Are You Experienced?

Barnaby Donohew and colleagues are reminded that it is the correct type of experience that matters

Published:  04 January, 2015

In the context of automobile diagnostics, the uninitiated often have a misguided concept of the benefits of 'experience' and the ability to diagnose a fault, whether that is experience gained from spending a life-time in the trade, a regurgitation of collective experience assembled within an online forum or repeated experience of exposure to the same faults within a one-make dealership.

The Peugeot's problems began when its timing belt snapped causing metal-upon-metal interactions of the unwelcome kind. The head and valve-train had been repaired but the engine was left with an erratic and unstable idle. The Peugeot's PCM was reporting a host of camshaft timing/solenoid DTCs (P0010, P0011, P0012 and P0075) and a clutch of fuel mixture and oxygen sensor fault codes (P0170, P0132 and P0135).

Prior to arriving in our workshop the Peugeot had been fitted with a new intake manifold assembly, consistent with trying to remove an air leak that might cause an erratic idle. It was also reported to have had fitted various other components which were removed when they were found not to immediately rectify the faults - these included a camshaft control solenoid, oxygen sensor and a PCM unit. In addition, the bottom end of the engine had been stripped and rebuilt to ensure that the oil pressure (used for cam timing control) was sufficient. Before and during that process the twin-cam/crank timing was checked and found to be ok. Clearly a lot of effort had been spent on the problem but we were unsure as to their diagnostic process.

We started by checking the PCM's serial data. It became apparent that when cold the pre-cat oxygen sensor was reading 3.75 volts (alarm bells!) and after the engine warmed up, the vehicle's operating mode would switch to a cyclic pattern of a high idle RPM, at which time the sensor would read 0.05 volts. A rapidly 'hunting' idle RPM oscillating at around two cycles per second followed, with the oxygen sensor voltage flicking between rich and lean in phase with the RPM (though note that the fuel system was not under closed-loop control). Tony checked the oxygen sensor and found that the heater circuit was electrically open. Upon removal of the sensor it was clear that it had at some stage been subjected to substantial force as its outer casing/body was bent at an angle.

Narrowing down

Figures 1 and 2

Figure 1

Figure 2

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