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Old 10-22-2013, 11:23 AM   #1
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Normal Turbo Sound?

I've recently had a lot of work done to the coach by Cummins, but I don't think this question has anything to do with that b/c the noise was there before, and it's still there. I'm wanting to know if this is 'normal'.

When I take off from a start, if I accelerate at anything above about 1/2 throttle, I can hear a whooshing sound (sounds like a big air leak) coming from the engine. Not a normal turbo whine. I'm a novice with diesel engines, but I think I understand there is no blow off valve used for the variable geometry turbo I have, right (Cummins 6.7 ISB)? So it's not that. The whooshing persists until the transmission shifts into 3rd or 4th gear and the boost pressure drops. FWIW, the max boost pressure I'm registering is around 28 psi. Can someone enlighthen whether this is a normal sound or if I potentially have a boost leak somewhere? Does the VGT making an odd noise under those conditions?

(Wayne, I'm assuming you're watching, so I just wanted to get a second opinion since your situation is a little different than mine.)
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Old 10-22-2013, 12:19 PM   #2
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I feel something may be amiss. Yes, turbo whine is normal and if you had a very small boost leak say at the CAC, it would have the tendency to screech under a high boost load in a manner more pronounced than the normal turbo whine. However a "whoosh" almost sounds like a loose band clamp at the inlet or outlet of the CAC or at the compressor outlet of the turbo itself. The sound of quickly escaping pressurized air lends itself to a failure in one of the rubber/polymer sleeves or their clamps that makes the connections to these components. You are correct, there is no "blow-off valve" (it's actually called a waste gate) with a VGT turbo. I'm assuming the discharge you are hearing is not from the air brake purge system. That would tend to repeatedly "pulse" if it was active during you initial acceleration. 28 PSI (~56"hg) seems a bit low, but when you factor in the sound you are hearing, it would seem that the boost pressure would be even lower if the failure is where I described.
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Old 10-22-2013, 01:51 PM   #3
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Dan, thanks for the response.

Regarding the air brake purge system, is acceleration something that would cause that to occur on a repeatable basis? It is an odd sound, not quite a consistent whoosh, but somewhat of a pulsating whoosh. (wish I could better describe it)

I will check all the sleeves and clamps to any obvious leaks.
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Old 10-22-2013, 02:01 PM   #4
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You know, I'm watching this thread because my Freightliner does this under hard acceleration. I hear this rather consistent pfft, pfft, pfft. It sounds like its coming from underneath the driver seat, not back around the motor.
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Old 10-22-2013, 05:15 PM   #5
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If the air compressor is engaged replenishing the air tanks at the time you accelerate from a stop, the air system will repeatedly purge once max pressure is achieved so long as the acceleration is taking place. I've witnessed this in my own coach and have heard it taking place in another coach that I happened to be behind. The repeated purges stop as soon as the acceleration subsides. The sound is a repetitious purge, about one every second until you let up on the pedal. I have no idea why it does what it does, but as soon as I hear the multiple whoosh's, I let up a bit on the pedal and it stops. My chassis is also Freightliner and the other coach that I witnessed doing the same was also Freightliner. I think the purge valve is located slightly to the rear of the centerpoint of the chassis.

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Old 10-23-2013, 05:18 AM   #6
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Quote:
Originally Posted by Pelledan View Post
If the air compressor is engaged replenishing the air tanks at the time you accelerate from a stop, the air system will repeatedly purge once max pressure is achieved so long as the acceleration is taking place. I've witnessed this in my own coach and have heard it taking place in another coach that I happened to be behind. The repeated purges stop as soon as the acceleration subsides. The sound is a repetitious purge, about one every second until you let up on the pedal. I have no idea why it does what it does, but as soon as I hear the multiple whoosh's, I let up a bit on the pedal and it stops. My chassis is also Freightliner and the other coach that I witnessed doing the same was also Freightliner. I think the purge valve is located slightly to the rear of the centerpoint of the chassis.

Dan
This is a very good description of the sound my ISL has always made under the same circumstances, and I think it is normal since it's occurred since my coach was new.

Regarding the issue Bryan H describes, I also heard something very similar for several weeks - then it got extremely loud all of a sudden and I found out the compressor inlet hose had split wide open. It almost sounded like the fan blades were hitting something when that hose blew open, but I suspect there was a small hole in it for a while so it wasn't so noticeable until it let go.

You may want to locate and carefully check your compressor inlet hose and the clamps retaining it. On my coach, it's a small curved hose on the right side of the engine. Only about $35 for the hose if that turns out to be it, but it cost me over $400 to have Cummins replace it because I simply could not locate the problem by myself.

Good luck!
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Old 10-23-2013, 07:00 AM   #7
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Big wooshing sound? Sounds like the cooling fan is running at full bore.

Does it do this all the time from a start, or just when it is cold?

Mal
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