How to resolve an uneven idle?

[lo_jack]

Originally Posted by synapseuc

Hello everyone I have been visiting this site for various reasons for about a year now and finally decided to sign up. I have a 2004 4.4 X5 that I am completely in love with. One thing that has me slightly concerned is that it idles unevenly, I would not say a rough idle but you can just see the tachometer vary slightly only while running at rest (and of course you can hear it). Also at night the interior lights oscilate slightly in intensity while idling. As soon as you give it a little bit a gas it purrs perfectly. I have done a little research and think it may be either the Idle Control Valve or the O2 sensor. Any help would be appreciated because (like all of us) I hate replacing a $150 part only to have solved nothing. Also I am new so if this is posted in the wrong section just let me know! Thanks again

How many miles are on the car, what sort of mpg are you getting at speed, and why do you suspect 02s? I believe it is less likely to be 02s than ICV, but if you do not have any SES codes currently, it may just as well be the turn up stuff that mth posted...specifically A) not enough gas, B)not enough air or C)not enough spark. These would probably manifest themselves at other times too though, not just idle and you would probably notice a decrease in miles per gallon or power, or both.

With the idle situation, you need to see if you can pry apart if it is running poorly only at idle, or at all conditions, as the ICV directly controls intake air only for idle, and could be the problem if everything else is operating within tolerances. The other thing I can think of showing up at idle only is a vacuum leak of some kind, or a malfunctioning vacuum booster or something of that nature.

If it were my car, I would check the plugs, check the intake for oil, keep track of the mileage it is getting and under what conditions, and see if I couldn't narrow this down.