How can I diagnose intermittent low boost on a turbocharged diesel?

Question

When driving around 120/140 kmph and applying max gas, normally the force/torque can be really felt from the turbo. However sometimes its just not there / no extra pull, also the consumption meter doesn't go up, its like the fuel control doesn't let it. What could be the reason of this?

In lower gears or at lower speed this is never an issue, this is specific / non-deterministic to higher gears / speed.

• model: e46 2004 BMW 320CD
• transmission: manual
• milage: 220k Km
• notes
  • it has still the original turbo
  • lambda sensor just got swapped

I know there could be a lot of reasons, however maybe someone also experience something like this or any starting points to look at would be appriciated!

Update after checking with my mechanic:

There were some error codes reported, he replaced the lambda sensor but he thinks the turbo control is setting wrong parameters for the turbo geometry. Gonna update again after a few days of experience.

Update after nearly a year:

The problem didn't resurface, however we've also disabled the EGR (according to my guy, this is customary for these models, because its design is flawed).

The torque feels perfect since, the only downside the "check-engine" lights because of the disabled EGR, downgrading the firmware to a model w/o EGR could be a solution for this but I don't like downgrading. (edit: I've received a contact who supposedly can disable this alert from the firmware, gonna update after)

Update after nearly two years:

The problem resurfaced later on last summer (turns out for proper reproduction hot weather was also necessary) so I've scheduled some extra checks.

Turns out there was a leakage on my intercooler. Also the cooling grid had lots of pollution.

After fixing those issues + a long/hot summer testing I feel finally stable/strong performance from my e46.

Answer 1

Based on what you're stated in the question, it sounds as if your turbo is basically working – so the question is how to figure out what, if anything, is wrong and causing low/absent boost at times.

Ideally, you'd have access to a tool that would plug into the diagnostic port and enable you to compare the requested boost from the ECU with the actual boost being delivered by the turbocharger. For VW diesels (which is what I'm familiar with) the tools of choice for this is VCDS, BMW diesels are (as best I can tell) nonexistent in the US, but there appear to be similar tools available for them as well.

If you have access to a diagnostic tool, start by checking for any turbo related DTCs. Then, take a look at the requested vs. actual boost (either by logging the data or having somebody monitor the computer as you drive) to see how they match up. It would be interesting to see if the ECU is asking for boost and not getting it, or if there is some reason why it might not be asking for it.

Without the tool, the best advice I can offer is to methodically go through the induction system and the turbo control system checking for leaks or anything else that might interfere with its operation. Things to check:

1. Look for smoke under heavy acceleration, that would be a sign that the ECU is delivering fuel and expecting air that it is not getting. If you don't see smoke, I would take that as a sign that things might be working "correctly" or at least that fuel delivery isn't exceeding air supply.

2. The condition of the turbo control vacuum lines, and the engine vacuum system in general. Any leaks in the vacuum system may result in not having enough power to activate the turbo under some circumstances. On our cars (11 and 14 years old) the vacuum lines were dried out and leaking. I replaced them with silicone hose from McMaster-Carr.

3. The actuation of the turbo control (vane adjuster, waste gate, or whatever you've got), you can use a MityVac or similar device to vary the vacuum and observe actuation. This will let you confirm that the turbo control system can hold a vacuum and that the controller gets actuated.

4. Inspect induction system for leaks between the turbo and the intake manifold. Cracks, loose clamps, a damaged intercooler are all possible sources of leaks. Since the problem is intermittent, I'd look closely at hoses and clamps to see if something might be opening up as the engine is loaded or as the engine compartment gets hot. Look at the engine mounts to confirm that they are all intact and holding well – engine movement could cause cracks, or hoses to open up at times. Since there is likely to be oily EGR/PCV fumes in the intake you may be able to find cracks by looking for oil residue on the induction pipes.

If none of the above proves productive, then it would be time to starting checking things like the MAF (mass air flow) sensor and the MAP (manifold absolute pressure) sensor to confirm they are providing reasonable (and expected) inputs to the ECU.

Another possible problem would be a restriction in the exhaust system – a clogged catalytic converter or DPF (diesel particulate filter) are obvious candidates, but anything that could prevent an increase in exhaust flow would also prevent the turbo from spooling up.

Answer 2

I have no expertise about your specific car however I want to point out that a typical root cause for this behavior would be a damaged vacuum line controlling the vacuum actuator of the variable vane turbo.

Chances are that the line got damaged (possible by some marten).