My Polo has only done around 1400 kms so far, but I am noticing that as the performance of the engine improves when warm, the cold starting - and to a lesser extent warm idle - are getting worse. When the car was brand new, the cold starting was actually smooth (ish), well certainly not rough or hesitant in any particular way. But after a couple of weeks, I noticed that the engine cold start was slightly rough as if there was a tiny misfire, but only until it reduced the idle speed down to normal (which I guess takes just under a minute or so) - at which point it was fine.

I never thought much about this since I felt this was par for the course and it wasn't really bad enough for me to make a mention of it at the 1500 km inspection last week. But over the last few days it seems to have gotten worse again and coincidentally or not, this coincided with me getting the car back after it's first inspection. Now the engine continues to run a bit rough at cold start as it did before but now as the idle speed drops to normal, it gets quite close to stalling. I have also noticed that the idle speed seems to be different after the first inspection - it seems to vary a bit more, generally being slower than before but faster than before when the air conditioning is on. And although the engine now runs noticeably better than ever before when warm, the warm idle is slightly more rough than it used to be - and again this seems to have happened after the first inspection.

It is possible (though I have not verified this yet), that the worsening of the cold start began after I put my first tank full of fuel in the car. I imagine it had BP 95 octane in it from the dealer and my first tank full was BP Ultimate 98 (as was the second tank full). Out of curiosity I am going to let the current tank get to almost empty, then put about $10 worth of Caltex Vortex 98, let it empty again and then try BP 95 octane, just to see if the different fuels make any difference to the cold running problem and slightly lumpy warm idle.

I guess what I am asking is whether this behaviour is normal for a Polo that is running in and improving in warm performance, or whether there is something wrong that needs to be looked at. I wasn't really worried about this until today when the car really hesitated quite noticeably as the idle dropped after the cold start. I am also wondering whether I just happen to have less than perfect fuel in the car, as the local BP stations dont get nearly as much traffic as the Caltex does and the BP service stations are quite old (unlike the two local Caltexes which are almost brand new).

I was also going to buy a bottle of injector cleaner and stick it in the tank as well, but I wonder if this would help given the warm performance is very good (definitely better than ever before), except for the very slight lumpiness at idle that it did not have before.