[Non Ford] Intermittent Rough / Sluggish Running

[Citroënbender]

Got a car that’s doing my head in.

Alfa 2 litre “Twin Spark” 147 hatch that intermittently runs terribly and is then weak, almost stumbling, on light throttle. 246K on the chassis, a bit less on the motor.

Number 3 cylinder seems to be the culprit, based on its plugs. It has headers with 1+4 and 2+3 and separate primary oxygen sensors. The 2+3 sensor is reading over 3V when at stabilised operating temperature. This flags a fault that doesn’t illuminate the engine light.

New coils
New correct NGK plugs
ECU tested by substitution
Fuel pump (genuine Bosch from reputed vendor) six months old
Full tank of 98
Hot compression 205-220 PSI on all cylinders

On #3 I have tried used plugs of good colour. I have swapped in a spare injector. I have swapped good running plug boots/leads from my other car with the same engine.

Wiring checked
Charging voltage checked
Throttle percentage seems to correlate and doesn’t misread (glitch) in live data. Clutch is not slipping.

At road speed and an indicated 8-12% throttle the effect (when it occurs) is of losing power as though compression braking - but a shade rougher. Instantaneous fuel consumption rises heavily. At idle, if it’s “having a moment” it will rattle and shake heavily - like when the Spirit of Tasmania is berthing. Yet it almost seems that briefly flicking off any of the coil connectors worsens this behaviour.

When you blip the throttle, you can see the intake trunking flex in response to throttle blade movement, but there is a small lag in engine response - a “soft” response as though either spark timing is wrong or fuelling isn’t adjusted quickly to compensate.

What I haven’t done (car is out on loan again):
Substitute tested lambda sensor
Leakdown testing
Fuel rail pressure check (odd fitting like an AC valve)
Vacuum read

I’ve also googled heavily, to no avail.

Any suggestions for diagnosis? When it’s running well, it’s a high sevens per hundred car at freeway speeds.