spotted this at the bottom of the forum with no replies so thought I'd give it a go - at least it'll count as a thread bump for you.
Diesels are pretty good at showing you what they're doing, so....
black smoke - that would suggest your car is running rather rich. If you've ever seen a diesel running down the road and the driver boots it and it heaves out a heap of black smoke, you'll have spotted an over-fueling diesel. Audi's are really good at this. They show it after remapping particularly as a basic diesel remap or worse still resistor chips just pile on the fuel pressure and throw more fuel through the motor for more power. that has the affect of riching up the mixture and making it black out back.
First and simple question - does your car have a remap?
Second, have you replaced any of the boost pipes (theres only two)? if you've got a boost leak and are losing air this will turn the mixture rich and you'll get more smoke. If you've not experienced a boost leak before it's hard to listen out for but if you have had one you'll recognise the noise in an instant. A smaller boost leak would make the car feel a bit flat and smoke but you wouldn't always get an error with it. Only when you start to get more major leaks would the thing start to buck and cough and flash lights at you.
There is potential for the MAF to cause fueling issues although my experience is that as MAFs fail they go lean not rich.
White / blue smoke is different. Pure white tends to mean more that the head gasket has gone and is the steam created by trying to burn the coolant mix. That's not so common on diesels as it is petrols, I wouldn't consider that too much. The compression a diesel runs I'd expect you to blow caps and hoses off pretty quick if you were pressurising the coolant system through a blown head gasket/warped head or whatever.
Blue smoke means oil burning - either failed piston rings (more of a sign for an N/A engine) or on a turbo motor it is more likely the turbo bearings / seals are leaking oil into the combustion mix / exhaust.
Next suspect is, unfortunately, your turbo. What the mechanic said about oil on the undertray just isn't sound advice. Both the hot (exhaust turbine) and cold (air compressor) side of the turbo are sealed systems and turbos leak inward not outwards, so you wouldn't really see any evidence of anything on the outside or around the engine bay unless you've cracked the turbo housing and I've only ever seen that done once.
99.9% of all turbo failures are due to the internal bearings on the shaft which connects the compressor to the turbine. These are oil fed by the motor to keep them well lubricated. Turbos spin upto around 200,000rpm in some cases, so they have to be well fed. Eventually these bearings fail and develop play in them, which then lets the oil from the lubricant circuit through the bearings / seals and into the turbo. This can mix with the hot exhaust gases and cause whitey bluish smoke out the back or if it gets added to the air intake side it can go as far as to actually feed the engine leading to whats known as a run-away.
A run-away is when the oil / air mixture resulting from a turbo failure is pumped back into the engine and combusts to essentially fuel the engine. This then turns the motor, which then pumps the exhaust across the turbo and it becomes a self sustaining reaction that can literally explode the motor. The motor will continue to run, even with the ignition off, simply fed by its own oil.
Blue smoke is typical symptom of oil being burnt, so I would be suspicious of the turbo.
Best way to check your turbo is on the cold side (when the engine is off and cold - don't do it when the engine is hot or even warm, you'll just burn yourself). Take the pipe off the turbo, reach in and wiggle the compressor / turbine shaft. You may get a very small amount of push pull motion on it but if you can move the turbine side to side or up and down at all - its broke.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso