What's wrong with my car's A/C?
July 30, 2007 9:33 PM   Subscribe

I have a 2003 Jetta with around 75,000 miles. It is hot here, and I have been running the air conditioning nearly every day for the past month or two, and I have noticed some strange occurrences. First, if I am parked with the A/C running, even for a short amount of time, the car seems to vibrate rather forcefully. In addition, when I pull away from the parking spot, there is always a fairly large pool of water where my car was. Also, when I am driving fast, say 60-75 mph, and I change the speed of the A/C from high to low, there is a noticeable jerk in my acceleration. It is most unusual...

So, worried about the A/C, I decided to take a gander at the owner's manual. The section about the A/C also mentions the "air recirculation" button. The manual says not to use this button when the A/C is running. But I, reckless as I am, decided to see what would happened if I pressed it. Lo and behold, when the recirculation button is depressed, all of my A/C woes go away! The car doesn't vibrate, it only leaves what I would consider to be a normal-sized pool of water, and the acceleration is no longer affected by my changing A/C speed. On top of all this, the A/C is colder and more powerful than I have experienced before! Everything about this air recirculation button seems so right! Why would the VW manual instruct me to let it be? Am I doing damage to my car? Also, do the symptoms I have described above sound serious? What could be happening?
posted by foxinthesnow to Travel & Transportation (17 answers total)
For what it's worth, I've never heard of there being a reason to not use the air recirculate while the AC is on--if it was that big of a deal, don't you think they would have made it impossible to have both on at once?

I always have air recirculate on, and it makes my AC literally twice as effective.

The pool of water is pretty normal, too...most cars I've seen do that. I think it's condensation of some kind.
posted by DMan at 9:38 PM on July 30, 2007

Best answer: Well, the pool of water is normal; that's the condensation produced from warm, moist air hitting the cold side of the A/C coils. (You'll probably notice that it decreases if you have 'recirculate' turned on, since then it's not warm moist air from the outside hitting the cold coils, but already-cool air from inside the car.)

I'm not sure about the vibration when the car is idling. My '05 GTI doesn't have much noticeable vibration when I'm using the A/C and not moving. That, if I were you, would concern me a little, since vibration in moving components means increased wear.

The note in the manual about not using the recirculate button when the A/C is on is interesting. I admit I never really studied them manual for my VW, but I do that all the time. A/C plus recirculate is the same as the "Max AC" setting on most American cars. You get much colder air than regular A/C, because you're cooling air that's coming out of the cabin, not from outside. Why they advise against it, I have no idea (maybe they're afraid of the coils icing?). But I do it all the time, particularly when I'm in heavy traffic, to keep from smelling diesel exhaust. (I've always wondered if, used long enough in a fully-loaded car with 5 people, if you could use up all the O2 in the car ... but I've never heard of this happening. I guess cars just aren't air-tight enough.)
posted by Kadin2048 at 9:45 PM on July 30, 2007

Response by poster: Wow. You have really quelled my fears. Thank you.
posted by foxinthesnow at 9:51 PM on July 30, 2007

Warm air holds more water han cold air, so unless you're in a very dry place, any air conditioner will condense a steady stream of water out of the air. This is normal. Most cars just dribble it on the ground.

[various now-redundant comments deleted on preview]

b1tr0t, pulling the water out is a side effect of the AC, not fundamental how it works. (That is, the AC will still work fine if you give it perfectly dry air.) Swamp coolers work by letting water evaporate into the air, but that's a different technology from heat-pump style air conditioners and refrigerators.
posted by hattifattener at 9:53 PM on July 30, 2007

How forceful is "rather forcefully"? Unless it's really forceful, it's probably normal. In a small car with a small engine, a strong idle in order to maintain the power to run an air conditioner running at full blast certainly might be felt if the car is older and not quite as tightened up as it was new. You could ask a mechanic about it, but practically it would probably be hard to fix and you probably don't need to worry about it.
posted by jcwagner at 9:54 PM on July 30, 2007

Response by poster: jcwagner- Forceful like, um, it feels like a semi-truck driving by. Does that make sense? It just sort of rumbles every few minutes.
posted by foxinthesnow at 9:59 PM on July 30, 2007

I don't know what's wrong with your A/C, but the vibration when you're stationary is the only unusual-sounding thing. It could be your idle setpoint is too low? The extra load of the A/C might be bringing it down to near-stall, causing some rough running. But I'm no car mechanic.

The manual for my 03 Passat says it's best to run AC + Recirc for a few minutes to cool down the car the quickest, then switch from recirc to ventilate mode.

