My car has an idle issue but it just doesn't happen when it's cool and only happens after driving it for 10 minutes then stopped. My mechanic's first reaction is a vacuum leak and the dealer reckons it's a dead o2 sensor.

I've just noticed this - the car idles fine most of the time and whenever after the fan kicks in, that's when the idle starting to hunt between 1300-1800rpm. It won't snap out of this. The fan then stay on for a couple of minutes after it's off.

My battery (oh and the MCS) is 6 years old so I am guessing, the battery doesn't hold as many amps as it once did, the fan kicked in (dropping rpm right down to 800rpm or so) drawing a fair bit of load so the alternator kicks in constantly which causes it to hunt as the alternator needs the extra rpm for generating power?

Or am I looking at the wrong thing altogether? There's no leak (at least I've tighten pretty much everything in the inlet bar the PCV lines or the tube leading to the supercharger as I can't access it) and no CEL/SES sort of lights.

Any ideas?

Thanks

 

Actually, a weak battery can cause idle hunting when there is a large current drain on the system. This will happen with a good battery, but does not start quite as soon. This happens because the alternator cannot keep up with the accessory draw at idle, so the battery must fill in at this time. With a fresh, fully charged battery, the voltage does not fall as fast under load when called upon. The ECU is programmed to kick the idle speed up if the electrical system voltage falls below a certain level. Once the battery/system voltage 'catches up', the idle will return to normal. Since a battery often 'dies' slowly by losing capacity, the battery will not only lose it's charge sooner, but will charge back up faster.

6 years on the original battery is a long time. At least have it tested for capacity. I would replace it simply because the originals historically are short-lived. More than 3-4 years and you are gambling on it starting your car the next time you are going to use it.

 

My stock battery has made it 6 years, and according to the guys at Autozone with thier battery diagnostic cart, it is in perfect shape. Since I'm poor, I'll leave it at that. I park at the top of a parking garage backed up against the wall pointing downhill - So if my battery fails to start the car, I can push/gravity start the car.

 

I have a 05' MCS and I've been having the same issue happening to me for about 6 months intermittently when coming to a stop ill see my idle go up to 1500-1800 rpms and then back to normal. i thought i had a vacuum leak as well but figured it should happen all the time which it doesnt nor can i find a leak anywhere. ill be checking my battery in the next few days as it has died awhile ago after sitting for 2-3 days.

 

I didn't think that worked on newer cars like MINIs.

 

you can push start any car with a clutch

 

So long as the battery is not severely discharged.

Thanks for your reply. I did my own search and got the info I've seen somewhere before.

http://en.wikipedia.org/wiki/Push_start

 

I had to gravity start my car today, remember I park backed up against a wall on the top level of a parking garage facing down hill. Anyway, today autozone said my battery and alternator are good.

 

FYI. if you dont lock your car it will drian your battery. locking the car puts the electronics to sleep.

 

But with the factory alarm, when you lock the doors the car is armed. The light on the wiper stalk blinks and the interior motion sensor is working. Is this less of a drain than with the doors unlocked?

 

that isnt anything of a drain to the battery...the body control module is more of a drain and goes to sleep when the door are locked.

 

I have been experiencing similar problems in my wife's '03 Outback. For the last two weeks we thought that our car was having transmission problems because the car would jerk as I drove. After I replaced the battery today, the "jerkiness" went away. Thanks for the explanation Greatbear.