Thanks for the further help guys -
Well, the car doesn't run over 100 degrees ever, and only pings once it's running at its normal ~85 degrees. But the problem did first come on in hot weather - coincidence?

I do burn a heck of a lot of oil. Do you know how the carbon deposits on the valves causes pinging, or how the deposists are caused?

Barry - I see the ignition module, with 2 plugs. The larger is the Electronic Ignition Control Unit Plug, and the other is like the ABS plug in my final picture, so must be for some low voltage signal.
There's no resistors there, and no switch for the octane adjustment.

For the record, here's the resistances and positions as shown by Bosch's ESI CD, for anyone wanting to adjust or play who doesn't have the switch:

EZL EZL-ECE (no catalyst) EZL-KAT (catalyst cars)
1...........S.............................1....... = infinite resistance
2...........2.............................2....... = 2.4 kohms
3...........N.............................3....... = 1.3 kohms
4...........4.............................S....... = 750 ohms
5...........5.............................5....... = 470 ohms
6...........6.............................N....... = 220 ohms
7...........7.............................7....... = 0 ohms (short-circuit)

S = for Super-unleaded hi-octane fuel
P = Premiem fuel

On all switches the resistance values are the same, only the markings of numbers or S's and N's are different. With a catalyst if using Super fuel, you need 750 ohm R16 resistor or switch in 'S'. Without a catalyst you need no R16 resistor or switch in 'S'

later. and please keep helping..!

__________________
190E's:
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black 2nd owner
http://www.maylane.demon.co.uk/190esmall.jpghttp://www.maylane.demon.co.uk/190esmall2.jpg