While it might look cool, smoke is actually just unburned fuel. Smoke means you aren’t taking advantage of all the power and efficiency of your TDI. If your TDI smokes under medium or heavy acceleration, and/or feels underpowered at low RPMs, here is a list of things to check.—
Faulty Turbocharger Actuator – the VNT mechanism inside the turbocharger is operated via a vacuum-controlled actuator. The VNT is most heavily used during acceleration at low RPMs. Unfortunately this actuator can get stuck, leaving the VNT mechanism unable to move.
Faulty Vacuum Modulator/N75 Valve – the vacuum modulator (called the N75 valve in VW/Audi world) controls the vacuum to the turbocharger actuator. If you have confirmed that the actuator is healthy, the N75 is the next most likely culprit. The N75 valve fails slowly over time, gradually reducing the power your engine makes, so you might not necessarily notice it has gone bad – that is until you drive a car with a healthy N75 and you’ll wonder how much power you’re missing!
Vacuum Hose Leaks – sometimes the simplest solutions are the best, and the entire vacuum system will be affected if the vacuum hoses have leaks. Think about replacing vacuum hoses at regular intervals, like timing belt replacements, to limit the chances of vacuum leaks.
Leaking Intercooler Hoses – intercooler hoses, or boost hoses, are designed to expand and contract as boost pressure increases and decreases. However, over time and thousands of miles and heat cycles, the hoses get brittle and can split open. They most often split in places that aren’t visible, like inside an elbow bend. Since the hoses expand and contract, the crack may be hidden when the hose is not under pressure, only to open up once pressure builds. Check especially around connections as newer style hoses use metal clips and o-rings to seal, and new intercooler hose seals may be all you need!
Faulty Check Valve – the vacuum system is divided up by a series of one-way check valves that prevent one part of the vacuum system, like the brakes, from robbing the rest of the system. The ALH has one white-and-black check valve between the N75 loop and the rests of the system. If this valve is faulty turbocharger boost pressure may be unreliable. Moreover, most vehicles have check valves built into the main line between the vacuum pump and brake booster. If this valve is faulty or clogged it will prevent vacuum from reaching the rest of the system.