Flapping Noise From Engine???

[WileyCoyote]

During a recent 3000 mile trip towing a 8000 pound trailer, my 2001 ETH/DEE would make a "flapping" noise after downshift to pull a hill.



The noise only occurred while engine speeds were above 2000 RPM, usually 2200 and above.



The noise seemed related to the beginnings of the start of the fan, but was hard to say for sure. When the fan did come on, it was steady and easy to hear. The flapping did not occur when the fan was on for sure.



Otherwise the truck did fantastic, with average mileage about 13-14 while towing. I did discover that the truck pulls hills much better than its brakes can go down them.



Thanks for the help!



Harry

 

[KatDiesel]

Do you have an airdam on your truck? I ordered mine without an airdam and there is a rubber "flap" that could be the culprit? Not sure if the trucks with airdams have the flap or not? Just an idea?

 

[EMDDIESEL]

My 01' has an airdam and the "Flap". I have never heard any flapping noises though.

 

[Texas Diesel]

Check your air cleaner box snorkel. Mine would make whooping/flapping sounds at high RPM. Mine was uselessly sitting outside the air box. I worked the lip of the snorkel inside the air box and no more noise.

 

[SEmerson]

I have noticed a flapping noise. It would happen if the turbo was spooled up and then a fast drop in pedal. It would make a flapping and then a gush of air. It is normal. It is the pressureized air rushing back from past the turbo and then shacking the air box.

 

[Extreme1]

Originally posted by SEmerson

It would happen if the turbo was spooled up and then a fast drop in pedal. It would make a flapping and then a gush of air. It is normal. It is the pressureized air rushing back from past the turbo and then shacking the air box.

This is the cause, wait 'till your truck is BOMBed.



Try to avoid it because the turbo is being twisted pretty hard in opposing directions when you hear it.

 

[Evan A. Beck]

In technical terms, the sound your describing is called "stack pulsation. " Basically, it is the excess air in the intake system trying to escape through the turbo in the opposite direction. If you let off real fast under high (30+ lbs) boost, you can snap a turbo shaft, so be careful.

 

[Texas Diesel]

Evan: I let up on the "gas" and dont shift until my boost drops to 10 PSI, is this good or am I being too "easy" on it?

 

[merryman]

My '01 ETH DEE, with no air dam and with the snorkel between air box and fender gone (along with the air box, as I have a K & N Cone filter), still flaps, and it flaps not only on a down shift, but also while pulling hard in a fourth or fifth . It flaps only under those two conditions, and does so on most substantial hills when pulling a load.



Evan, could the stack pulsation occur other than on a shift, for example while pulling with a load on a good hill? I gross about 19. 5K?

Thanks for the info.



Vaughn

 

[WileyCoyote]

It sounds like what some of you are describing is called compressor stall in jet/turboprop aircraft engines and/or "surge" in other applications of centrifugal compressors.



My understanding of these phenomena is that the mass flow through the compressor drops to zero and back flow occurs. In jet engines or turbines it is accompanied with a loud "bang. " In a very large industrial gas turbine used for pumping natural gas I once worked on there was a loud noise.



Is this what some of you are describing??



I have had both the Cummins NTC 400 and the Cat 3406E in Peterbilts and never heard anything like this even with the engine brake slowing the engine rapidly.



Thanks.



Harry