Curious Noise

[Andy Clark]

This seems like such a dumb question to ask, but I have run out of ideas- At idle I have this slight "jangling" noise coming from the engine bay (maybe when running too, but can't hear it). It sounds about the same regardless of where you stand (either side and rear) of the coach. I have stethoscoped the turbo, the head (THAT'S a loud one in the ears!), and anywhere else I could think of. The noise seems loudest at the sheet metal cover over the gear train on the passenger side rear of the engine, but still not very loud. It sounds almost like the noise that a sheet metal disc riding loosely and rattling on a spinning shaft would make. Any ideas?

Thanks. Andy

[Gerald Farris]

Andy,
It is usually impossible to diagnosis an engine noise from a description. However, one thing that may help in getting some useful suggestions is listing the engine that your coach is equipped with. I think that your coach is running a 300 HP 8.3 liter Cummins, is that right?

Gerald  

[Andy Clark]

Correct. 1995 Cummins 8.3C.
Sorry to have omitted that info.

Andy

[Bill Sprague]

Andy,

I have the same engine.

I had a strange noise for about two years.  I would not say it "jangled", but it was strange and not identifiable to with my ears.  I would say it "clanked" more than it "jangled".  My wife said she could hear it when we were at cruise speed if she were in the back.  Once she took my video camera back there to record the sound.  It still didn't sound to me like anything that made sense.  

One day I stopped at a Freightliner store and asked if they would give it a "listen".  They pulled an experienced mechanic off his assigned job.  In ten seconds of listening he pronounced I had an exhaust leak and should get it in the shop.  With further analysis in the shop, it was determined to be a failed exhaust manifold gasket.  $2500 later, the noise went away.  The exhaust manifold was warped causing the gasket to fail.  It is the only significant repair I've had to do in the chassis/drive-line system in ten years and 100,000 miles.

Experienced mechanics can tell what those unusual noises are.  There were other indications I ignored.  One was some minor "soot" accumulation in the bedroom and closet.  Another was that he PacBrake seemed a little weak.  In fact, I had hot exhaust gas blowing into the engine compartment.  

Don't let this go on.  It may not be a minor and odd noise.  It could be an indicator of something more significant.

[Andy Clark]

Thanks, Bill.  Last time the Freightliner mechanic who listened to the engine had me rev it up, but proclaimed that he couldn't hear anything significant, so the mystery continues. Maybe I should take it to Motor Trucks in Mt. Vernon for another independent analysis.

Andy

[Steve Huber Co-Admin]

Andy,
Per Bill's suggestion, you may want to check on possible exhaust "sooting" in bedroom and closet if you haven't done so already. I had a slight amount of sooting around my engine access in bedroom and thought it was due to a poor hatch seal but found that it was caused by a faulty manifold gasket. Had no noticeable engine noise though (CAT 3126).
Steve

[Andy Clark]

No sooting anywhere that I have seen, Steve. I still could have an intake leak, though. Kinda hard to run alongside the coach while the engine is developing boost. In my younger days, but now....  
Andy

[Keith Oliver]

Andy;  I read Bill's post with interest, as I have a similar noise on my 1997 infinity QX4.  Seems the exhaust manifold is attached to the block in such a way that the least accessible bolt fails, a leak develops, and the fft, fft, fft.... noise is audible at low revs.  The mechanic that had a look at it told me that is common on those engines (including all Pathfinders of that vintage) and to fix it would cost more than the car is worth. As well he doubted I would notice any changes in the noise, and it has no effect on performance.  About once a year my wife wonders aloud if it might just be worth spending some money on, but is quickly dissuaded when I remind her of the estimated cost.
On a diesel, I am sure the noise is different, and likely more jangling than ffting.  Good luck with it.  Hopefully it is more accessible on your rig.

[Joel Ashley]

If it is indeed a warped manifold, much of the repair cost is in the labor.  A learning experience 25+ years ago for me was a warped right manifold on our Pace Arrow/454 Chev.  

Having never approached such a problem before, since Dad's farm equipment always ran so well, it took a lot of making do with what tools I had, and questions to those in the know as well as repair manual reading (before the internet no less!).  Innovative thinking re. tools and access came into play, but I repaired the problem myself, eventually replacing the warp-prone manifolds with Banks headers, and doing other things to mitigate the heat differential causing the warping.  Removing a wheel and tire gave some access to the broken bolt that resulted from the warp, but it was frozen in place and I had to very carefully drill a centered hole in it, over several days' time, then letting penetrating fluid seep around its threads as I spent hours hammering micro-vibrations into it and the surrounding block head.  Now those were fun times.  Eventually the bolt extractor bit in and steady pressure, fluid, and vibration began turning the stub, and I got it out, to great fanfare from family, friends, and myself.  Total stop and go operation took a couple of weeks.

That 454 had a reputation in that application for warped exhaust manifolds and snapped mounting bolts, and people had come up with aftermarket solutions.  Of course a bad gasket can be the cause also, and is more easily fixed than if one has broken bolts stuck in the head or block.  Admittedly a diesel is a heavier unit to deal with.

My point is that you can save a lot of cash if you are in a position to make the repair yourself, have the time, and are willing to learn.  And also, at the time of repair a person should take steps to mitigate future bolt and gasket failures and warpage.  Research solutions that improve heat management specific to your engine, or solutions that use more tolerant components, and implement them as part of the repair.  I also advocate making sure you allow a hot engine to idle and cool, never shutting it down in an overheated state, which only exacerbates, if not outright causes, the warping.

Joel

[Robert Mathis]

Bill, I have the same engine and had a similar noise in the same location. I had almost decided it was a bad baffle in the muffler until I got in there to see how to remove and replace it and found the sheet metal heat shield over the muffler was loose. Just enough the rattle when idling.

[Andy Clark]

Robert- I checked the muffler heat shield- no dice. The "jangling" noise, because it's very faint, still escapes isolation, so I'll try to ignore it until or unless it gets worse.

Bill, I very carefully inspected and stethoscoped the area of the exhaust manifold. At idle when cold I could just barely hear (with the sciope) and detect (with my fingers) slight gas flow from the forward side of the #6 (front, as installed in coach) exhaust mainforl connection. It went away as the engine warmed up. #5 looks suspicious too, but no sounds from there.  Further, I noticed a very slight amount of sooting on the flat surface below these connections, so it appears that I do have an incipient exhaust leak. I'll monitor it until it either self-heals (I hope, I hope <G>) or gets bad enough to spent $2500 on a repair.
Looking forward to seeing you at Spruce Goose and Back to the Future this Sept.

Thanks for all the wise advice, gentlemen.
Andy

[Bill Sprague]

Andy,

The first two symptoms were the noise and soot.  The third was gradually diminished PacBrake effectiveness.  Part of the $2500 repair cost was a new exhaust manifold because ours was warped.  You may not have to replace yours.