On the off chance that this proves useful to someone. Before my 1.4 petrol A2 fell into my lap, I owned a 2002 1.4L 75bhp TDI AMF with < 100k on the clock. It lacked aircon and suffered badly from paintwork eczema (lacquer peel - non-metallic red finish). My lovely wife never took to it but I very much liked the brute 
It got embarrassing when it started producing a white smokescreen on winter mornings as I was driving off to work. It would still start straight away, felt a little sluggish/hesitant pulling away - sometimes I had to give it extra revs to get going. The smell of the smoke was unmistakably unburnt fuel. (Oil consumption was normal btw, it hardly needed topping up in between services.). The smoking would stop very soon - always less than a minute down the road, sometimes literally seconds later, and it would start running normally. It would do it again at the end of the day in works car park. The problem was also semi-intermittent. Some mornings, no matter how cold, it would start without smoke and sluggishness. On warmer days (+10 C ish) it would sometimes smoke, sometimes not. The MPG did not seem to be affected, always around 53-55 in winter (60 - 65 in summer). I checked the glow plugs and they were fine. I tried diesel cleaner addtive and that made no difference. My mechanic friend suggested replacing EGR valve but it made no difference. Diesel Care Services in Findon said it was 99% certain that the injector wiring harness needed replacing but that also turned out to be a waste of time and money.. In spring when the average temperatures rose the problem was no longer apparent. I had to decide whether to keep the TDI or the 1.4 petrol and these troubles tipped the balance. The TDI was sold without ever getting to the bottom of this problem.
Now I've had to deal with a coolant temperature sensor fault in the 1.4 petrol and it reminded me that the TDI also had something very similar. The dashboard temperature gauge would sometimes start reading nonsense - either stone cold, or below 90 degrees. This would be more common on a fast dual carriageway run but could happen any time. Sometimes normal service would be restored by itself after a while without switching the engine off, and pretty much guaranteed after a restart. I used to turn the engine off at the lights and turn it on again and the temperature gauge would instantly return to vertical for 90 degrees. I don't know if the coolant temperature sensor drives the dashboard gauge directly, or via the ECU. Either the vibration of engine starting was reliably restoring circuit continuity, or restarting the engine was causing the ECU to start taking the coolant temperature sensor signal seriously again
I recently had similar coolant temperature gauge symptoms in my 1.4 petrol - intermittent dropping out or below-actual reading in the dashboard temperature gauge. Eventual return to normal by itself or with a little help by tugging the coolant temperature sensor connector wires. The car would occasionally become difficult to restart with engine not completely cooled down. It turned out that the coolant sensor pins were too thin for the connector, resulting in poor contact that was affected by vibration. A replacement part had visibly much thicker pins and has cured all the symptoms.
So I now wonder if I could have saved myself £300-400 by treating temperature gauge symptoms and white smoke as connected symptoms. The temperature sensor is a 4-wire type, providing separate temperature signals to dashboard gauge and to the ECU. It stands to reason that if the dashboard signal is failing, then likely the one to the ECU is similarly affected. If the dashboard feed produces spurious readings, probably the one to the ECU does also. Perhaps what was happening with the TDI was this - the coolant temperature sensor was reading so far off that it caused the ECU to over-fuel at start. This would then quickly sort itself out as vibration would tend to restore / improve contact (or ECU became convinced by the amount of garbage coming in from the sensor that it should just ignore it altogether and assume a default temperature). Occasionally the reading from the coolant temperature sensor was probably OK at starting, resulting in normal starting with no symptoms.
I will never know now
It got embarrassing when it started producing a white smokescreen on winter mornings as I was driving off to work. It would still start straight away, felt a little sluggish/hesitant pulling away - sometimes I had to give it extra revs to get going. The smell of the smoke was unmistakably unburnt fuel. (Oil consumption was normal btw, it hardly needed topping up in between services.). The smoking would stop very soon - always less than a minute down the road, sometimes literally seconds later, and it would start running normally. It would do it again at the end of the day in works car park. The problem was also semi-intermittent. Some mornings, no matter how cold, it would start without smoke and sluggishness. On warmer days (+10 C ish) it would sometimes smoke, sometimes not. The MPG did not seem to be affected, always around 53-55 in winter (60 - 65 in summer). I checked the glow plugs and they were fine. I tried diesel cleaner addtive and that made no difference. My mechanic friend suggested replacing EGR valve but it made no difference. Diesel Care Services in Findon said it was 99% certain that the injector wiring harness needed replacing but that also turned out to be a waste of time and money.. In spring when the average temperatures rose the problem was no longer apparent. I had to decide whether to keep the TDI or the 1.4 petrol and these troubles tipped the balance. The TDI was sold without ever getting to the bottom of this problem.
Now I've had to deal with a coolant temperature sensor fault in the 1.4 petrol and it reminded me that the TDI also had something very similar. The dashboard temperature gauge would sometimes start reading nonsense - either stone cold, or below 90 degrees. This would be more common on a fast dual carriageway run but could happen any time. Sometimes normal service would be restored by itself after a while without switching the engine off, and pretty much guaranteed after a restart. I used to turn the engine off at the lights and turn it on again and the temperature gauge would instantly return to vertical for 90 degrees. I don't know if the coolant temperature sensor drives the dashboard gauge directly, or via the ECU. Either the vibration of engine starting was reliably restoring circuit continuity, or restarting the engine was causing the ECU to start taking the coolant temperature sensor signal seriously again
I recently had similar coolant temperature gauge symptoms in my 1.4 petrol - intermittent dropping out or below-actual reading in the dashboard temperature gauge. Eventual return to normal by itself or with a little help by tugging the coolant temperature sensor connector wires. The car would occasionally become difficult to restart with engine not completely cooled down. It turned out that the coolant sensor pins were too thin for the connector, resulting in poor contact that was affected by vibration. A replacement part had visibly much thicker pins and has cured all the symptoms.
So I now wonder if I could have saved myself £300-400 by treating temperature gauge symptoms and white smoke as connected symptoms. The temperature sensor is a 4-wire type, providing separate temperature signals to dashboard gauge and to the ECU. It stands to reason that if the dashboard signal is failing, then likely the one to the ECU is similarly affected. If the dashboard feed produces spurious readings, probably the one to the ECU does also. Perhaps what was happening with the TDI was this - the coolant temperature sensor was reading so far off that it caused the ECU to over-fuel at start. This would then quickly sort itself out as vibration would tend to restore / improve contact (or ECU became convinced by the amount of garbage coming in from the sensor that it should just ignore it altogether and assume a default temperature). Occasionally the reading from the coolant temperature sensor was probably OK at starting, resulting in normal starting with no symptoms.
I will never know now
