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Old 10-13-2004, 07:20 AM
Coming back from burnout
Join Date: Jun 1999
Location: in the Pacific Northwest
Posts: 2,274
How do I distinguish between bearing knock and nailing?

On my 85 300D, I really got the engine running quietly except for a sound which is bearing knock or nailing. I can't tell the difference and dont know the difference.

I do have some good data. I was fooling with the IP pump timing and I also stuck 20% gas in the tank for an unrelated reason. When I retarded the timing and started the car, the nailing/bearing knock sounded like someone was hitting the inside of the valve cover with a metal hammer. All H__ broke loose..when I raced the engine, the hammering got incredibly embarrassing...
When I finally took the timing back to Optimum, and drained all the gas from the system, the car suddenly settled out and ran like a swiss watch except for a very hushed nailing/bearing knock that you have to listen for. I would venture that this is what the cars sounded like out of the factory (minus the nailing/knock) .
Another description of the sound is that on acceleration the engine used to put out a large Rat-a Tat- tat like one of those old muscle cars with a Holley carburetor and resonantor mufflers...
I have read all the posts about nailing, if thats what it is, and I am about to do a Diesel Purge, get some new heat shields and play with my injectors (swap them back and forth) The injectors and IP are realtively young, I switched the IP two months ago.

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Old 10-13-2004, 07:38 AM
tomm9298's Avatar
Diesel and WVO ..
Join Date: Sep 2004
Location: South Carolina NO RUST
Posts: 502
Thumbs up Knock vs Nailing

I am fighting the same battle! After reading this info from the following link, I am convinced it is nailing. I have done diesel purge, replaced injectors, though come to find out my "new" injectors will be as bad or worse than the ones I took out. Off to the local injector rebuild shop.

Read this, great info about rod, wrist pin, and bearing noises...
1984 300D Turbo Sedan (Hilda) 272,115 miles.....
Anthracite Gray/Palamino Leather, I am 3rd owner

2001 Dodge Stratus (Silver) (wifes) 55814 miles...

1982 280TE Wagon Astral Silver Metallic/
Anthracite Velour
260,512 miles (Eva)

1969 230 Sedan Olive Green/Black MBTex
4 Speed Manual
84,213 miles ????
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Old 10-13-2004, 09:30 AM
TwitchKitty's Avatar
Я не хакер
Join Date: May 2004
Location: Varies
Posts: 4,628
If it is a fuel knock it should stop when you crack open the injector line for that cylinder.
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Old 10-13-2004, 10:01 PM
sixto's Avatar
smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,800
Get a big nail and a big hammer, hammer the nail into concrete. That's the sound of nailing. Rod or bearing knock will get worse with load. Put the car in gear, step on the brake pedal, rev the engine, listen for the knock to get louder.

If it's nailing, do a search on delivery valve seals.

95 S420
87 300SDL
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Old 10-13-2004, 10:31 PM
TwitchKitty's Avatar
Я не хакер
Join Date: May 2004
Location: Varies
Posts: 4,628
Originally Posted by TwitchKitty
If it is a fuel knock it should stop when you crack open the injector line for that cylinder.
A bearing knock may stop when you crack open an injector line also, so this test may not be conclusive.
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Old 10-13-2004, 10:43 PM
Registered User
Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
Bearing knock is lower (crankshaft) and gets louder with load. Injector knock is obviously in the head, and gets quieter or does not change with load.

Main bearing knock is a deep sound, as if someone were banging on the inside of the block with a large hammer. Most obvious at low speeds. Rod knock is sharper (more like a framing hammer on the side of the block) and gets MUCH louder under load. Listen for an old Chyrsler 2.2L about to go, they clatter terribly while still running for a long time. Both will have a ringing sound as well, usually.

Injector knock sounds like someone driving a large nail into an oak block with a wooden handled framing hammer -- no ring, just a very loud "snap" sound. won't sound metallic like bearings slapping around.

Check your chain stretch too -- the chain will make an almost identical sound as it slaps the chain guides when it get so long the tensioner cannot keep it tight (or the tensioner is bad). Definite metallic (although mufled) sound that does not go away with cracking injector lines.


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1987 300D Turbo killed 9/25/07, 275,000 miles
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