Fluctuating idle exhaust note.

[jswansti]

Fluctuating idle exhaust note. Need help!

So i've noticed recently that when i'm just sitting at idle the engine seems to fluctuate in idle sound and it's noticeable in the exhaust tone. It'll idle steady, then drop, then compensate for the drop and jump up a bit, then idle steady and start the cycle over again. I plugged in my AP and watch the idle for a minute or two, it was usually right around 830-860, but would go from 780 when it drops up to 920 when compensating for drop. I've heard it could be caused by the cold weather...? Some times the exhaust note drops so much it sounds like the car almost stops running for a fraction of a second, and then it jumps up again. Any info on this would be greatly appreciated! And I havent talked to my tuner Harvey about it. When we had his fan running during the dyno runs this happened a bit, but we checked to see if the idle leveled out when the fan wasnt blowing on the engine and it did.

 

[jswansti]

Gonna see if Harvey can take a look at it after work this week some time. Im not too engine savvy and wouldnt really know where to start looking for leaks.

 

[jswansti]

I've been reading that if I have a boost guage that can read vacuum, and I do, the vacuum should read wrong and mine reads normal. This leads me to believe its not a vacuum leak, unless this info is false in the first place...Other things i've seen are: bad front o2 sensor, clean maf, issue with fuel pump, boost leak, something off in the tune. Any other helpful places to look appreciated!

 

[jswansti]

If the front o2 sensor was going out would that cause the A/F ratio at idle to be funky? Or its also possible that theres something in the tune causing the A/F ratio to be off at idle. I emailed Harvey so we'll see what he says. New o2 sensor is a little pricey, so I wanna try to make sure that will fix it before i get one.

 

[jswansti]

Do you have a Wideband? If so, what are the AFR's doing when the idle fluctuates?

I don't have a wideband yet....thats on the to do list for future mods.

 

[jswansti]

I like my AEM uego. Let me know if you need help wiring the SOB up. I have a usb drop cord that pops out from the center console at the perfect spot to plus into a laptop.

Sweet, thanks i'll let you know when i get one!

So here is the data I have so far:
I forgot to mention that this is the worst when doing short drives, and in cold temps.
-The vacuum is consistant with normal readings at ~ -15 in hg.
-Boost is hitting peak at 15psi like it should be
-RPMs on AP read between 830-860 typically, but will drop to ~780 and jump to ~920 consistantly.
-NO wideband so i cant tell you what the A/F is.

The idle just doesnt feel right, like its choppy, and i've noticed a "hesitation" sometimes when shifting between 2000 and 2500 rpms from 1st->2nd and 2nd->3rd, but its very inconsistant and only has happened when its super cold out and the car isnt completely warmed up yet. Maybe im just being paranoid with the idle, but it just seems off.

 

[jswansti]

Took a couple data logs on the AP last night. One is a 20 minute log of driving home through the city, no hard acceleration or boosting. The other is of the car just idling for three minutes with a couple revs. Gonna email them to my tuner today, he said he'd look them over to see if something was off.

 

[06STI360]

My old STI did this really bad for a while, it would drop so low in idle it would make a chatter noise then kick back up into the 900 rpm range. Ended up being a small vacuum leak, hope your tuner figures it out for you!

 

[jswansti]

Thanks! I think it might be a small vacuum leak, but it never did this before the tune so im starting there. How'd you find your leak?

 

[jswansti]

So I took a data log the other day of the car idling for a few minutes. Figured I might as well post it up here and see if anyone with tuning knowledge (Holy?, Fuji?) might see something funny in the data for why my idle is weird; but i cant seem to attach an excel spreadsheet...Any help on how to upload the log to the site would be appreciated!

 

[jswansti]

Well since I cant upload the file right now, i'll just say whats noticeably off on one part of the log.

After a minute or two of idle I revved the car up to about 2300 rpms and let it drop back down to idle. Nothing weird on the log. Waited just a little and did it again up to 2600 rpms. THIS time the log shows there was Dynamic Adv of -1.5, and knock correction of -2, both at 2100 rpms. So, what could this mean?

On the data log for my drive home there are also instances of Dynamic Adv, and a couple instances of knock correction and knock learning.