No heat at idle and engine vibratio at low RPM

[windyctyaudi]

I recently got by car back from having the thermostat replaced after it failed. Now after my car has been warmed up and I'm at a red light stopped or in the parking lot at idle my heat stops blowing hot air. If I rev the engine the air will get hot again only while my foot is on the gas. It has been pretty cold in the mornings when I go to work between -5 and 0 but I don't remember having this problem in the past. I have also noticed when my RPMs start to fall below about 1500 the engine is making a rumble/vibration noise until it settles at idle. I did notice on my paperwork from the dealer stated I received an update for my ECU when they replaced my thermostat.

[cW6mT mKviGLi]

I'm thinking maybe the system wasn't bled properly. There's probably an air pocket and when you rev the water pump pressure builds creating more positive pressure through the heat exchanger.

Also, i don't see how the thermostat would affect the idle of the motor unless it was stuck full open and the car revs higher than normal because it's having issues warming up. About how many miles have you put on the car since the ECU update? Maybe it needs more time to fully adapt or it may just be something with the new file having a revised OEM tune.

[MrFunk]

Yeah - they need to bleed it again.
I had sort of the same issue with my last A4. There was actually ice blockage...

[windyctyaudi]

about 150 miles on it since update.

[DGVR6]

Do you have the "AC" on or not?

[windyctyaudi]

Lol no the ac is not on.

[DGVR6]

Turn it on. Check if it makes a difference.

[schirm]

Lol no the ac is not on.

I always run the heat with my "AC" on. No specific reason just never turn it off.

[windyctyaudi]

the AC always comes on when you turn on the front defroster but I always turn it off. I don't know why it does but I'm sure there is a reason

[DGVR6]

Lol just a temporary fix for the nuisance.
It will blow hot on idle.

Everyones comment is pretty much on point, air in the system.
Worst case; weak water pump.

How do I know? I just fixed it lol.

[anerbe]

From what I recall, AC always comes on to avoid fogging and frosting up your windows in the winter cold with excess humidity in the interior. AC will reduce the humidity in the car, whether settings are on cold or warm.

If you have the AC on Auto, the standard door setting is to the defrost vents. Turning off AC will soon fog up the windows.

If you've ever tried recirc in the winter without AC, the car will fog up real fast.

[Dr GP]

I always run the heat with my "AC" on. No specific reason just never turn it off.

Many do, but IMHO, if you do not need A/C you do not need to have the compressor running. Lowers gas mileage. I only hav A/C on when I want A/C

[mcoomer]

The AC does indeed work to remove humidity from the cabin to prevent fogging of the windows. It's not always necessary to prevent this but I typically leave it on. As far as your original complaint, did you have the car serviced at the dealership or DIY? If someone else did the work I'd be squawking at them to fix their mistake.

Mike

[schirm]

Many do, but IMHO, if you do not need A/C you do not need to have the compressor running. Lowers gas mileage. I only hav A/C on when I want A/C

Good info thanks

[windyctyaudi]

The AC does indeed work to remove humidity from the cabin to prevent fogging of the windows. It's not always necessary to prevent this but I typically leave it on. As far as your original complaint, did you have the car serviced at the dealership or DIY? If someone else did the work I'd be squawking at them to fix their mistake.

Mike

Dealer worked on it under warranty

[DGVR6]

Dealer worked on it under warranty

Get them to bleed the system again.

[S4'ed]

Many do, but IMHO, if you do not need A/C you do not need to have the compressor running. Lowers gas mileage. I only hav A/C on when I want A/C

Keeping it off when you don't really need it also prevents wearing-out the AC compressor. The stupid Auto climate control defaults it to ON, even when you want to heat the air.

[Crucible35]

On my A4 I have to keep the A/C on all the time.....if I turn it off in the winter (or anytime really) my windows will fog up to a point that it's dangerous.

[S4'ed]

On my A4 I have to keep the A/C on all the time.....if I turn it off in the winter (or anytime really) my windows will fog up to a point that it's dangerous.

The windows fog-up because you have been running the AC in the full Auto mode. If you always disable the AC in the winter, the system will soon dry out the accumulated moisture in the system, and then the windows will stop fogging up. Try it for a few days...then you won't need the AC to defog all the time.

[levigarrett]

the AC always comes on when you turn on the front defroster but I always turn it off. I don't know why it does but I'm sure there is a reason

AC comes on to reduce humidity and prevent fogging. Seriously, the AC used for defrost is exactly how the system should work. Don't turn it off, you'll have better results.

[anerbe]

The windows fog-up because you have been running the AC in the full Auto mode. If you always disable the AC in the winter, the system will soon dry out the accumulated moisture in the system, and then the windows will stop fogging up. Try it for a few days...then you won't need the AC to defog all the time.

Interesting. What happens with all the humidity that we create by breathing?

[S4'ed]

Interesting. What happens with all the humidity that we create by breathing?

All cars have "flow-thru" ventilation : the air is pulled from outside, thru the heater/AC cores, then into the passenger compartment, and then it exits thru the rear deck, to the outside thru vents to the outside. Any moisture is purged. The recurc button closes off the outside air intake, and only recycles cabin inside air. If you keep recurc ON, you will get fogged windows if the AC is off.

I always turn the AC off, after selecting the AUTO mode, and have never had window fogging issues in the winter.

[mcoomer]

Yeah, make sure that you don't run your AC for extended lengths of time in recirc. Moisture will accumulate in the vents and if you don't run fresh air through it and can eventually grow mold and mildew. Ever get into a car that smells like swamp ass when they turn on the air? Smells terrible and it's difficult to clear up.

[anerbe]

All cars have "flow-thru" ventilation : the air is pulled from outside, thru the heater/AC cores, then into the passenger compartment, and then it exits thru the rear deck, to the outside thru vents to the outside. Any moisture is purged. The recurc button closes off the outside air intake, and only recycles cabin inside air. If you keep recurc ON, you will get fogged windows if the AC is off.

I always turn the AC off, after selecting the AUTO mode, and have never had window fogging issues in the winter.

For the most part, I agree. However, the system has an option of Auto Recirc, which it may turn on while your AC is manually set to off. I don't know if the system detects when the car goes in recirc and kicks back on AC. You get a faster heat-up of the interior if you keep the system in recirc, and the car will fog w/o AC as you stated.

Also it's good to run the AC at least for a bit during the winter to keep the oil flowing in the system.

The system is designed to minimize fog-up/reduce humidity in all conditions (remove user error regarding settings). It's not smart enough to determine when to turn it on or off. If you have below freezing/no outdoor humidity, I agree the fresh setting will eventually rid the cabin of moisture with AC off (AC can't remove humidity from fresh incoming air if it doesn't have any humidity).

Personally, I turn it off after warm up in very cold temps. However, I don't immediately turn it off when starting from cold, as I do recirc as well. I leave it on when I need a heater and it's above freezing outside and higher humidity levels, as I hate the humid feel.