Screaming sound on hard acceleration - PeachParts Mercedes-Benz Forum

2.5Turbo · #1 06-05-2006, 02:56 PM

My 300D 2.5T sounds like somebody is strangling a rat under the hood under hard acceleration. The sound doesn't gradually increase in volume, it comes on abruptly as soon as I give it more than about 1/3 of the pedal travel. I popped the hood and reved the engine in park, and I'm 99% sure it's coming from the vacuum actuator for the EGR valve. When the hood is open, it's pretty loud, louder than anything else that makes noise under the hood. With no load on the engine, it comes on at about 3500-4000RPM, but it starts at around 2200 when driving, but only when the go pedal is at about 1/3. Before this started happening, there was a little whistle that starts under the same circumstances. The whistle doesn't sound like turbo whine. It starts when the ALDA starts giving the engine more fuel. The car never wistled before I cleaned out the ALDA lines...but before that it was agonizingly slow (0-60 in about 18).

Any ideas? Bad EGR solenoid?

Motorhead · #2 06-05-2006, 04:38 PM

I can give you info on a simular experience with a different turbo charged diesel. When I worked at UPS, we had some ONAN inline 6cyl diesels in some package cars. They would do the same noise as you describe when the ehxaust manafold was loose. From what you are saying that you increased your power after cleaning the ALDA line, you now are putting more fuel to the engine and also generating more boost. I would ck ALL ehxaust manafold bolts and look for any blown gaskets on the ehxaust.
These ONANs would SCREAM with a loud squeal/whistle when this happened.
Man were those things a P.O.S.

sixto · #3 06-05-2006, 04:41 PM

Are you sure it's not the serpentine belt slipping?

Sixto

85chedeng300D · #4 06-05-2006, 11:07 PM

Quote:

Originally Posted by sixto

Are you sure it's not the serpentine belt slipping?

Sixto

yeah! might be. or a bad pulley bearing.

2.5Turbo · #5 06-06-2006, 01:02 AM

Yeah, I'm pretty sure it's not the belt. It's not the slipping belt squeal, it's more like a screeching noise. Kind of like a bad bearing, but it shouldn't be a bearing. And there aren't any bearings in the EGR

I'm 99% certain it's coming from the EGR actuator, since I've been able to change when the noise happens by momentarily removing and plugging the vac line going to it.

A little more background on the problem...I did a more thorough cleaning of the ALDA pressure lines before it started. I took all the rubber fittings off and cleaned them out with some wire...and now, it screams (literally)

Maybe I'll do a recording of it tomorrow. And before I post the recording, I know, I know...I need to do a diesel purge to get rid of that little injector rattle

rchase · #6 06-06-2006, 02:26 AM

Might be your turbo on its way out. A friend's Volvo C70 High pressure turbo Convertible did the same thing right before the turbo failed. Not a very common thing to happen especially on a diesel. You might invest in a stethescope and place it on different parts of the engine while at various RPM's to see exactly where the noise is coming from. Belts and other parts can scream as well. Good luck. Hopefully its a belt or tensioner rather than a turbo.

Oh also on the stethescope thing use a bit of caution. Ears are very sensitive to loud noises.

2.5Turbo · #7 06-06-2006, 07:02 PM

Quote:

Originally Posted by Motorhead

I can give you info on a simular experience with a different turbo charged diesel. When I worked at UPS, we had some ONAN inline 6cyl diesels in some package cars. They would do the same noise as you describe when the ehxaust manafold was loose. From what you are saying that you increased your power after cleaning the ALDA line, you now are putting more fuel to the engine and also generating more boost. I would ck ALL ehxaust manafold bolts and look for any blown gaskets on the ehxaust.
These ONANs would SCREAM with a loud squeal/whistle when this happened.
Man were those things a P.O.S.

