Mysterious Ford Fiesta ST running problems

We were asked to investigate a problem Ford Fiesta ST 1.6 turbo. The customer had recently purchased the car second hand and had immediately experienced problems where the car would go into a limp home mode with the engine warning light coming on making the car difficult to drive. Initially the car was booked into our local Ford main dealer to investigate the fault. Our involvement was after the Ford dealer had failed to fix the fault despite having changed multiple components including the air mass meter, manifold pressure sensor and stripping out and checking the inter cooler this process had taken five weeks and had left the customer very frustrated.

Mysterious Ford Fiesta ST running problems

Our initial assessment of the vehicle revealed a fault P006A-62-6C Manifold absolute pressure sensor -mass or volume air flow correlation which probably explained the parts that had been replaced as they all related to air mass, air flow and manifold pressure. On road test we were able to replicate the fault when driving at part load just as the vehicle came onto boost .Back in the workshop we conducted an inlet pressurised smoke test to prove the integrity of the inlet system. The air mass meter and inlet manifold pressure sensors were tested on our Pico scope for any errors, they all tested OK and working correctly. Having tested hundreds of these vehicles on our rolling road and modifying quite a few we decided to carry out power testing of this vehicle to see how it performed and if it had been modified.

Rolling road results..
ford fiesta st Rolling road results                  

After testing we could immediately see that this car was not standard and had some form of engine software installed. We were suspicious that this modification was causing the default running issue there was quite a spike in the torque curve and the car was running lean at this point ( Blue line is the original software ) . To prove this we reinstalled the standard factory software and re tested the car on the rolling road.

The results were immediately apparent with a much smoother power and torque curve and no default running mode. After extensive road testing we were happy to report back to the customer that his car was fixed. (total time taken two and a half hours)

We are seeing more faults of this nature where poor software modifications are causing or worse still masking under lying faults making diagnoses difficult especially where fault codes that we would normally see and aid diagnoses have been written out of the software.

Conclusion :- Yes we could fix the Fiesta ST a combination of methodical testing and the added advantage of testing on our rolling road helped us locate the cause quickly .