I had a problem with very rough idle one my 380SL and could not isolate the problem until I saw a Northern Equipment ad one day that had a special price on an Infrared Thermometer with a laser pointer. It measures temperatures without having to touch the object being measured. Bought one and after warming up my rough-running engine I measured the exhaust manifold temperatures of all 8 cylinders as close to the cylinder heads as I could/ I found that not one but TWO cylinders were running about 180 degrees cooler that the others. I had already checked the compression on all cylinders and had found them all fairly close together, so I decided that it must be fuel injection related. I cracked the pipes where they connected to the fuel injector nozzles and found gas was arriving there, so I ordered two new fuel injectors and the associated rubber bushings. After changing the injectors I now have all 8 cylinders running smoothly again! That Infrared Thermometer saved me a BUNCH of money!
Hauptman Udo
1985 380SL
2005 E320 CDI

A modern update of an old technique; when an unusual RPM drop is noted while performing aircraft engine mag checks, a grease pencil was (and still is) employed to find the cool cylinder(s).

Yes and amen, cheapest cylinder test you can do other than listening to them with a stethascope, wait...how much is a stethascope?

I keep my thermometer in my trunk box, mostly for ac work but its also handy testing to see if you've successfully decreased resistance in the blower circuit on your 80s HVAC system, ohm tests aside.

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg

This method is not really new when my boss was in the military he was a tank mechanic and they tested it the same way for misfiring. On your 380SL it has a diagnosis plug and with the tool from Mercedes it could be detected right away. The computer controlled cars now the computer tells you exactly.