to flush the system again.

So like I said, it's not thinking you're dumb, it's a matter of sometimes we do silly things through ignorance......and if your pump impeller is, say, 1/2 gone, it could easily give you the problem you're experiencing.

 

[Aharami]

yea. i used VW's G12 pink coolant. 50/50 mix with water.