The car had compression of around 175lbs on cylinders 1-5, and 115lbs on 6. The engine was rebuilt at around 210,000km and we suspected busted piston rings on no.6 cylinder.
But then the head gasket blew, and revealed corrosion and slight warpage on the head around cylinder 6 - probably because the head was not torqued down properly after the previous owner had rebuilt it.
The head has just been resurfaced and new valve seals fitted, plus one new valve on cylinder 6. The mechanic believes that this will have solved the compression problem, although it has not been tested.
After the lumpy idle and misfires, the mechanic found that two of the new (China-made) Bosch plugs that he fitted were oiled up. He wondered if he had a bad batch and replaced them with NGKs but I don't know what the temp range is. The idle seems a bit better now but I suspect it will revert to the original lumpiness soon.
I was wondering if ignition or electrics are faulty, causing erratic plug sparking? The lumpy idle occurs no matter how warm the engine is.
Does that help?
1977 W123 280E