In the last few days my 2001 audi a4 1.9 t diesel has seemed to be down on power once reaching working temp ( about 3 miles )
Left over night it is back to normal then when temp up it looses power, 50 on a straight is about max. Any ideas on what to look for? Seems as if the turbo does not work when heated up.
Regards, John

An air leak somewhere would be my first thought. As it's when up to temp perhaps something swelling with temperature is leaking. I'd start by checking the air hoses around the turbo first for splits or maybe looseness. The turbo itself can fail and need rebuild/replacement, but I suspect you'd need a garage to diagnose that more effectively.

A work mate of mine had similar on a Passat tdi but it would start and run fine on a cold engine and as soon as it got warm wouldn't have any go and then if turned off it wouldn't start again until cool. It turned out to be the water pump impellers has disolved as they have a tendency to as the anti freeze eats them over time and there only plastic. I can't remember why it didn't show up on the temp gauge but I'm fairly sure that it just read normal but can't remember why but a new water pump (and cambelt while he was at it) and that solved the problem. The car was a 2001 Passat tdi pd130 if that's any help

Try the airflow meter very common problem on early Passat

I would also go for airflow sensor. Very easy to test. Disconnect. Drive. Connect. Drive. Any difference apart from the warning light.
(Reason they often don't give a warning light if connected but failed is the reading may still be within normal limits. )
Could also be engine temp sender for ECU. Most of the 1.9 VAG diesels have separate circuits and senders for gauge and ECU but combined into one unit. One sender in the double unit can fail whilst the other still works fine.

[Edited on 9/9/16 by Dusty]

The age of the car is right for the waterpump issue VW found Nylon 66 wasn't a good choice as a water pump impeller material as it gradually became loose on the shaft. A faulty MAF will severely restrict the power but I don't think it is a MAF issues as that would normally not account for the symptoms changing as the engine warms up. Disconnecting the MAF works as a test on some engine types but not others MAF output can checked directly and MAFs can be cleaned
VW are very good for plug in diagnostics so I would want to connect a VW specific scan tool and look at any codes and live data before doing anything.

I'd check the obd for codes, sounds like limp mode to me.