Sudden onset random rough idle/ rough running issues
'98 Rover 420. Had the cambelt work done midway last year. 10,000+ kms since then, been running great, more economical even.
Was running perfectly fine one day. Next day, suddenly rough idle at startup. See sawing between normal idle to almost cutting off and then back to normal and so on, till the engine warmed up. Was a bit juddery when running. Eventually smoothed out though.
Next day still, no rough idle upon startup. Running fine too. Roughly between 3rd and 4th gear, suddenly juddering again. Then smoothing out (but not completely) as the revs went up.
Have been advised that it might be issue with glow plugs, especially considering the rough idle when cold. Still puzzled by the very sudden onset though. And by the juddering when warm.
Doesn't heat up, needle stays in mid. Car has always smoked when revved above 4k. Thick black smoke. Hasn't affected the driving or the mileage.
Might change the glow plugs anyway. But what else could cause this?