Troubleshooting Low Turbo Boost

Question

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:

1. 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.

2. 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.

3. 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?

Answer 1

Some thoughts on how to verify if you have...

• an intake obstruction

  Disconnect the air filter (keep the MAF in place) to expose the compressor inlet and log boost values with the car at idle and under load.

  If the boost numbers are close to what the engine computer is expecting then this would confirm that something upstream the MAF is obstructing the intake.

• an exhaust obstruction

  The most convenient spot to probe for exhaust back pressure would be the upstream O2 sensor location. Unscrew the O2 sensor and put in a pressure gauge to probe how much back pressure there is.

• carbon build-up on the turbine vanes

  Spinning the turbo shaft by hand won't tell you much besides confirming that the bearings are OK at very, very low RPM. I can't think of anything short of a turbo teardown to confirm if carbon build-up is an issue (unless you can get a borescope in there).

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Possibilities

• not enough vacuum from the engine

  (Based on the comments, this is pretty much ruled out)

• incorrect VGT actuation

  (Based on comments, you're pretty sure this is not an issue)

Answer 2

You can rev it up and see if the vane boosts it to 100% at any moment. If not, it's a ECU not allowing to reach the pressure. So the issue is somewhere before your turbo, like detonation, pre-ignition... If it doesn't open a boost fully, you should see your Lambda sensor readings.

• Do you have knock sensors?
• Is the smoke white or black?

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If the smoke is black, it will be a mechanical issue of loosing a boost, and not decreased by ECU/ECM. So if you have black smoke, it can be an air/vacuum leak, VGT valve, exhaust pipe, etc. If the turbo's shaft bearing is worn, you should hear it.. It might feel a bit loose when you touch it cold by hand, but it'll be fine on the running engine.

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Lovely description!