First: General car buying advice
The Alfa 166 represents very good value for money on the second-hand market, and they are a very nice car to drive, but the car never quite got out of the Alfa 164's shadow, the 166 shares the same floor pan as the 164 but there are many changes to the suspension which improve ride and road holding, as with any car it is necessary to check for accident damage evidence of this can be seen in the poor fit of panels, overspray on the trims around the doors and windscreens as this is also an area where accident damage will show, there may also be paint chipped off various panel fastenings or nuts and bolts and the colour match between panels as this is another area that garages make mistakes that can help tell you if the car has had an accident. If the car has had paintwork done it is necessary to find out if it was a superficial accident or if it was a more major incident, this can be done by checking the welds and trim behind the repair for signs of damage.
Alfa 166's are reliable cars and if treated well and will do two hundred thousand miles but it is necessary to keep them in top condition as they will deteriorate rapidly if neglected, this is a very important point when you are going to view a car. All Alfa Romeo engines use oil and the V6 can use up to 1 litre per 1,000 miles and this is normal, consequently when you go view a car check the oil level and if it is below the minimum the car has been neglected and this can cause major engine damage so it is probably best to walk away. All 166's have a recommended cam belts (Cam Belt Change Offer) change interval of 72,000 miles, however, since 13th November 2006 Alfa Romeo recommend that the belts should be replaced at 36,000 miles on 4 cylinder petrol engines (we recommend changing the belt at 60,000 on the V6) this is a very expensive job costing between £400 and £700 depending on which engine is fitted, if the car has not had the belt changed use this a bargaining point and get this cost deducted from the price and get the belt changed on the 2.5 V6 it is best to change the water pump as they have plastic impellers which crack causing the engine to overheat at speed. Another common problem is the air flow meter (MAF Diagnosis Guide) this will usually manifest itself as a lack of power higher in the rev range and an unwillingness to rev a new airflow meter costs from £117 to £175 depending on where you get it from.
Clutches and gearboxes are very reliable, a worn clutch will tend to bite at the top of its travel and as they are self-adjusting a high biting point signifies that a new clutch will be needed soon, gearboxes are generally trouble-free but do check that all gears engage smoothly without crunching. The Auto box does not usually give any trouble but I find the ratios too widely spaced for the 2.5V6 and it only really suits the torque of the 3.0l V6.
The suspension on the 166 is double wishbones on the front with a high upper wishbone, this wishbone tends to wear out and it is worth checking it for play, it may also creak and this sounds like it is coming from the dashboard, The rear suspension is a multi-link design and does not cause any major problems however on high mileage cars it would be prudent to check all the bushes.
This is a brief outline of what to look for in an Alfa 166, once you have decided that the car is suitable for your needs it is worth getting a specialist on the marque to check the car over for you, most of the UK specialists will do this for a very reasonable fee especially if you can get the vendor to take the car to them.