Odd noise after turbo boost (audio/video link)

usa591 · **Posted:** Tue Oct 04, 2016 2:33 pm

Hey Team,

Ok for the last few months I've been noticing this odd noise after I give 'er a little gas when I'm at low speeds such as driving around parking garages in 1st or 2nd gear.

Right after a slight acceleration under a load, and as the RPM's are going down, I hear a slightly metallic growling for about a second or so. No other odd noises coming from turbo or transmission at other times.

I was able to recreate this sound by standing on the brakes and revving up--check it out:

http://piersonjacquelin.com/permalink/C ... eNoise.mov

It's especially clear at :17 and :22.

…any ideas?

flash7210 · **Posted:** Tue Oct 04, 2016 3:40 pm

Dont know. May not be turbo related.
But it wouldnt hurt to pull the turbo inlet hose and check the turbo shaft for excessive play.

mass-hole · **Posted:** Tue Oct 04, 2016 4:17 pm

usa591 wrote:

Hey Team,

Ok for the last few months I've been noticing this odd noise after I give 'er a little gas when I'm at low speeds such as driving around parking garages in 1st or 2nd gear.

Right after a slight acceleration under a load, and as the RPM's are going down, I hear a slightly metallic growling for about a second or so. No other odd noises coming from turbo or transmission at other times.

I was able to recreate this sound by standing on the brakes and revving up--check it out:

http://piersonjacquelin.com/permalink/C ... eNoise.mov

It's especially clear at :17 and :22.

…any ideas?

Do you have torque pro and can you monitor boost when you are powerbraking like that?

Couple things I can think of, maybe the EGR is dumping the exhaust pressure to help kill the turbo, or possibly you are hitting the compressor surge line since the RPM's are dropping pretty rapidly due to the load but the turbo still has momentum. Surge is basically where the turbo is making a lot of pressure but has nowhere to put the air because the engine cannot accept it(RPM's are too low) so it chokes.

diesel_guy86 · **Posted:** Tue Oct 04, 2016 7:30 pm

Sounds normal to me, its just the excess boost venting out backwards through the compressor. Every turbo-diesel will do it.

usa591 · **Posted:** Tue Oct 04, 2016 9:10 pm

mass-hole wrote:

Do you have torque pro and can you monitor boost when you are powerbraking like that?

Negative--I should look into it, I'm sure there's a raft of great data to be had.

mass-hole · **Posted:** Tue Oct 04, 2016 9:26 pm

usa591 wrote:

mass-hole wrote:

Do you have torque pro and can you monitor boost when you are powerbraking like that?

Negative--I should look into it, I'm sure there's a raft of great data to be had.

For sure. It's enough to help me self tune mine.

It could just give you an idea if you are getting a boost spike when you let off. Mine makes some intake noise when I let off the throttle but not like that.

usa591 · **Posted:** Tue Oct 04, 2016 11:53 pm

diesel_guy86 wrote:

Sounds normal to me, its just the excess boost venting out backwards through the compressor. Every turbo-diesel will do it.

In my mind, I'm hoping it's just the sound of what my boater-mind would call "cavitation" on a propeller.

I'm just a little nervous because it sounds so metallic.

This weekend I'm gonna take the air cleaner off and see if I can feel any wiggle in the turbo vanes.

Sent from my iPhone using Tapatalk

diesel_guy86 · **Posted:** Wed Oct 05, 2016 8:02 am

My liberty did the exact same thing when i had the stock turbo on it. Watch your intercooler tube swell in your video, you can tell that turbo is making plenty of boost. If anything you could have worn rockers, this would cause the valves to open later/ close early not letting all the air through that it was designed for. This would definitely cause more air to rush back on decel like that.

WWDiesel · **Posted:** Wed Oct 05, 2016 10:56 am

Could possibly be the left (drivers side) motor mount? They are hydraulic mounts and yours seems to be making a noise when the rotational torque is released by the engine. This would make the noise much more pronounced in the lower gears due to higher torque values produced by the engine under those conditions which in turn would make the left motor mount expansion / contraction much greater, i.e...more noise produced each time it is unloaded and collapses! :roll:

I know your signature shows mounts have been replaced, but that does not mean one cannot go bad... :shock:

usa591 · **Posted:** Wed Oct 05, 2016 2:29 pm

WWDiesel wrote:

Could possibly be the left (drivers side) motor mount? They are hydraulic mounts and yours seems to be making a noise when the rotational torque is released by the engine.

Yes, that does seem like a possibility…and the mileage interval might make sense.

usa591 · **Posted:** Mon Dec 05, 2016 4:13 pm

Hi Team,

I changed the engine mounts last week--and the sound is still there.

I still haven't checked the turbo for play, and I'm following the logic that it could be an over-boost related issue.

My gut says it's not a bad turbo because I've always been aware of that unique high pitched turbo "spin down" sound, and that particular noise has remained constant throughout my ownership.

BUT, I'm curious if anyone thinks it could be transmission related??? Like, torque converter stuff?

I was doing some wheeling this weekend and this noise was really obvious at low speeds in 1st gear when I'd "spike" the accelerator a little for a little thrust and then lay off as the Jeep came up to speed--all below 10mph.

