Have had my 2018 diesel mondeo for about 6 months now.  About 2 months ago it start idling strangely but only intermittently.  Every day or two it revs from the usual ~800rpm to just under 1000 up and down over and over for a few minutes and then a few minutes later goes back to normal - a nice steady 800rpm.  While it's doing the uneven idle, fuel consumption is almost double as well.  This is happening regardless of if the auto-stop is turned on so often what it does is that it idles unevenly for a few minutes (presumably charging?) and then the auto-stop kicks in and it switches off.  If you turn the auto-stop off to make it run, it idles just fine.  This seems to happen on a warm engine only.

That was kind of weird but the worse thing is that it has stalled at low speed on a number of occasions. Usually just manouvering at parking time but has cut out three times while out and about.  Once slowing down to almost a stop for a tight turn and twice at the lights - one time was while it doing the uneven idle described above.  It usually restarts OK but sometimes takes 2 attempts of ignition off->start to go again.

The mechanic read the OBD and there are no error codes.   He wrote it off as bad fuel but it has been refuelled many times and still doing it so I am skeptical. I've replaced the fuel filter and air filter and am currently running injector cleaner but unless the cleaner does the job it's still a problem and I don't get why it would be the injector if it only does it intermittently and only for a few minutes.  I expect it to repeat again any day now.

Any advice would be appreciated.

It sounds like it's trying to regen

Next time it does it take it on a good journey and hold the revs at about 3,800 RPM for a few minutes and see if that cures it

Thanks Dave.  I suspected that might be a possible cause.  I did take it for a big 2 hour highway drive in the hopes that might cure it and I thought it didn't happen for a bit longer than usual after that but wasn't sure if that was my imagination.   Still - maybe I'll try that again with holding the revs up as much as I can.  Cheers.

54 minutes ago, unric said:

Still - maybe I'll try that again with holding the revs up as much as I can. 

It's all about engine speed to do a good regen cycle, not vehicle speed.

About 20 minutes into a journey so it's warm enough

So I found out what the problem was - the hard way.  800km from home the transmission error light came on.  The next day it transformed into the check engine light.  Turns out the EGR was sticking/broken.  Several thousands dollars and a new EGR later the problem seems to have gone away.  A cautionary tale for anyone experiencing similar symptoms.