I'm starting to suspect that the smoke and low power (I'm pretty sure it is not "limp mode" just a lack of enthusiasm) I'm seeing in our 2002 Jetta TDI (ALH engine with Garret VNT-15 turbocharger) may be a turbo issue. Here's what I'm seeing:
- Vacuum system seems good – vacuum pump (no manifold vacuum on a diesel) is strong and I don't see any leaks on the lines when I test with a MityVac.
- Turbocharger output (the "actual boost" value shown using VCDS) is well below the boost requested by the ECM – for example while climbing at about 3,000' at full throttle the ECM was asking for about 1.5 to 1.8 bar (from memory) and the turbo was only able to produce about 1.0 bar.
- At idle at about 6,500' the ECM asks for about 1.0 bar and the turbo MAP (manifold absolute pressure) and actual boost values are at atmospheric pressure (0.805 bar).
- This is a vacuum actuated variable vane turbo and the control system seems to be working (the ECM is able to modulate the amount of vacuum, and the actuator moves in response to the ECM's commands.
All of this leads me to think that the turbo should be working – in other words the problem I'm seeking is not in the control system (despite getting a diagnostic code, P0245/16629 that suggests a control system problem). So, I'm wondering what can go wrong and how can I troubleshoot. Here's what I'm thinking so far:
An intake obstruction. I've had the whole system apart and replaced the air filter. I don't think there is an obstruction unless it is in the intercooler. I would think that an input restriction would manifest itself as a drop off of boost at higher loads, rather than`an overall lack – seems that I'd be able to get enough boost at idle for example.
An exhaust obstruction. Haven't looked at this at all (other than noting that there is some exhaust coming out…). Is there a way to do a meaningful check without taking the whole system apart? Again, I'd expect this to get worse as exhaust volume built, so that I'd be able to get some boost at idle.
Something (carbon build up on the turbine side, or coked up bearings?) causing enough drag to stop the turbo from spinning.
I'm leaning towards number three, and thinking that I could check this by pulling the intake hose and trying to spin the compressor – I assume it should spin easily and keeping going. Is there any possibility of the shaft failing, so that the turbine is no longer connected to the compressor?
Any other ideas?