WWDiesel · **Posted:** Mon Dec 05, 2016 4:56 pm

I have listened to that darn video over a dozen times now and it is real hard to say exactly where that noise is coming from!!! :banghead:

It still seems to be mechanical in nature to me...

Does it seem to be getting any worse with time?

You may want to enlist the help of another person and use a mechanic's stethoscope to pinpoint exactly where (area) the noise is coming from.
If you determine it is coming from the TC area; I would be checking the torque converter to flexplate bolts and make sure none are loose.
I have also seen a flexplate develop a crack in it and make some weird noises at varying engine loading... :roll:

usa591 · **Posted:** Mon Dec 05, 2016 5:22 pm

WWDiesel wrote:

I have listened to that darn video over a dozen times now and it is real hard to say exactly where that noise is coming from!!! :banghead:

It still seems to be mechanical in nature to me...

Does it seem to be getting any worse with time?

Yeah, weirdly mechanical, right?

Getting close enough with a stethoscope is a challenge--since the only way to recreate the sound is to rev up while braking…and I would really have to trust the guy on the brakes if I were under the vehicle! I'll think of something, tho. That's going to have to be the next step.

As for worse over time--hard to be completely objective, but I'd say once I noticed it, it basically has stayed the same for the last six months or so. Even with the sound, everything else is working fine. Shifts fine, no other noise, etc.

mass-hole · **Posted:** Mon Dec 05, 2016 8:24 pm

Still sounds like the turbo barking to me. Disconnect the turbo outlet CAC hose so you dont build pressure and see if it still does it. You can see the hose expand from all the pressure it builds and then deflates when you let up.

usa591 · **Posted:** Mon Dec 05, 2016 8:47 pm

mass-hole wrote:

Still sounds like the turbo barking to me. Disconnect the turbo outlet CAC hose so you dont build pressure and see if it still does it.

A-ha, good one. I'm thinking disconnect it from where it connects to the intercooler, 'cause it's easier to access. Possibly hose clamp a piece of old T-shirt around the now open intercooler intake to keep junk out? Or is that over thinking it?

mass-hole · **Posted:** Mon Dec 05, 2016 9:15 pm

usa591 wrote:

mass-hole wrote:

Still sounds like the turbo barking to me. Disconnect the turbo outlet CAC hose so you dont build pressure and see if it still does it.

A-ha, good one. I'm thinking disconnect it from where it connects to the intercooler, 'cause it's easier to access. Possibly hose clamp a piece of old T-shirt around the now open intercooler intake to keep junk out? Or is that over thinking it?

I would just pull it off from the intercooler and rest it on top. I dont think you need a t shirt for the 5 minutes if will be disconnected.

It may be smokey when you powerbrake it but thats normal without the turbo pumping air in.

rankom · **Posted:** Mon Dec 05, 2016 10:13 pm

i hope is not excessive play in piston wrist pin ? i seen this before noise will come on ones torque is released , but again its just a thought lol

usa591 · **Posted:** Tue Dec 06, 2016 4:55 pm

mass-hole wrote:

I would just pull it off from the intercooler and rest it on top.

Bingo! Noise gone… at least, I'm pretty sure it's gone--there's a lot of air noise coming from the turbo, but I'm pretty sure I can't hear it anymore. Thanks for the idea Mass-Hole.

So…what does this mean again? Excess boost pressure not being vented correctly?

mass-hole · **Posted:** Tue Dec 06, 2016 5:42 pm

usa591 wrote:

mass-hole wrote:

I would just pull it off from the intercooler and rest it on top.

Bingo! Noise gone… at least, I'm pretty sure it's gone--there's a lot of air noise coming from the turbo, but I'm pretty sure I can't hear it anymore. Thanks for the idea Mass-Hole.

So…what does this mean again? Excess boost pressure not being vented correctly?

It doesnt really mean anything. Basically you are building a lot of boost, probably 15+psi, and the turbo is spinning very fast and has a lot of kinetic energy. When you quickly let off the gas the engine RPM's drop extremely fast because of the brake load, but the turbo still has all the speed and energy and suddenly has no where to send the air/pressure it built up so it backs up and the turbo basically cavitates. When you go from say 2000 rpm to 1000 rpms, the engine should really only accept 1/2 the air flow at the same boost pressure. If your turbo is still trying to pump in the same amount of air at 1000 rpm's then the pressure must increase.

Boost is created by resistance to flow. If you put a drink straw between the turbo and the intake, you would need a massive amount of boost to cram a little bit of air through the straw. Reducing your engine speed kinda does the same thing. The air first has to fill the cylinder before it can leave the engine as exhaust, so if you reduce the RPM you are filling the cylinder less times per minute and therefore need less flow.

I wouldnt say its a good thing, you shouldnt try to do it, but its probably not a big deal occasionally.

usa591 · **Posted:** Tue Dec 06, 2016 6:26 pm

Cool--thanks Mass-Hole!

Just curious, but can anyone else reading this thread recreate this sound by power breaking while stopped and revving the engine a little?

Odd noise after turbo boost (audio/video link)

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