A/c Help

[Maha]

I have a '99 GMC Sierra. There is an intermittent hissing noise that seems to be coming from under the dash. (at least that's what it sounds like from the drivers seat) I haven't checked under the hood to see if it is there, but when the a/c or defrost is on, about every 5-10 seconds there is a short burst of what sounds like air under pressure escaping. When the a/c is off and I first turn it on there is a longer burst of air. The burst lasts for a few seconds and then goes into the regular 10 second burst pattern.

[Norseman]

I have a '99 GMC Sierra. There is an intermittent hissing noise that seems to be coming from under the dash. (at least that's what it sounds like from the drivers seat) I haven't checked under the hood to see if it is there, but when the a/c or defrost is on, about every 5-10 seconds there is a short burst of what sounds like air under pressure escaping. When the a/c is off and I first turn it on there is a longer burst of air. The burst lasts for a few seconds and then goes into the regular 10 second burst pattern.

 

Is it still blowing cold?

 

It does sound like you might have an evaporator leak

[Maha]

Yes still blows cold. And it doesn't do it all of the time. Sometimes I can start it up and get rolling and no noise, but if it starts it continues until I shut off the truck. At first I thought maybe something happened after I had the system charged before driving it from Oregon to Missouri, but I remember that it did the same thing once before I had it charged.

[Carder09]

In my old 94 chevy I had the same problem but it turned out that it didnt effect anything at all and worked fine even when I sold it so unless it actually stops working or you just get tired of the hiss, then I wouldnt worry to much about it

[MikeNH]

TSB text found on another forum-

 

HVAC Hiss Noise with AC On - kw GMT800 GMT805 GMT806 GMT820 GMT830 idle blower compressor line hose #PI02148 - (Jun 21, 2004)

HVAC Hiss Noise with AC On

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Condition/Concern:

Customers may comment of a hiss noise coming from the dash area with the A/C on. The noise can come and go or change/ intensity depending on A/C system loads (outside temp, engine RPMs, humidity, etc.)

 

Recommendation/Instructions:

The noise is normally only heard while stopped with blower on low and radio off. The noise is coming from the A/C orifice tube and is normal. No repairs should be made.

 

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Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance. This diagnostic approach was developed for the vehicle with the VIN you entered and should not be automatically be used for other vehicles with similar symptoms.

 

Models:

(03-04 Chevrolet All Full Size Trucks and Utilities) and (03-04 GMC All Full Size Trucks and Utilities) and (03-04 Cadillac All Full Size Utilities)

[Maha]

Thanks. So far this is better than just advising that I turn the radio up..

[MikeNH]

My 03 made the noise too and I got that TSB way back then from a friend at a GM dealer but had since lost it.. It also would stop putting out cold air when stopped in warmer weather and that was something I never was able to figure out. I worked in a shop at the time so I checked the level of the refrigerant and it was okay, pulled vacuum to check for leaks and that was okay, no damage to the condenser or blockage between it and the radiator. It was weird. The truck was out of warranty by the time I noticed (bought the truck with 32k on it in the fall...) so I just dealt with it.