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Discussion Starter #1
I have an '06 with 105K miles that I bought new. It's in excellent shape and these days, I only drive it during MI winters as I have a "summer" car as well.

This year, as the weather is turning, I started driving it and there's an intermittent hiss/vacuum leak sound coming from the dash. It sounds like it's on the passenger side but, hard to pinpoint. The sound only exists when the compressor is running which, of course, happens when the defrosters are on, etc. I run the climate control on auto 100% of the time.

I've tried to listen for the noise from under the hood but, the engine sound overwhelms it. It's definitely something leaking in the interior. The A/C worked great this summer and everything seems fine functionally. I would just like to stop the hissing noise from happening while driving.

Any thoughts out there? Any help is appreciated.

This vehicle goes down as one of the best I've ever owned. Other than a couple of minor rust issues that were repaired by Acura, it's been 10+ years of maintenance only driving. Nothing has ever failed.....so far.
 

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Have you changed the cabin air filter recently? When I bought my car and had any air flow on it made a loud jet engine sound. I changed the cabin filter and found it full of junk clogging the air flow. It's better now. There is a video on YouTube I followed the instructions.
 

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I agree that a clogged filter can induce all sorts of noises - on my MDX (when I first bought it), I was able to diagnose the noise as a clogged filter by just moving around the filter access "door" (under the dash in the passenger footwell). You DO have to drop the glovebox and a metal brace to change the filter (silly design, IMHO) but it's really not a hard job at all.
 

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Discussion Starter #4
Thanks for the replies. I did change the cabin filter a year, or so, ago. I don't think that's it since this sound only occurs when the compressor is running. So, I can choose the vent and turn the fan on high in manual mode and there is nothing but normal air-flow sound.

When in full auto mode or forced into AC, this is when the sound can be heard. When in full auto, it's intermittent since the compressor doesn't run continually. It literally follows the compressor on cycles however, it is NOT coming from under the hood. This is an interior sound under the dash.

Some things I've read suggest a control valve for the vents. I'll be taking a look into that short of any other ideas.

Thanks again for the replies.
 

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The hissing/vacuum sound is coming from the evaporator behind the dash which happens to be on the passenger side. This symptom is a common isssue due to low/insufficient freon levels in the system.

You have a leak or the AC just simply needs an evac/recharge.

Check the low side rubber hose going to the compressor (visible from the top) as they tend to leak. If it is leaking you will see it as the hose will be covered with compressor lubricant oil.
 

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I agree with CurrentDraw, as I had that problem about a year ago. Turned out there was a leak in the high pressure hose under the hood. It cost me about $500 to have done at my local independent shop. A/C works awesome now and no noise.
 

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Discussion Starter #7
Thanks for the feedback. Interesting. I don't drive the vehicle much in the summer but recently, had the timing belt, water pump, serpentine belt, etc., replaced at the dealer. Come to think of it, the sound started after I got it back. I wonder if they did something during the disassembly process. I'll take a look at the hose(s) for any leaks as well. When I last drove it in warm weather, the AC seemed to work fine. Hmmmmm....

Thanks again.
 

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Discussion Starter #8
I checked my records and the AC evac/charge was done in May 2015. That doesn't mean it couldn't have leaked but, it seems a little unlikely. I wonder if topping it off with a can of refrigerant from an auto parts store would be an inexpensive way to test the theory.
 

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It's best to use at least a low-side gauge when adding gas to your A/C - you don't want to over-charge the system or it won't work as well (and could potentially blow off your over-pressure valve, which is there for just that purpose). Some of the charging kits include a gauge which is generally OK for very simple diagnosis (that is, they'll tell you if your system is very low on gas). Generally you won't hurt anything bringing the gas back up to the indicated level on the cheapo gauge, and even before adding any gas, you should be able to tell that the system is low on gas, or not...
 

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Discussion Starter #10
Thanks habbyguy.

I have one of those semi-cheapo gauges that I've used on my kids cars, etc. The one I have actually has some adjustment for ambient temp. The problem is, here in Michigan, it's about 40 today. The gauge only allows for 65F at the low end. I guess I could pull it into my building and let it warm up for a few hours. Then, open the garage door just high enough to let the exhaust out and try it.

Will the can of gas transfer properly even at the low temp? I can just turn the defrosters on to force the compressor to run.
 

