Rough running 300sd

[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.

[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.

[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.

[Oliver]

Thanks both of you for the suggestions, they are much appreciated. I haven't had much time outside of work to get a compression test done but that's still on my to-do list. I have watched lots of the Mercedes Source videos but I'll be sure to look for those ones in particular. Are they free on YouTube or will I have to pay?

[amilio]

The videos introducing the subject are free on YouTube, in depth guides as PDF's or videos are $5-20. Great source of parts, as well as PelicanParts. Highly recommended cross shopping.

I'm personally doubtful it's a compression issue, especially given your cap dance is good. I know you said the fuel system is clean, but did you replace the fuel filters or know how long they've been on the car for?

[spyboing]

Even though you've replaced one bad glow plug, it's worth checking all of them again. A single malfunctioning glow plug can cause rough starts and poor running until the engine warms up. Verify that all glow plugs are working properly and that the glow plug relay is functioning correctly.
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[Oliver]

I replaced the black filter between the lift pump and injection pump. I bought the small clear pre filter but haven't installed it because the existing filter looks really clean. I know I should also replace the tank pickup screen but haven't yet.

One thing I recently tried was cracking the injector lines while it idles to see which cylinder(s) are having trouble and the rear two don't seem to be firing when it's doing the rough idle.

I really don't think it's the glow plugs since the initial fire up is fine and to my knowledge that's what the glow plugs are for. Also it hasn't done the glow plug light blinking thing since replacing the bad one.

Thinking of redoing the valve adjustment even though I triple checked my work the first time I did them

[amilio]

Have you cleaned the check valve on the injector pump? Sounds like there may be some air in the system that is preventing those two injectors from getting fuel upon start-up. Was a quick job, took 20 minutes to take it off, take it apart, clean it, and put it back on. Depending on the wear, you may need a new ball bearing for inside the valve.