Air hissing only under load accelerating seems air compressor and boost related

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Freightshaker03, Apr 11, 2021.

  1. Freightshaker03

    Freightshaker03 Bobtail Member

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    Hi I’m new here, I’m really frustrated with my 2003 Freightliner Classic. Detroit 60 series 12.7l
    I’ll try to explain it.
    Driving on the highway, the compressor builds up to about 125 and cuts out, then if I tap the brakes a couple times til the air is down around 105-110psi, it makes a hissing air leak kind of sound when I hard accelerate but when I let off the pedal the noise stops completely. This goes on until the instant the governor cuts back in and starts building pressure again to 125 psi.
    Then it happens all over again. Air down, hiss on acceleration, and quiet when I let off.
    Also seems related to my turbo boost. I don’t get quite as much boost when I hear the leak.
    I have leak tested all my boots and intercooler etc, and all good there. It seems associated with the compressor/ air dryer? Any ideas?
    I hear the compressor is somehow connected to turbo boost?

    many ideas would be greatly appreciated!!
     
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  3. mnmover

    mnmover Road Train Member

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    The air compressor gets filtered air from the intake charge air cooler system. The leak you hear when pulling hard is either the hose going to the air compressor or a leak in the charge air cooler system. Have the charge air cooler system checked by taking apart the system where the turbo supplies air to the intake air charger cooler and block up the system by the intake manifold. Pressurize the system by adding air into the manifold where the manifold pressure gauge, just a simple 3/8th fitting and an air chuck screws right into it. Plumb a gauge with the air chuck and pressurize the system and see how much air you loose in 15 seconds. you should be able to hear the leak st this time. 2 or 3 lbs air loss is normal, but more than that you have problems. The engine oem usually say 5 lbs loss in 15 seconds is normal, but it is so they can get out of warranty work.
     
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  4. windsmith

    windsmith Road Train Member

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    Carefully inspect the air compressor intake hose for damage or loose connections. Also check all of the pipes from the turbo outlet to the engine intake. If all seems OK, then perform the pressure test outlined by mnmover above.
     
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  5. McMahons Trucking

    McMahons Trucking Light Load Member

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    A spray bottle with soapy water will find your leak
     
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  6. lester

    lester Road Train Member

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    I'd think maybe the pop off valve on compressor or purge valve on dryer.

    You said it quits when the compressor engages and starts building air again, even if on the throttle hard?
     
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  7. bad-luck

    bad-luck Road Train Member

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    Check the pop of valve located on top of the compressor, check the purge valve under the air dryer
     
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  8. Freightshaker03

    Freightshaker03 Bobtail Member

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    for some reason I found the electrical plug was off of the purge valve... looks like it’s been like that a long time... I cleaned it up and put it back on, seems to be better but I think there’s still a bit of a leak. I’ll try checking into the pop off valve!
    Thanks for the help!
     
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  9. Freightshaker03

    Freightshaker03 Bobtail Member

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    Apr 11, 2021
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    Lester, It happens only when the compressor is unloaded and around 110 psi or less before it reloads. The harder the pull the louder the leak. But as soon as the compressor starts building the leak completely stops!
    How would I check the valve on the compressor though?
     
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  10. Inderjit

    Inderjit Light Load Member

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    What dryer do you have?
     
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  11. lester

    lester Road Train Member

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    I think, could be completely wrong, the electric plug on the purge valve is for heat, to keep the valve from freezing in the winter.

    I would just change the pop off valve and the purge valve.
     
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