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Rough running 300sd

Started by Oliver, 01 July 2024, 07:52 PM

Oliver

Hi all, I'm new to the forum and owning classic Mercedes. I've had my 1980 300sd for a couple of months now and I'm loving the car, great overall condition, drives beautifully. But I'm having a hard time diagnosing and fixing a problem with the engine. On cold start it runs very rough, I'm guessing a couple cylinders aren't firing. It makes a sizable cloud of blueish gray smoke and will sometimes even stall. After it gets warms up with some driving it runs better but still a bit rough at idle. Under power it runs great, smooth and surprisingly peppy, a little black smoke if I floor it, different than the cold start smoke. So far I've replaced one bad glow plug, done a diesel purge (fuel system seemed to already have been quite clean), oil change, valve adjustment, and recently some rebuilt injectors from eBay. The injectors definitely helped it feel more powerful under acceleration but if anything they've made the idle worse. With all this in mind, does anyone have any thoughts on what the issue (or issues) could be? Oh and the car is at just over 200k miles. Thanks

daantjie

Compression test I would think is needed.
Daniel
1977 450 SEL 6.9 - Astralsilber

Oliver

Yes I agree and I will try to do that soon. It is worth noting that it passes the 'cap dance test' with flying colors, little to no cap movement. Do these engines ever have problems with valves not sealing?

daantjie

Check out mercedessource videos on YouTube. He has tons of videos on diesels and not too long ago there was a whole series on a rough running diesel.
Daniel
1977 450 SEL 6.9 - Astralsilber

amilio

I would check your injection pump for two issues; one is a check valve that can get gunked up or corroded and let fuel leak back through when left for a long time, which may explain the rough cold starts. Also, and this is not usually on W116 SD pumps but mine had one, but the idle adjustment bolt may be worn out and not moderating the start up mixture correctly. As noted above, Ken from Mercedesource has videos on both these issues. You may also want to look at your throttle linkages and ensure they are all in spec in terms of length; if they've stretched over the years your idle may just be a little too low now.
1979 300 SD "Liesel", 282k Miles