BMW 325Ci idle hunt when coming to a stop

Question

Car details: 2002 BMW 325Ci, manual transmission.

I’m having an idle hunt issue with the car specifically when stopped in traffic, at a stop light/sign, etc. The issue does not occur when the car first starts up, only after it’s been running for a few minutes.

Sometimes when coming to a stop, when I put the car in neutral, it’ll pulse for a couple of seconds between 1-1.5k RPM, then find itself and stop. However, sometimes in the same scenario it will jump between 500-1k RPM and do that continuously until I take off again.

It hasn’t stalled yet as a result of this, but sometimes it feels like it might. Even after it finds where it wants to be RPM-wise, I notice that if I turn the wheel it hunts again for a second before it calms down.

This issue is also somewhat intermittent. If the car has been running for a while and is up to temp, I’ve noticed it has a higher likelihood of not hunting when I come to a stop.

I’ve checked all of the vacuum lines and cannot find a leak. Cannot see any cracked or compromised hoses. I replaced the alternator and MAF recently as part of another maintenance. Cleaned the throttle body recently as well. I can say I’m 99% certain there is no vacuum leak after having looked it over extensively on multiple occasions.

Does anyone have any advice?

Answer 1

Not sure if you have a scan tool, but you may want to look at live data while it's hunting for idle. I would check MAF voltage, ECT voltage, IAC, anything you can think of that has to do with idle engine speed. Also, I have kind of a similar problem on my truck - idle will bounce between 500 and 1k but not always. Ended up being due to an intermittent misfire on cylinder 8. If all other roads are a dead end, you may need to check basic run functions like compression, spark, fuel. That's rare, I know, but it happened to me.

Comments

Unfortunately all I have is a very basic OBDII reader, although I’ve been meaning to invest in a better device for that. Perhaps now is the time.

The MAF voltage point is interesting, because about 3-4 weeks ago it was throwing a P0102. I got that to go away by cleaning and re-seating the MAF harness and it hasn’t resurfaced since- but perhaps even though the code hasn’t triggered it could still be a voltage issue.

Your other suggestions regarding voltage are interesting too, considering I don’t think this problem appeared until after I replaced the alternator. Could it be possible the alternator is causing the issue?

I had also considered replacing the IAC as well because I’m pretty sure it’s running on the original, 18-year old unit. I might replace that simply for the sake of doing so (but not looking forward to it lol, that’s a ***** to get to on these e46s)

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Hmm....I wouldn't think it would be caused by an alternator, especially on an older model. Perhaps revisit the MAF for a moment.....did you clean just the connectors or did you clean the sensor as well? I've seen one or two interesting issues that have been caused by MAF goofiness.

If you can gain access to any scanner that will read some live data, try pulling up some voltage and resistance info online on each sensor you look at, and verify the readings you're getting from your car are within range.

Here's another wacky scenario I've seen: I don't know if BMW is engineered this way, but on Hondas the power steering pressure switch can cause unusual idles - when the PCM sees a pressure increase such as when turning the wheel, it bumps the rpm up a little to spin the pump a bit faster and assist you. if the switch goes bad (or if it's plugged in BACKWARDS which unfortunately is possible on those), it can send erratic readings. The likelihood of that being your problem, again, is pretty low. But just throwing it out there.

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I cleaned the sensor as well, and didn’t see any corruption on the prongs. I actually tried swapping the MAF with a new unit as well, which didn’t help. I have a voltmeter that I’ve been meaning to take to the harness to see if all of the wires have a proper connection, which I haven’t had a chance to do yet.

I have a friend with a scanner that will capture live data, so I’ll bribe him with a beer and try to get him to work with me on that while I save up for a scanner of my own. Thanks for the advice!