On Board Diagnostics Related to Volkswagen Turbo Engine Codes
Volkswagen on board diagnostics (OBD) is a second generation system. There are several engine lights that might light when system is out of spec. Of course, it depends on whether the car is a a diesel or gas and what year. Most VW's made after 2001 use the these. This hub specifically deals with the VW TDI, a diesel car.
There several engine lights that may indicate a malfunction: the Glow plug, an Electric Power Control (EPC), the Malfunction Indicator Light (MIL), and Fuel Cap. Whenever these are lit, something is wrong and an associated engine code is recorded and retrieved by using a computer tool. Generally, the lights either flash or are steady. All codes start with a letter: P = powertrain, U = vehicle communications, B = body, C = chassis. The third digit further defines the general malfunction area, while digits 4 and 5 indicate precise components.
The engine lights can reset by themselves if the issue is resolved for whatever reason. If so, they will turn off. When one of the lights are lit, there could one or many codes recorded indicating issues that triggered it. Since many situation could trigger them to light, repairing the situation does not mean the light will not come on again in another day for a different reason.
With a turbo, there are eight codes (P0234 - P0246) that can trigger the MIL light to come on. Indirectly, six codes (P0068 - P0103) dealing with the MAF, downstream from the air filter, may impact turbo operations.
Whenever the engine lights are lit, in many cases, the car will enter into limp mode, meaning you cannot go faster than 40-50 mph no matter how you step on the fuel. It is a safety issue to protect the engine. To get out of limp mode, stop car, turn off engine and restart. The system will be normal again for another minute before limp mode returns.
If you are getting engine codes relating to the turbo stating boost is intermittent, it means there is a vacuum leak somewhere in the lines or the actuator is having a problem. If the code states boost is low (you know this when you cannot pass or go up a hill faster than 40 mph) the things to check are: clogged intake manifold, actuator functions, vacuum leaks, or a turbo that is sticking because of soot buildup from diesel. If the code is boot is high or exceeded, check the N75 solenoid on the firewall, to the right. It will have 2-3 tubes attached to it. Check the actuator for vacuum leaks, check turbo for sticking and vacuum lines. Sticking occurs when the turbo vanes are clogged by soot. If you have a wastegate, is it stuck open?
Even a slight slit in the rubber hose will cause a vacuum leak enough to trigger the MIL. Sometimes, with engine running, you can hear the hiss or air. You car usually will either have boost or not, and when you do not, passing, climbing hills becomes a slow affair.
A turbo can last 100k miles or more, but not much more. If you have a BRM engine code, if the actuator fails, you will need a new turbo ($700-1100). Other engine types can buy a actuator for $200. The N75 runs about $170.