Need some help diagnosing an engine fault please

[siggy_7]

The Fiesta has developed another issue. The symptoms are occasional lumpy running at low revs when in traffic and intermittent loss of power when accelerating with wide open throttle from low (~1.5k)RPM. Several times a fault code P2237 has been raised (O2 sensor positive current circuit/open (Bank 1 Sensor 1)), which clears and takes a while to return. The exhaust generally smells quite rich.

The car has done 95,000 miles, so my first thought was a faulty pre-cat oxygen sensor. I've changed this for a new one, but things haven't improved markedly. I have run the car with a diagnostic box recording several times to try and diagnose the fault. I am convinced the issue lies somewhere in the Lambda sensor control - the issue happens on closed loop control, but not when the engine is cold (on open loop). Also, I noticed that during one test drive the engine went into open loop control (system fault), after which the issues didn't resurface during the remainder of the drive.

I have a mass of data recorded from the diagnostic interface, but I can't really identify clearly what is going on. Some things that appear anomalous to me are:

• The loss of power is generally preceded by a large negative short term fuel trim (-25% or greater in some instances). I was led to believe that fuel trims are generally a lot smaller than this. This is happening when the accelerator is wide open. Shortly after the negative fuel trim, I can see other symptoms on the diagnostics that are being caused by the negative fuel trim (e.g. loss of intake manifold pressure, air mass flow rate etc.)
• The second oxygen sensor trace looks odd - not very stable, generally hovering around 0.8V but when the power loss is happening frequently dipping to around 0V. It also shows signs of large fluctuations when engine conditions and throttle position are stable.

All suggestions on a postcard most welcome. 

[siggy_7]

I eventually gave up on this and took it to a local independent. They have diagnosed the problem as petrol in the oil system - somehow about 6l of petrol has made its way into the oil since I last serviced 3,000 miles ago. The petrol in the oil was enriching the mixture. The most likely source is apparently the high pressure fuel pump which sits on top of the rocker cover; the garage's opinion is that if it were an injector leakage of that magnitude the engine either wouldn't run or would run very badly. The car is currently ok having had fresh petrol-free oil in it, but given the loss of lubrication it's now sounding a bit rattly and has potential turbo performance issues. So probably looking to swap out for a second-hand engine.

[vikingman95]

Hey, what was the diagnostic box that you used? OBD connected or something else?

[siggy_7]

I used an EOBD interface (ELM327) from Gendan.

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[siggy_7]

All sorted now with a second hand engine. As a warning to others in the future, if you are having issues with rich mixture that don't appear to be lambda related or your stop/start system is sometimes not cutting the engine out cleanly (ours would sometimes refuse to cut-out completely which I reckon in retrospect was due to fuel vapour getting into the combustion chamber from the sump) then I would get the fuel system checked out ASAP and inspect the engine oil for signs of petrol before it does serious damage.