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Can't say I have any experience working on an A/C system in freezing temperatures (hey, we don't even allow those here in Arizona!) ;-) I'm sure it should be warm enough in your garage to at least get a good idea of whether or not you need more gas in the system or not. I can't think of any reason you couldn't top off a system with the ambient temperature in the 50's or 60's. I wouldn't get too twisted if you can't get quite up to 65° for the test - you should be able to interpolate where the actual temperature would be on the scale, roughly. If the pressure is obviously below that, try adding some gas - if not, maybe not.
 

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Discussion Starter #12
So, I want to say thanks again. This forum and it's members are truly awesome. The same goes for the ones I belong to for other cars and a motorcycle.

Today, I drove to my office and then into the building. It's an office with a small warehouse area. The warehouse area is heated to about 65 degrees. I let the MDX sit (starting with a warm engine compartment) for about 4 hours. I installed the gauge with the valve closed just to check the level. I found some results that are odd to me. When the compressor is engaged and running, the needle pegged at the minimum. When the compressor kicked off, it rose to the low side of the normal (green) range. This repeated with each compressor cycle. I turned on the AC button on the dash and dialed the temp down until it said low and the compressor came on and stayed on. The needle on the gauge pegged low again.

So, with the vehicle running, I pierced the can of R134a with the T valve and opened it. I followed the instructions by tipping the can from 12 to 3 o'clock continuously and shut the T valve about every 20 seconds or so. After several iterations of adding gas, the gauge started to increase into the low range when the compressor was running and relatively higher in the normal range. I stopped when the gauge indicated just below normal with the compressor running since it was approaching the high side of normal when it was off.

Get this. The hissing/vacuum sound I was hearing inside is gone. I drove it home at lunch and after sitting outside for almost an hour it was silent when I restarted it. I pretty much always makes the sound when first started after sitting a while. I will be certain after it sits overnight but, I definitely think adding refrigerant has caused a change. This is good news and bad news. The good news is, I found a problem and it can be fixed. The bad news is I found a problem and it needs to be fixed. The other bad news is I had the system evacuated and refilled in May 2015. Since then, myself and others have had experiences with this shop that are definitely suspect. One quick example, a buddy has a BMW 328iX and this place told him his oil filter housing was leaking and needed to be replaced. They quoted ~$800. I sprayed some brake clean in the area and had him watch it for a couple days. It turned out to be the o-ring on the plastic oil filter cap. They change the oil on the car but, the o-ring was flat as a pancake. We replaced the o-ring (15 minutes and $3) and the leak is gone.

Sorry for the long post. I do have a couple questions.

1. On the cheapo AC low-side fill gauge, should the reading be in the normal range when the compressor is running or off?
2. If the answer to 1 above is both, what does it mean when there is a difference between compressor on and off?

I suppose it's time to take it in to someone that can properly check for a leak and properly fill it.
 

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The pressure is checked with the compressor running, so it's clear your system was really low on gas.

I wouldn't get too twitchy about the leak until it happens again (that is, you start getting that hissing noise in the cockpit and your A/C starts warming up). You can always inject some UV dye into the system (there are kits for that at any auto parts store), and then see where it's leaking after a few weeks / days / months. It could be an easy-to-reach connector, or something sinister like a bad compressor. OTOH, many auto A/C systems "just leak a little", and a can of gas every spring will keep them running just fine. It's hard to get too excited about fixing a leak if you can keep the A/C running with a can of gas per year.
 

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Discussion Starter #14
Thanks habbyguy. Agreed on all points.

Since my long winded post above, I did more research and learned the correct time to read the pressure is when the compressor is running. A not so subtle point left out of the cheapo gauge fill instructions and most all web discussions and youtube videos. Amazing.

Anyway, the hissing is gone. When I started it this morning, sweet silence outside of normal sounds. The pressure is at ~25 psi when the compressor is running and this is right at the transition between low and normal on the cheapo gauge. It may be due to the temp being closer to 60 or even less after a few minutes with the garage door open.

Your recommendation to monitor the situation and go from there is exactly what I intend to do.

Thanks so much again. I spent $13 on a can of refrigerant and about 15 minutes of my time in total to at least temporarily remedy the situation. Again, these forums are great. The service manager at the dealer said, "in 18 years here I've never heard of low refrigerant causing any sound. I'm guessing its your cabin air filter." This is after telling him I replaced the cabin air filter a few months back.
 
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