Help! '76 300D Has Intermitent Knocking - Mercedes-Benz Forum

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post #1 of 4 (permalink) Old 05-26-2007, 01:04 PM Thread Starter
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Date registered: May 2007
Vehicle: 76 300D
Location: Colorado
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Question Help! '76 300D Has Intermitent Knocking

Hello All,

I'm a newbie here and greet you all with a problem ;(.

Driving from Massachusetts to Colorado this weekend. My76 300D had been sitting for a year in storage in MA. It was tough to start, and I used a tiny tiny amount of starting fluid to prevent a complete battery drain...There was a brief clanking 20-30 seconds after she turned over, but the initial idle was normal. Everything seemed OK driving around town for a few days. I heard a bit of diesel knock on hills, and used some Red Line just before begining the trip West.

On the highway, intermitent knocking became apparent. On later fill ups, I tried Lukas and Sea Foam additives, with no change. The knocking is erratic/intermitent, and not rhythmic like the knocking of a cold motor. If I push down on the accelerator on the flats at 70MPH, it is not present, or maybe there will be a light "click" or two. Sounds like just one cylinder or injector to me. It does sound like a typical diesel knock if I accelerate onto the highway before it has reached 180 degrees. OTOH, motor sounds great when throttling on and off at speed.

I have replaced both the primary and secondary fuel filters today with no change. The old primary filter was clear. Mostly, the pinging, (kind of a light rattle, like a gasoline engine with bad timing) comes and goes at steady speed. When it occurs the frequency is almost like the random sound of popcorn starting to pop. It goes away for 5-10 minutes, then starts up again. Not present at idle, or revving it while looking under the hood.

The car has always run very smoothly, and has never had this issue before. Oil changed before the trip looked fine. Valves adjusted 5K ago. I'm wondering if it's NOT the top end, yikes! Maybe a clogged injector? Or is it possible I really messed something up with the starting fluid? My bad. Any and all suggestions or ideas wouild be greatly appreciated.

Feel free to email me as I can check my Yahoo account via my cell phone. Currently in IN on I-80.

Thanks very much in advance.


P.S. On my last fill up after posting this, I did not use any Lukas additive. The knocking was worse, still varied in its timing, but more consistently there. Added some Lukas, and it's quieter now...In Illinois

Last edited by pjelliffe; 05-26-2007 at 05:53 PM.
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post #2 of 4 (permalink) Old 05-26-2007, 05:39 PM Thread Starter
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OK, figured out it's quieter when the revs are up--above 70mph. 55-60 is pretty bad. Doesn't seem to be any power loss, no smoke. Maybe injector pump needs attention? Anyone?
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post #3 of 4 (permalink) Old 05-29-2007, 10:49 PM
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Date registered: May 2006
Vehicle: 1976 300d Mercedes Benz
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Be Careful This Sounds Like A Repeat of What happend To Mines Og Motor go through and change all the fluids And Look Through It Just To Be Sure Mine Just Seized And Wouldn't Turn one Day When My Mom Went To Start it just clicked even With A Beaker Bar The Engine Just seized Solid And a new diesel engine was out lots of $$$ so just becareful and try not to drive it to much and too hard till it is fixed Just be careful sry i couldn't help more but check the timing, injectors,injector pump and Make sure You Are Using The Proper viscosity Oil it sounds like timing WAY advanced But Thats Just Me... Dont Know Too Much About Diesel engines

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Last edited by 76'benz-ford-chev; 05-29-2007 at 10:52 PM.
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post #4 of 4 (permalink) Old 05-30-2007, 10:32 PM
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Patrick have you tried biodiesel in the fuel? The cetane may be low.

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