Got to the root-cause. Two DTC's stored, one was P0170 fuel trim malfunction, the other was P0115 Coolant temperature circuit malfunction. After replacing the CTS and other maintenance, the engine temp is a lot slower to rise, doesn't get higher than about 84-86 degrees (thermostat and cooling system working well), the fuel mix isn't as lean, all the jolting, stalling and low power from idle has pretty much disappeared. All that remains in handling issues is a little blip in the throttle at around 1000 rpm. This is always at the same point on the pedal, so putting this down to a poor sensor on the pedal or poor connection. Will get to this soon.
Just to explain the poor running, for anyone else having this problem:
- OK when cold
- Temp would rise quickly
- During this rise, the car would lose power at low revs
- Revs would dip below idle then a short struggle to return to idle
- ECU could not manage the throttle when using the clutch alone, I would have to rev to move away from junctions
- Low power at idle when hot, this would cause a more 'chugging' idle which would disrupt the oil pumping, reducing the pressure between pump max pressure to the point where the oil indicator light would flicker
- The engine would run lean, so perfectly fine at high revs, just rubbish at low revs
- The revs would drop fast when heating up so caused jolting on deceleration
- The power at lower revs on increase would suddenly kick in with the airflow, this would result in a jolt on acceleration, so longer, smoother clutch control needed in gear changes to avoid breaking a mount
- The engine management would struggle to correct the air fuel mix, so P0170 fuel trim malfunction would be raised
- P0115 Engine Coolant Circuit Malfunction was eventually raised
- No problems whatsoever starting
- No ignition overrun (usually indicates over-fueling if it continues to turn over when ignition is off)
- Problem could be intermittent, though more often than not. Sometimes it would run for up to three out of four, six mile journeys through the day with little or sometimes no problem, this was less often though
- Disconnecting the pre cat O2/lambda sensor doesn't help (don't try this for too long or you could end up ruining your cat - meow - it could get fat and not do what it's meant to - scavenge)
This isn't a complete, nor correct symptom list, but something that you might consider. Cheapest, simplest route first: a CTS will cost you around £10; an O2 sensor will cost you at least £60 and is often the blame for many underlying issues and not in itself faulty.
There are a few other causes to contribute to poor running, most of which I've resolved now but some I still need to work on. At least it has earned itself a further two months lease on being owned by me. It's a fun car to drive when it works well, cheap too. I would like it to be good enough to sell cheap to someone else to get another year or two out of it.