I think you hit it right on the mark. I did some further investigation today as it was sunny and I could see deep inside the engine compartment without much difficulty. I stuck my head down in there, listening for where it could be coming from. I pushed the throttle linkage all the way open for about 1/4 second in park so it would make the noise, and I noticed a wisp of exhaust smoke coming out from near the turbo/manifold area, along with a bit out the tailpipe, normal for a short burst of WOT. After doing that a few times, I narrowed down the source to the gasket between the manifold and the turbine housing. A closer look at the gasket area revealed a small amount of carbon deposit, a sure sign of an exhaust leak. The gap between the two parts looked a little too big as well. So I went to check the bolts sandwiching the gasket between the two parts, and wouldn't you know it, the top two nuts are missing! I'm having a hard time believing that these two nuts could have just fallen off under normal driving conditions. And for as long as I've had the car, my mechanic hasn't done anything that would require the removal of the turbo or the exhaust manifold, so he didn't do it. That leaves me with the previous owners, which happen to be my grandparents

. They always took it to the dealer for anything and everything...could it be possible that the dealer forgot to put these back on for something? I'll have to dig through the service records for any items regarding the exhaust system.

Meanwhile, this shouldn't pose any danger to the car, should it? I wouldn't think so since the leak isn't big enough for me to even detect inside the passenger cabin. It's just annoying, and it makes people on the sidewalks think "What is wrong with that car???"

And I'm glad that it isn't the turbo!

Motorhead · #8 06-06-2006, 07:13 PM

The problem with letting an ehxaust gasket/turbo gasket leak go and keep driving is that the mating surfaces can "erode" and cause a burned area on the cast iron. This will prevent the new gasket from sealing properly and will allow burn out sooner.

It is NOT uncommon for ex manafold and turbo bolts to come loose. They are constantly exposed to HOT and COLD expansion and contraction.
My DODGE Cummins loosened the ex manaflod bolts.

I would drive it as little as possiable until you fix it. I haven't R&R'ed a 2.5 turbo but the 3ltr 5cyl wasn't that big of deal to R&R. My $.02

peterhardie · #9 10-10-2006, 08:53 PM

i have a cali wagon (turbo on top) which has had a bad seal and oil leak out of the joint between the turbo and the intake manifold. When considering all the little bits needed to pull the turbo unit and install the o-rings, I discovered that a recent screeching noise at 2500rpm (now starts at 2100) turned out to be the manifold gasket. Telltale soot present, nut missing on one of the studs.

Myy guess is that a couple missing nuts on the support bracket have led to other joints working loose.

I will tear it down tomorrow, hope I haven't done too much damage--had to do a 8 hour hway trip, with the exhaust screeching!

I'll let folks know if I find something different.

mrwvo · #10 01-24-2007, 09:46 PM

Hey know this is an old post, but its one of the searches I found when researching my problmem. I have oil under my air cleaner assembly, and have a very loud screeching noise like mentioned above. Its dark outside now, but are these nuts hard to identify? And can I just replace them without having to pull the turbo and re gasket?

peterhardie · #11 01-25-2007, 05:59 PM

Mrwvo:

We don't know what kind of car you have, but the quick answer is if you have a hole in your manifold gasket, you need a new one: whole assembly off, make sure mating surfaces are clean and sealable, re-assemble. If you have a leak at the turbo-manifold joint, same deal.

Oil under your air cleaner may or may not be related to the noise; on mine it was not. Check your turbo and air cleaner drain tubes for leaks, tighten your turbo u-tube (non-Cali 123). Put a clamp on the breather tube at the valve cover, if none resides there now.

Rev your engine while looking and track down the sound. If a manifold leak, you can feel the leak blowing as well.

You MIGHT just have loose bolts, wouldn't hurt to tighten them. However, I pulled everything out between the exhaust downtube and the head, and am glad I did. Replaced all the relevant gaskets and seals (there's a kit, don't have the number, the dealer should), as well as a number of missing nuts and bolts holding the whole assembly together

. And did the motor mount for good measure. It took a few hours, but worth the time, and no more noise.