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sprinkles08, ASE Certified Technician
Category: UK Car
Satisfied Customers: 23110
Experience:  ASE Master and Advanced level certified. Factory trained with 17 years dealership experience
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I have a 2010 Fiesta 1.6 tdci with a lumpy idle and

Customer Question

Hi. I have a 2010 Fiesta 1.6 tdci with a lumpy idle and occasional slight hesitancy when accelerating from slow speeds. It also has black smoke when revved from idle when the idle is lumpy. It starts and idles perfectly from cold, but within a couple of minutes the engine starts “hunting” as though the idle speed is dropping too low and the computer is compensating for the low rpm and constantly repeating the cycle. Although this can alter with the engine changing to a perfect idle sometimes until I rev it and then it goes back to the lumpy idle, but this isn’t consistent
I thought it may be the EGR valve although there are no fault codes. I did remove the valve to clean it and there was no build up of carbon and the valve seat was seating fine. I unplugged the connector to the EGR valve when the engine was idling perfectly, so in effect taking the egr valve out of the equation and it still had the lumpy idle when it warmed up a little. The EML did illuminate after removing the conne
Submitted: 2 years ago.
Category: UK Car
Customer: replied 2 years ago.
I read that a low alternator voltage may cause these issues, so I checked and it is producing a maximum of 13.2 volts with no load and 12.8 volts with lights, heated screen etc on. Could this cause the problem? I am planning to change the fuel filter next as I have read a clogged filter could also cause a problem. Any possible suggestions to point me in the right direction would be most appreciated.
Expert:  sprinkles08 replied 2 years ago.

Hello and welcome to JustAnswer!

Charring voltage may be a bit low but you'd really need to monitor commanded voltage with a scan tool against actual voltage to see if there is really an issue. Charging voltage won't cause what you're having though.

It's possible that you need to replace the fuel filter as far as maintenance goes but a restricted fuel filter will cause a lack fuel, black smoke is excess fuel.

If the EGR valve isn't leaking and no fault codes are setting then I suspect an injector issue. If there are no codes store then you can be relatively sure sensor signals are ok, most likely what you're feeling is a cylinder misfiring due to a mechanical injector issue. When injectors wear they will inject excess, unintended fuel and it will cause smoke and rough running. An injector return test or removing the injectors and sending them out for testing would be necessary to know for sure.