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