The reason is, VW's have an air filter in the recirc path. Using it unnecessarily means that you have to replace it sooner. Recirc should be used when you're driving through some really dusty (or smelly) area. Otherwise, ventilate should be adequate.
posted by ctmf at 10:00 PM on July 30, 2007

Response by poster: What is an idle setpoint? How would I correct it?
posted by foxinthesnow at 10:04 PM on July 30, 2007

Just to elucidate a bit on the points made above: when idling, your car maintains a constant RPM, so when you put a power draw on it (the AC), it has to burn more fuel per stroke to supply the needed power and will thus be more rumbly. The fact that this rumbling is lessened by turning on recirculation (thus lessening the load because you are starting with cooler air) indicates it is probably just the load on the car that is causing the vibration. I don't know what kind of engine you have, but my diesel Jetta is very rumbly at low RPM with any kind of load.
posted by ssg at 10:07 PM on July 30, 2007

I don't know why your manual thinks A/C + recirculation is a bad idea.

I have a 94 jetta and I'm going to take a guess. Using the a/c and the recirculation button for long periods will make your car smell like an old sock the next time you start it up.
posted by fshgrl at 10:16 PM on July 30, 2007

If it rumbles every few minutes, ctmf is probably right: it is probably coming near to a stall. But IANAM either.

If you do want to adjust your idle setpoint, you'll need to either take the car to a VW dealer or find a mechanic that has some third-party software to communicate with the car. You can buy the software/cable yourself (VAG-COM) but it is spendy.

@ctmf: The cabin filter (at least on A4 Jettas) is in the path of any air distributed into the vehicle, from outside or recirculated, so using recirculation will presumably keep it cleaner.
posted by ssg at 10:18 PM on July 30, 2007

What is an idle setpoint? How would I correct it?
It's the adjustment for how fast the engine runs when it's idling. On my (ancient, pre-computerized engine control) car, it's one of the things you have to tweak periodically as part of regular maintenance. On a modern car, it's probably controlled by the engine computer, and is (what is the phrase?) 'not intended to be adjusted by the end user'.

Most manuals have a troubleshooting section (with entries like "IF YOUR PROBLEM IS: can't see through windshield POSSIBLE CAUSE: windshield is dirty SUGGESTED RESOLUTION: wash your damn windshield"), have you checked whether it has a section on the air conditioner?
posted by hattifattener at 10:33 PM on July 30, 2007

So, your underpowered 4-cylinder engine is idling at, oh, 700 rpms -- nice and smooth.

Then your A/C compressor kicks on, because the pressure in the A/C system is starting to drop. The sudden increase in engine load strains the engine so much that it drops to, oh, 450 rpms -- rough, baby, rough.

Almost immediately, the engine computer notices the drop, and kicks the idle back up. A moment later, you're idling at 700 rpms again -- nice and smooth.

Eventually the A/C system is repressurized, and the compressor disengages. The engine speed temporarily rises a bit, but there's no associated roughness with that, so you don't notice, and the engine computer pulls it back down to 700 rpms a moment later.

Lather, rinse, repeat.

Four-cylinder engines are rough, and a drop in idle speed, even momentarily, is always a bit of a rough event. The question is, does the engineer who designed the system consider the amount of idle drop you're getting to be normal, or is it excessive?

Here are some reasons it might be excessive, only some of which are fixable (and most not worth it):

1. High mileage car with parts starting to wear enough to reduce compression a bit;

2. A/C compressor getting old and starting to show the first early signs of eventually seizing up (although not for a long time to come, probably);

3. Dirty fuel filter, worn spark plugs, or other wear items not quite delivering the power they used to, so at idle the engine computer has to deliver a bit more power to keep the normal idle, and so the additional A/C load at kick-on requires an even bigger jump;

4. Design trade-offs in the original engineering of the engine et al.

If you've owned the car since new, and this is recent behavior, I'd get it looked at, but don't worry too much -- and if you're relatively new to the car (or had it since new) and it's always done this, don't sweat it at all.
posted by davejay at 10:36 PM on July 30, 2007

PS: my 1985 Jetta did exactly what you describe. So did my 1998 Mazda B2500SE, 2001 Sentra GXE, 2004 Sentra Spec-V, and 1993 Mazda Protege -- although arguably that last one always runs rough at idle. Four-cylinders all. On the other hand, my 2005 Mazda MPV with a Ford V6 doesn't do it at all.
posted by davejay at 10:38 PM on July 30, 2007

My `03 Golf vibrates like crazy with the AC on but only at or around idle. It's odd, I wish it would stop.
posted by knowles at 4:17 AM on July 31, 2007

The reason they say not to drive with the AC on recirculate full time is that an ice block can form in the AC ductwork from the air being too cold, eventually blocking the airflow. It happens in my Durango, although it never happened in previous vehicles. It won't cause any damage, you'll just need to run the AC on fresh air for a while until the ice block melts. It took about 30 minutes in my Durango.
posted by COD at 5:04 AM on July 31, 2007

This is normal VW behavior - normal in most cars, but definitely normal in a VW.

VW mechs have told me that you should run without recirc on until the car is cool on the inside, and then switch it on.

If you STOP seeing the pools of water, you need to pay attention - especially if you start seeing water vapor come through your vents on humid days. It means that your A/C drain tube is blocked. The fix takes about 30 seconds and will result in a lot of water on the floor. Problem solved.
posted by TeamBilly at 2:26 PM on August 4, 2007

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