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Car:

2012 BMW 118i, UK spec. 115K miles.

Issue:

Fault code 120308 - 'Boost pressure control, plausibility too low'

Symptoms:

Car seems down on power under hard acceleration. Fault usually occurs under hard acceleration, particularly when going up hill.

Checks:

  1. Visually checked boost pipes/intake pipes for loose connections or damage - can't see any issues there.
  2. Cleared fault twice. Fault has re-occurred.
  3. Logged Boost pressure set point and actual boost pressure values. Excel sheet can be seen here.
  4. Car doesn't seem to have any symptoms at cruising speeds. Not noticeable during a 100 mile motorway commute twice a week.

Update:

I found a crack in the cam cover breather tube (post MAF). I have replaced this.

MAF and MAP sensor values look reasonable and are steady/zero with engine off.

Cleaned MAF sensor.

Car was due for a service so oil, oil filter and air filter have been replaced. I don't believe any of these items have a bearing on the issue anyway.

VANOS adaptions reset.

Problem still persists and I'm not sure what else I can check. Is it feasible that that turbo is simply worn out? I can't feel any play in the spindle from the air compressor side.

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3
  • Does this model have a variable pitch turbo or is it fixed pitch?
    – MTA
    Commented Jun 13 at 13:45
  • @MTA - Not sure but pretty certain it will be fixed. It's the Prince engine, BMW 'N13'.
    – Joe
    Commented Jun 13 at 13:56
  • Then my possible solution of freeing a stuck vane actuator does not apply. My only other thought is to analyze output from the MAF and if it's not consistent and plausible, then clean or replace it. A crumb of dirt on the screen that protects the MAF element can cause wildly inconsistent readings that the ECU may not report as a MAF problem. If you also have a MAP sensor, best to check that it's reading your local barometer pressure with engine off and plausible pressures at idle and at high rpm and load. If MAP readings are derived from MAF output, focus closer on the MAF as the cause.
    – MTA
    Commented Jun 13 at 14:56

1 Answer 1

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This turned out to be the pressure converter valve located under the inlet manifold (BMW part number 11657599547). The valve controls the vacuum supply to the wastegate actuator.

Initially, the valve was intermittent and worsened as the engine heated up. Eventually, the valve became faulty most of the time and the fault wouldn't stay cleared for long.

I replaced the turbo CHRA and the diverter valve before this as another two codes were logged: "122201 - Diverter valve, mechanics: Suspicion of an open stuck diverter valve" and "123201 - Wastegate, control: Suspected fault in the wastegate control". The oil feed to the top of the turbo was leaking which led me to believe that the turbo spindle may be worn from oil starvation.

During diagnosis, I measured the vacuum pressure from the reservoir in the engine cover to eliminate the vacuum pump and the vacuum feed from the pressure converter which was reading a slight vacuum when the wastegate actuator was supposed to be closed suggesting a sticky or stuck pressure converter valve.

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