Thanks for all the replies.
Firstly, Steve. I hear what you are saying about the effect of oil on the hoses. That's why I flushed the cooling system and am keeping an eye on it, in addition to monitoring a possible head gasket problem. Further to this, on the weekend I had a long hard look at the motor and as best I can tell it looks like it may even be a newish head gasket. Possibly it had failed and been replaced without all the oil being flushed from the cooling system. The top of the head under the valve (rocker) cover also looks very clean. Regarding the noise of a belt idler damper, I know that one, did one not long ago on my M103 together with the tensioner. Receipts with this car show the belt tensioner to have been done not long ago. The noise in the M102 definitely sounds like the regular tick-tick of a lifter to me. Its repetition rate is directly proportional engine speed. Only comes and goes during warm-up. Once hot it never appears. Should I worry or not? How would I determine which rocker is the noisy one anyway? Also, should I worry about timing chain and/or guide rails etc. at 212,000km? There seems to be different thoughts on the M102 with double row chain (and on the chain in my M103 with 175,000km for that matter).
Russ and John. I too was of the impression the M102 (like the M103) was pretty tough. My wife's parents have a 1983 W123 230E with the M102. At around 250,000km it has needed nothing other than a timing chain and a water pump. Not even valve stem seals or a head gasket. Theirs has a single row chain whereas mine is double row. Owning both an M102 and M103 which are virtually 4 and 6 cylinder versions of the same motor, it is interesting to compare them especially when you realise both cars (190E & 300TE) have similar power to weight ratios. If the 102 was a 2.0 rather than the 2.3 it would share the bore and stroke of the 103 (mine only share stroke). They do differ somewhat in characteristic. The 4 is very torquey and is happy to lug at low revs and makes good use or the tall 5-speed gearbox. In the middle to upper rev range it is a little coarse. Only approaching redline does it become smooth again. The 6 on the other hand is turbine smooth. Not much down low but strong and willing to rev smoothly into the red if you let it. This is reflected on paper too with the 103 producing its peak torque and power at higher revs than the 102.
Russ. Regarding the 190E in the Sportline, I love it. I can only compare it to a 1992 180E (190E 1.8) manual 5-speed I owned 5 years ago. The 2.3 seems to have a nicer gear change than the 1.8 and is much more flexible with the bigger engine. Both have overly long cluth pedal travel. The ride is not as bad as I would have expected reading the comments of others, only getting choppy on badly broken surfaces. Cornering is very flat and adhesion limits high on the 205/55s on the 7" rims. The 2.3 is also quieter than the 1.8 which lacked some of the sound insulation of the 2.3.
Thanks again for the replies and I am still open to suggestions from anyone else. In the meantime I will monitor the situation and continue enjoying the 5-speed Sportline.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.