wing0

I may have never noticed this since I always have the radio on.
When A/C is on, I can hear this periodic hissing noise(last for like 15 sec) that comes out from front driver end of the car. It sounds like you let air out of a ballon, but slowly.....
Is there something to worry about?

Tammy

First, check that the green colored cap to the EVAP test port is snug. It may be loose; or cracked from too tight.. It is located behind the air filter housing.

Otherwise,

I would suspect a split hose or broken fitting leading into your vacuum tank. It is located below the battery, about the size of a 12 Fl. Oz. can. Made of black colored plastic.

Check the vacuum diagram located on the left side under hood. Pinch the hose with flat pliers and listen for the leak to stop. If needed, gently work loose the edge of the hose from the plastic connecting tube, prior to turning the hose on the tube. This reduces the chance of twisting off the connecting hose from it's tank, while removing the tube..

And as to getting it repaired, I would consider it.

It's sucking in unfiltered air into the vacuum system, controlling the emission EVAP. This removes the vapors from the charcoal filter in the rear, behind the axle. It additionally, vents the tank filler, allowing the fuel filling to vent. There are also vacuum switches that may also clog and fail as they suck in fine dust..

You may be getting a check engine condiction as the leak continues.

wing0

Thanks...i'll check it out once the car is back...
hope nothing is wrong with any of those...this count as powertrain?

How do i check for leaks on the hose? Have a person sit in the car and turn on the A/C?

rxdriver

What you're describing actually sounds normal to me, because I've heard it too. What I think you're hearing is the compressor coming on and building up pressure in the system.
Does the A/C work OK? If you have a leak, it probably would not work. These systems are very touchy about the amount of refrigerant in them, and either too little or too much will cause it to stop cooling effectively.

salimshah

Air conditioning is based on the principal that PV/T is a constant. So a highly compressed medium (almost liquified) is brought into a chamber and it is allowed to expand (almost like letting air out of a baloon) and it eminates a pssssst sound. As the medium expands in volume it takes the enegy out of its surroundings and thus lowers the temprature.

If you are hearing th sound when the AC compressor clutch engages and goes away when the thremostat releases the clutch, you are hearing the expansion valve working.

One of the problems with quite cars is that you can hear a pin drop.

Salim

Tammy

Check under the hood for the hissing sound and listen inside the left front inner fender housing, steering wheel turned to the right. Then the right front inner fender housing, steering wheel turned to the left.

Have the condictions set... meaning A/C on, set temp for cold.

Eyeball the sightglass for excessive bubbles in the liquid freon, while the A/C cycles on and off. A few bubbles are normal.

The sight glass is in front of the radiator/condensor, below the hood latch.

wing0

I just went out and started up the car....put A/C on.....went in the cabin and heard the hissing noise coming from front dash..(like..i dunno...uniformly?)

When I tried to find the noise in the front hood....the only suspecious hissing noise came from the black plastic housing above the alternator, but when A/C was turned off. It still remains.

I gotta go out again to check the freon sight glass tomorrow....
It's going to be 36C tomorrow here with humidty. It'll feel like 41C....the A/C better not break or else the 2 hr drive will be torture....

Another question I have is, do you think RX has a weak A/C system? I mean....It takes a while to get the air cold from the vents and it doesn't seem to cool the card fast enough even with recirculate mode selected.

rxdriver

No, I wouldn't call the RX's A/C system weak, but in this high heat every A/C system seems weak. The tiny vents can only supply so much air, so it takes a while to cool off a relatively large space inside the car that's pretty hot. That's why the recirculate mode helps - it will cool air that it has already cooled to some degree, which is better than slightly-cooled hot air from outside.

Definitely check the sight glass and see what's happening.

Another measure of system performance is the duct temperature. Put your hand right on the vent and feel the air. It should only take 2-3 minutes of max cooling for the air inside the duct to be noticably cooler than the ambient air temp. There is such a thing as an A/C duct thermometer, which looks like a miniature meat thermometer with a dial on a long metal stick. I believe the duct temperature should be lower than 50 F (10 C), although I can't find that number in the service manual. The manual only talks about testing the system with pressure gauges.

yoon

does your hissing sound keep coming on and off? i know some of the hissing noise is normal, but i think i might have the same problem as you to a certain degree. mine hisses for a few seconds, turns off, and does it again for the whole time

Lil4X

As others have noted, the hissing sound would seem to be either the expansion valve working normally, or a vacuum leak. The engine vacuum is used to move the various flaps and doors that distribute heated / cooled air into the cabin. You might try pushing the appropriate buttons, directing air flow from the floor, to the dash, to the defrosters and see how or if the sound changes.

The Lexus RX 300 has, IMHO, an excellent AC system. If a couple of weeks in central Nevada and Utah weren't enough, I spent a week in North Texas where the ambient temp was showing 110 - 114 degrees. . . . .and this was with only the stock tint on the windows. It took a bit of time to cool down after parking in the sun for a few hours, but after a few minutes at highway speed with the windows down to extract the superheated air, it took only a few minutes to become comfortably cool in the cabin with the AC on full and the "Recirc" button on.

My RX 330 with deeply tinted windows is even better, able to pull the inside temp down under 72 degrees as needed. My old Suburbans operated with the "Temp" set to "MAX" and the fan on "High" for eight months out of the year - and at temps over 95, you began to suffer. Of course down here, DUAL air is considered standard equipment.

On my RX's, I've learned to tilt the moonroof open with the shade pulled forward all but the last inch to help vent extreme heat from the cabin when parked. Of course you have to exercise good sense concerning security and weather conditions when you do this, but keeping the heat from building up is sure faster than extracting it and cooling the cabin later.

wing0

I've tried to change the direction of air and it does not chnage the noise.
The noise does come and go, but it's repetitive and I dun really recall this noise coming on when A/C was on......or maybe it's because i'm really focused on hearing this noise now that I noticed it without music/radio on.

Used the A/C for good 2 hrs today while driving to work and back home. It seems to cool down the cabin and such....just the noise i guess....

SilverSprt

I have the exact same issue with my RX. I hear an intermittent hissing noise when the A/C is turned on and it lasts. I've brought up this issue with Lexus 2-3 times. The first time they added additional freon and it went away temporarily but is back now. My A/C works perfectly fine as it should, it's just the hissing noise that is a little annoying (that is if the car is dead silent with the radio off). The service advisor said the hissing noise is the freon being injected, and admited it was a bit loud but later said it was "normal". Not sure if this is normal. At the dealership, they injected ink into the A/C systems to detect any leaks, but nothing could be found. So in the end, I am still stuck with the hissing noise which is a little annoying, but the AC system works perfectly fine. Maybe I'll try the pointers from Tammy and see if I can find anything. (I thought I was the only one with this problem until I found this thread!)

wing0

now there's two of us...WEE!!....out of how many RX?
lol...

HarrierAWD

The hissing sound is normal. It's simply R-134 being decompressed and released into the A/C evaporator. You hear the hissing sound because the RX300 is so freaking quiet. All car A/C evaporators make that noise.

cbejamimin

Same exact problem with my 2015 IS 250 Crafted Line. I’ve taken it in twice and they can never replicate the issue. I was then told that it is normal for the noise to occur. It’s quite annoying and find myself hearing it over the radio at times. I also have a clicking noise at the thermostat near the wheel and was told that’s normal as well since it happens on my normal setting.