I've got an opportunity to buy an inexpensive 300SD with 345 on the clock. When I started it, the engine has a "miss" and smokes. But it smooths out at a higher rev. A buddy suggested that it might be a leaky injector, which makes sense to me. Anyone here have some other ideas to check out? also, were the trunk lids aluminum? This car has great paint, but the vertical end of the trunk lid is 'crusty'. Looks like aluminum. I've not put a magnet to it yet as I don't have the car.

First thing I would check would be the valve adjustment, that cleared up my 300SD's miss when I did them. The trunk lid and hood are both aluminum.

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1971 R600 Mack toterhome (SuperDawg)
99 NU-WA Snowbird 34+2 SE 102 5er
If it's not diesel, it's not in my stable!
53 F100 617.951/auto
95 Dodge 3500 Cummins/5 speed service truck

thanks. I completely forgot about valves. I bent wrenches to do them, but still need to grind the sides a bit to get better lateral movement to tighten and loosen. Oh, my back hurts already from thinking about leaning in there. LOL

Also just running it helped mine clear up and run good. Both mine idle aroud 500 so I need to increase that to 700 and that should also help.
Also a flush of diesel purge.
Any air in the lines?

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83 300SD Dark Silver Dark brown inside

Do the things that do not cost much: As said before Valve adjustment, change filters and change the fuel hoses (eliminate air leaks). If still rough pull the injectors and check or rebuild them and try adjusting the Governor Idle Pin or replacing it.
If it is running OK but shaking look at the Motor and Transmission mounts and Engine Damper/Shocks.
While the valve cover is off line up the Camshaft Drive Gear timing mark to the one on the Camshaft Bearing Tower and take a look at the pointer on the Crankshaft Damper/Pulley and see what timing mark it points to. This can give you a rough idea as to the Timing Chain Stretch.
This is a split picture; Camshaft Timing Gear on the right; Crankshaft Pulley on the left but it is hard to read the degrees on the pic.

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84 300D, 82 Volvo 244Gl Diesel

As 911 mentioned.

However, with 350K on the clock, the condition of the valves will be heavily based upon maintenance. If nobody did the valves in 100K miles, there is a good chance that one of them is not seating correctly and one cylinder is not performing........and hence the rough idle. It really doesn't smooth out at higher revs..........you just cannot see the problem due to the engine speed.

In such a condition, adjusting the valves to spec won't cure the problem and the head needs valves. I'd be very doubtful that an engine with 350K could run well without valves unless the maintenance has been done regularly.

But, hope for the best.........maybe some inexpensive assistance will accomplish the goal.