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I was at BMW yesterday and had a software update put in to fix a battery drain problem. I posted this in a separate thread.

While there I had the tech take a look at an idle problem I've had since I got the car, day one. My 325i has only 7 miles on it when I picked it up but it had and idle that wouldn't still still and the tach would move slightly up and down radomly. It isn't just the tach as you can feel and hear the engine speed up and slow down. Also, my gas mileage sucks. With the current "winter gas", which is "oxygenated" all cars will have reduced mileage but my 325i with 5 speed manual is only getting 19.5 MPG. I've never been able to get more than about 23mpg but usually only get about 21mpg. I use 92-93 octane and about 90% of the time it's Amoco/BP fuel.
I suspect that part of my poor MPG is related to the poor idle quality.

I finally got the tech to come on a ride with me so that I could show him what the problem is. 2 times they've had the car and said "no problem found".
I wasn't taking "no" for an answer this time. An engine should idle smoothly and steady. If all systems and sensors are working correctly there is NO reason why the idle should go up and down even when using your heater/AC/rear defrost ect... The system is designed to take those loads into consideration and then adjust to maintain proper idle, SMOOTHY.
My engine idles around 650rpm but wanders down to 500rpm and up to 900rpm. It's random. Now, since it's got cold here in the Chicago area, I've been using my heater. I normally set it to around 72F. An odd thing happens when I turn the heat up to around 78F and above. The idle goes up to around 850-900 and stays there as if it's the new idle point. Very strange.
Then while sitting at this higher idle, it radomly acts erractic just as it does in it's typical lower rpm around 650rpm. The tech saw this. It's very very weird.
As the HVAC system was set to 80F the tach showed a 850-900 engine rpm.
I turned the temperature down to 74F and the tach and engine slowed down to 650rpm. I then turned the heat back up to 80F and the engine sped up to 900rpm. Very strange. All this time you can hear the engine speed increase and decrease. When the heater temp is up and I sit at a light, the engine is idling so high that I can hear it. And, cause it's random the engine speed goes up and then down and the tach shows that too.

So, sometimes the engine rpm is responding to heater temperature and sometimes it just does what it wants. I think it's an idle control vavle.
The tech is not sure and he was stumped. He said he will test another 325i 5 manual like mine to see if it does it too. And, he said he will call BMW tech support to see if they can shed some light on this.
My suspicions are that it's an idle control valve or maybe a poorly functioning O2 sensor or maybe a combination of things. My MPG is really bad.
I drive mixed city/highway with mostly highway type driving, steady throttle and such. I don't run to redline at every shift. My normaly shift point is 3000rpm or 4000rpm with the occaisional run to redling but that's not an everyday thing. So, my mileage shouldn't be this bad.
Something is wrong and I will stick with it and BMW to solve the issue.
I've read a lot of threads on unsteady and erractic idle. Maybe we can get some resolution to this.

BTW, just so you know, when the heater temp and idle thing do their strange dance, the AC is NOT one and the vent selection is face and floor.
If I turn the AC on with the heat, the idle doesn't change much even when the compressor kicks in. Also, with the temp down and AC one, such as warm weather setting, the rpm doesn't rise much, which is normal. When the sensor detects a load like AC compressor, the idle speed control will alter the idle and increase fuel/air to compensate for the load. You may feel a transient reponse but the idle should stay steady as that's what idle speed control is designed to do.

Whew, I know that was long but I just wanted to give as much detail as I can so that others can determine if their car is affected in a similar way.
Post your impressions if you've had or have a similar problem. Also, post if your problem has been fixed and what fixed it. Thanks.

TT
 
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