Starting problems on a Ford S-MAX. What's causing it?

Our car, an S-Max 2L petrol (2009), seems to have developed a fault but the garage’s diagnostics are far from clear.

About 2 months ago the engine warning light came on and the local Ford garage traced it back to the temperature sensor in the CAT. They could find no fault in the CAT, so one possibility was that the temperature sensor itself was faulty. They cleared the alarm and advised us to see how we got on. If the fault came back soon then they said they would suspect the CAT itself. The car has been driving fine and now 2 months later the engine warning light re-appeared.

This time I took it to a completely different garage and they confirmed the warning again came from the temperature sensor in the CAT. They did emission tests and checked the exhaust system and all was confirmed to be OK with the CAT. However they noted that the car was difficult to start, unless the clutch was pressed in when starting the car. We may have got used to it but yes one generally has to rev it once or twice after starting the engine or otherwise the rpm may drop. And in the first few minutes of driving the rpm can go up & down a bit, sometimes causing the engine to race a bit, until the engine has warmed up when everything seems to be OK.

This, they believe is due to the Dual Mass Flywheel not working OK. They hypothesis on the link to the temperature sensor warning is that if the flywheel doesn’t work as well as it should this can cause unburned fuel to end up in the CAT, then when this ignites this caused the CAT to report a high temperature.

I am no car engineer and with the car otherwise functioning OK it makes it hard to justify spending the quoted £1300 repair for clutch & DMF replacement. They say there is a possibility that by not doing the repair the flywheel problem (? if confirmed) could impact the CAT and eventually damage it.

Has anybody else come across this problem, first of all of the car not always starting well, the engine RPM being up & down for a while and getting faults on the CAT temperature sensor? I see no other issues with the flywheel, no rattling or grinding for example. Car drives well and doesn't make any funny noises.

Asked on 20 January 2015 by Geoidea

Answered by Honest John
You should always dip the clutch when starting a manual car because then the battery and starter motor only have to turn the engine, not the gearbox as well. But what they have identified is extra unburned fuel getting into the catalytic converter, burning and raising the temperature. Though the garage is probably right that the timing is affected by a failing dual mass flywheel and this could cause the poor starting, the fault might be throttle position sensor, idle control valve or mass airflow sensor. As you rightly point out a DMF and clutch is an arm and a leg, so I'd persevere for a while.
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