I never thought I would be typing these words, but after owning the car for two years (1998 P2 twinny) I have finally sorted my problem and the car is running properly for the first time. (fingers crossed - I hope it holds!)
OK start from cold, idle stable.
Once warmed up, erratic idle, especially after dipping the clutch (say while waiting at traffic lights)
Also when driving with 'feathered' revs, it felt like the engine was fighting with itself - surging, 'rising and falling power' feeling.
I had tried and replaced all the usual and the unusual:
- Airbox - checked many many times for leaks, none found.
- MAF - replaced with a Bosch new one, overall this improved things a bit, but core problem remained.
- Oxygen Sensor (Lambda) Only one on my P2, at first a poor trace on ECUDiag, so I replaced it, trace much better, and as with MAF things improved overall but core fault remained.
- Engine mounts, all checked, found to be ok so left alone
- Idle control valve - I bought 3x spare throttle bodies on Ebay, with ICV's attached, installed one of those*, still exactly the same.
- Engine temperature sensor and Thermostat replaced - still the same
- Exhaust checked for leaks/pinholes - none found
- Fuel Evaporation solenoid removed and tested - found to be working correctly.
- ECU changed! - bought a used ECU package on Ebay - swapped out - still the same!
Finally as part of a clutch overahaull, and while I had the throttle body out I decided to replace the ICV again with one of the 3x spares I bought, and BINGO! - car is a different machine. Seems like I must have installed a duff ICV at step * above, or at least one that was exhibiting the same erratic symptoms of my original one.
Now the idle and running is stable with no jiggy, jiggy when driving!
I have a feeling that the ICV, ECU, LAMBDA and MAF were all in a pitch battle against each other to reach their optimum settings, and were each affecting the others adversely, effectively creating a vicious circle of engine management.
It is this type of ICV: