Help wanted. 2 litre diesel 8P chassis. Driving along the other day. Loss of power. Limped home at 25 mph. No faults up. Been on VAG reader. No faults found. Starts fine. Ticks over with a slight surge, if revved to 1600 rpm approx surging increases. At that speed if you put your hand near the exhaust the pressure comes and goes. If revved hard there is excessive black smoke. All hoses seem to be intact. EGR valve appears to be working. It was fitted with a new throttle body servo motor last year and that seems to be working ok. Anybody got any ideas?