Hi

Our BKD engine is going into limp and showing overboost. I've read lots about it and have a few pointers to look at, but reading them made me realise I'd perhaps completely misunderstood how the turbo and ECU interact. What I want to do here is write what I now think happens, as it will help me (and maybe others) understand the context of some of the advice given, and hence spot when some generic or misinformed views on the general internet may be wrong and misleading.

Originally I thought the turbo worked by springing into life during acceleration to boost air pressure, and the infamous sticky vanes were "activated" to achieve this. But I couldn't understand how lazy / sticky vanes would then cause too much pressure (overboost) rather than too little. After further thought and reading I now think this happens (and would like to be corrected if not )...

  1. The turbo is driven by exhaust gasses.
  2. As the engine revs pick up more gas is whizzing through, and at a certain limit the turbo starts to spin.
  3. That "hot" exhaust side is connected to the "cold" air intake side, so in turn the impellor on that side pushes more air into the engine intake manifold, allowing more fuel to also be added to get the required power increase.
  4. The amount of air passing is measured by the MAF /AVM on top of the engine, and the ECU wants to regulate this to keep things in workable boundaries.
  5. Regulation is achieved by adjusting the vanes in the exhaust side so that they are "less active", hence reducing the rotation speed of turbo on both hot and cold sides.


In a fully operable turbo:
  1. The vane angle / engagement is achieved by an rod that passes from inside the turbo to a mechanical actuator on the outside
  2. The actuator is controlled by a vacuum line, and "at rest" the actuator has no vacuum applied
  3. The vacuum line runs to a vacuum controller termed N75
  4. On many VAG engines N75 is a discrete component (of differing part number depending on physical orientation of vacuum lines, etc), and there is a similar N18 for the EGR valve
  5. On BKD and some others the N75, N18 and others are combined into a single vacuum control unit behind the battery
  6. The ECU signals a desired vacuum to the N75 to regulate the vanes, and this is an increasing vacuum to "retract" the vanes
  7. The ECU measures the air inflow to make sure it is reducing to the required level


Where things start to go wrong:
  1. If there is a leak in the air intake pipework before the turbo it will draw air in that doesn't pass the MAF / AVM. The ECU will detect this as an underboost.
  2. If there is a leak in the air intake pipework between turbo and engine it will expel boosted air. ECU can't measure this, but running may seem odd due to fuel / air imbalance.
  3. MAF / AVM can be giving false reading. They never over-read, so this will only cause underboost error. [??? Is this true? Did I read about a contaminated one over-reading and triggering overboost???]
  4. If turbo vanes cannot be adjusted as much as required they will be "over active" and there will be too much air compared with what ECU wants. Actual air is measured by MAF / AVM, and when the requested vs. actual difference exceeds a limit (150 units?) overboost error is triggered.


Things that prevent turbo vanes moving correctly to adjust airflow, in turbo-N75 order:
  1. Vanes themselves are sticky, and won't move as far as needed. Very common. Full strip and clean or Mr Muscle treatment may be appropriate.
  2. Actuator rod is crudded / sticky. Not often mentioned as root cause. Rub down with wirewool and oil may restore.
  3. Actuator is failing due to vacuum leak. Hardly ever mentioned, and is not a replaceable part without full turbo.
  4. Hose from N75 to actuator has a leak, so can't pull as far as needed. Quite common. Easy to replace so worth trying.
  5. N75 unit is dodgy. Quite common when individual unit, far less so in combined BKD style.
  6. Central vacuum reservoir has fault.


So diagnosis paths with same numbering:
  1. Get access to turbo and measure ease of moving actuator. Even better hook up a Mitivac or similar and see if full travel happens between 5"Hg and 20"Hg (must get mbar equivalents). Not sure if a large enough syringe should be able to do this.
  2. Same as 1 really.
  3. Pull vacuum on actuator with Mitivac, syringe or mouth. See if it holds. Also blow firmly through pipe attached to actuator to see if that leaks.
  4. Just replace it, or use an additional length of hose as a stethoscope to listen for hiss of leaks.
  5. Measure input and output vacuum pressure with guage. Should be ~22"Hg on input and ~20"Hg on output.
  6. As 5


Is that about right?