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Old 06-14-2021, 10:07 AM   #1
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Should I hear air "leaking" when I shut the engine off?

Newbie question here, I noticed that when I park my bus and shut the engine off, I hear a steady "hiss" sound coming from the air dryer. Should it be "leaking"/purging air continuously for 1-2 minutes after shutting off the engine, or do I have an air dryer issue?

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Old 06-14-2021, 10:39 AM   #2
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Sounds like a dirty valve. I chased down all my air leaks and now the bus holds air for weeks.
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Old 06-14-2021, 10:41 AM   #3
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Sounds like a dirty valve. I chased down all my air leaks and now the bus holds air for weeks.

Yeah, that was my impression as well. Time to rebuild the air dryer, that's definitely where the hissing was coming from. I just wasn't sure if it was supposed to purge any air after shutting off the engine. Survey says: NO!
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Old 06-14-2021, 10:54 AM   #4
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No, air should never be "leaking" from the dryer. But the important question is where is the air you hear coming from on the dryer?

There's only 2 locations the dryer should be flowing air, the discharge port and purge valve exhaust. When the system is building pressure, air should be flowing out the discharge port, into the line to the wet tank. After pressure is built, the governor will cut off the compressor and initiate the purge cycle in the dryer. During the purge cycle, the purge valve will discharge the air contents of the dryer housing through the desiccant and out the exhaust port. This cycle will typically last 20-30 seconds, until the dryer housing pressure is depleted.

If you hear air leaking out of the exhaust port for 2-3 minutes after shutoff, or if you have frequent purging/compressor cycling, the check valve on the discharge port has likely failed and is allowing air pressure from the wet tank to flow back into the dryer, and then out the exhaust.

An easy way to inspect this check valve is to open the wet tank petcock, after air has stopped flowing from the dryer exhaust. If there is normal air pressure in the wet tank, then the check valve is likely fine, if it's empty or near empty, then you can assume the check valve has failed.

Depending on the dryer model, you can typically install a service kit that will replace the check valve. In older dryers, these kits can be obsolete, and the repair is either a reman'd unit, or better yet, upgrading to a modern dryer.

FYI, dryer check valve failure isn't something you'll see on the gauges. The only tell tale is extended purging and frequent compressor cycling.
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Old 06-14-2021, 11:04 AM   #5
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Originally Posted by Booyah45828 View Post
No, air should never be "leaking" from the dryer. But the important question is where is the air you hear coming from on the dryer?

There's only 2 locations the dryer should be flowing air, the discharge port and purge valve exhaust. When the system is building pressure, air should be flowing out the discharge port, into the line to the wet tank. After pressure is built, the governor will cut off the compressor and initiate the purge cycle in the dryer. During the purge cycle, the purge valve will discharge the air contents of the dryer housing through the desiccant and out the exhaust port. This cycle will typically last 20-30 seconds, until the dryer housing pressure is depleted.

If you hear air leaking out of the exhaust port for 2-3 minutes after shutoff, or if you have frequent purging/compressor cycling, the check valve on the discharge port has likely failed and is allowing air pressure from the wet tank to flow back into the dryer, and then out the exhaust.

An easy way to inspect this check valve is to open the wet tank petcock, after air has stopped flowing from the dryer exhaust. If there is normal air pressure in the wet tank, then the check valve is likely fine, if it's empty or near empty, then you can assume the check valve has failed.

Depending on the dryer model, you can typically install a service kit that will replace the check valve. In older dryers, these kits can be obsolete, and the repair is either a reman'd unit, or better yet, upgrading to a modern dryer.

FYI, dryer check valve failure isn't something you'll see on the gauges. The only tell tale is extended purging and frequent compressor cycling.

I don't have the "ear" to hear the compressor cycling yet, but I don't hear frequent purging; just the normal "psh" once the compressor cuts out at start up and after braking to come to a stop, then accelerating from the stop. Going down the road, I never hear it purging.
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