Idling too fast

Took the car out for a spin tonight. Started up and it was idling at about 1500rpm (rather fast for a TDi).

Carried on along the road, stopped at traffic lights, still idling at 1500.

Got some diesel in, started up and it was OK when I came to a stop to drive out of the service station (about 750rpm). Plenty of stopping on the way home, and it had risen again above 1000rpm.

Got home, got out to open the garage, got back into the car and put it away for the night, sat in the garage for about 60 secs - all the time the idling never dropped below 1500rpm. Not good for letting the turbo cool down. Had driven about 15 miles of mainly start/stop driving no more than 40mph.

I know the throttle position is controlled by the ECU, but it's nothing to do with the position of the accelerator pedal (I tried blipping it). Only done 800 miles since I got it - maybe it needs a good run! Anyone any ideas?

It's done it once before a couple of months ago, but next time I used it, it was fine. Drives no problem otherwise when it happens.

Thought it might be the throttle potentiometer, but unless it's faulty there can't be any wear in the system in a car that has only covered 800 miles!

Probably find next time I'm out in it it's fine. Intermittent faults are the worst to diagnose.

Thanks Cobain, will bear this in mind next time it happens. Was bit concerned about what it might do to the turbo if it was switched off hot when the engine was 'idling' at 1500rpm.

Nathwarr - registered Mar 07. Don't get the chance to use it much. Working away from Edinburgh most of the time and use plane/train.

Only thing is, it seemed to idling a bit too fast when I first started it from cold. So maybe this rules out the water temp sensor.

Know what will happen if I take it to the dealers - it will be fine, unless it starts doing it all the time.

Tried blipping the accelerator - makes no difference. Will give a good run sometime and see if it sorts itself out (could be a result of underuse).

Silly comment - if the battery is undercharged before starting up, maybe the car is clever enough to realise this and idles at higher than normal to charge the battery - You never know what clever devices might be fitted these days

Strange it may seem - but have just checked the battery voltage (last used Sunday evening with headlights on), and it is 12.00 volts. So rather discharged!!

Now on trickle charge and will let you know whether the fast idle is actually due to a discharged battery