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Vintage Cars/1983 300(T)D W123 Rough Idle


My 300TD shakes badly on idle, and it appears that there may be something specific going on that causes the engine to buck, but what that something specific is, I don't know. I wonder if you can help me diagnose it. The odometer has 260,000 miles on it, and I don't know if this is the original engine, but I suppose it is. First, I have done these diagnostic/maintenance steps:

1. Clean fuel tank, make sure no fuel restriction.
2. Adjust valves.
3. Change fuel filters, make sure clog is not starving engine of fuel.
4. 2 applications of Lucas Upper Cylinder Cleaner with Injector Cleaner
4. Ran 2 cans of Liqui Moly Diesel Purge through engine using fuel system bypass method.

While doing the Diesel Purge, I captured audio of the engine sounds, which are distinctive. A vicious tick often occurs on a period of about 2/3rds of a second, making the engine shudder. What do you think is going on? I should mention that the engine runs smoothly and knock-free if I just increase idle speed a little bit, and it sounds good and runs relatively smoke free at normal speeds.

Could you have a listen to the audio file, below, and give an opinion of what it might be or what a next step might be? Thanks!

Webb Mealy

I listened to the noise, I would look immediately at your timing chain and measure for stretch.  Sounds like chain slap , ignored can be catastrophic.

Could also be an injector that is not pulsing, or injection pump.  Use a mechanics stethoscope and listen over each area I have mentioned  

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I can answer questions on W110/W111/ W114/ W115 /W(R)107/W123/W126 , and Finny Mercedes models 1962-1985. I have rebuilt many of these cars and have problem solved many issues. I have access to much of the technical data and have detailed instructions on fixing the most common issues. Vintage US 1950-1989 and other foreign cars as well If you are going to ask me about car value, please provide ample information on service history, rust issues, and current "needs". Pictures are helpful. Disclaimer on value - I can only give you data on what the market is bringing, and may not be what you want to hear!


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