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Old 11-10-2014, 03:36 PM   #1
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Engine brake, air rushing sound

My coach runs fine but I have a question/concern about a noise I hear whenever I use the compression brake. I usually only can hear it when I am in a tunnel or passing close to a bridge wall or jersey barrier. It is the sound of air, a lot of air moving. Someone on the Cummins forum suggested it was a symptom of an exhaust manifold leak but when looking in that area with a flashlight, I see no evidence of soot or air leakage.

Is this merely a characteristic of the 2007 and later clean diesels? Do any of you with the ISL of that year or later hear the same noise? Is it some attempt by Cummins to quiet the compression brake....perhaps due to the DPF muffler?
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Old 11-10-2014, 04:54 PM   #2
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Try starting it and jamming a rag in the exhaust. You'll be able to track down any leaks from the back pressure.
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Old 11-10-2014, 05:12 PM   #3
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The newer diesels use a variable vane turbo. What you're hearing is the turbo vanes closing down to slow the engine which makes a loud hissing noise. It's perfectly normal. At idle, you may be able to hear whoosing sounds which is made by the vanes changing pitch.
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Old 11-10-2014, 08:10 PM   #4
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Quote:
Originally Posted by HicksRA View Post
The newer diesels use a variable vane turbo. What you're hearing is the turbo vanes closing down to slow the engine which makes a loud hissing noise. It's perfectly normal. At idle, you may be able to hear whoosing sounds which is made by the vanes changing pitch.
Loud hissing is exactly how I would describe it. Thanks.
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Old 11-10-2014, 08:17 PM   #5
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Quote:
Originally Posted by smlranger View Post
My coach runs fine but I have a question/concern about a noise I hear whenever I use the compression brake. I usually only can hear it when I am in a tunnel or passing close to a bridge wall or jersey barrier. It is the sound of air, a lot of air moving. Someone on the Cummins forum suggested it was a symptom of an exhaust manifold leak but when looking in that area with a flashlight, I see no evidence of soot or air leakage.

Is this merely a characteristic of the 2007 and later clean diesels? Do any of you with the ISL of that year or later hear the same noise? Is it some attempt by Cummins to quiet the compression brake....perhaps due to the DPF muffler?
There are a number of things that happen when your compression brake (Jake Brake) operates. One of them is a few seconds after the Jake turns on the engine cooling fan will turn on high. It has been estimated to add up to 40 more braking HP to help even more than just the Jake Brake. If you are in an underpass or tunnel the added noise is VERY noticeable! I am sure this is what you are hearing.

You are probably aware that the transmission will also attempt to downshift if it can without over revving the engine. The Jake works best at the upper end of the RPM range. The turbo will also make some changes internally.
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Old 11-11-2014, 06:04 AM   #6
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There are a number of things that happen when your compression brake (Jake Brake) operates. One of them is a few seconds after the Jake turns on the engine cooling fan will turn on high. It has been estimated to add up to 40 more braking HP to help even more than just the Jake Brake. If you are in an underpass or tunnel the added noise is VERY noticeable! I am sure this is what you are hearing.

You are probably aware that the transmission will also attempt to downshift if it can without over revving the engine. The Jake works best at the upper end of the RPM range. The turbo will also make some changes internally.
Barney, all makes sense. Mine is set to downshift to 3rd gear when it can. No question that the fan goes supersonic and it is noisy.
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