Occasionally my car's idle will dip to about 600 RPMs at idle (like when I pull up to a red light and stick it in neutral) and slowly creep back up to normal RPM. Never stalls, etc.


I also notice that the Air/Fuel spikes to around 16.5+.


Car is tuned by STM with Walbro 255, STM full intake and intercooler hoses and MBC, so pretty much stock.


Is this normal? Why would there be so much air in the fuel?

 

Have you changed your spark plugs recently ?

 

I have not. I purchased the car a month ago, 45k miles, I have put 800 miles on the car. I am changing the oil this weekend so I will check the plugs. Thanks

 

only in the Evo community is this "pretty much stock" LOL

anyway, could be a lot of things.

fuel pump
injectors
relay
wiring to fuel pump or injectors
alternator
BISS out of adjustment
IACV
error in the tune

and more.

start with the simple/free stuff and go from there. good luck, dude!

 

It's great, isn't it?


Thanks, I will start with plugs and go from there. If plugs are fine I will continue driving it, and if it gets worse I will take it to STM and have them diagnose. Like I said, never stalls out or anything major like that, but I have noticed that it is happening more often.

 

In STi land that will prob. be something like a stage 5.

 

LOL. true story.

"stage blah blah"
"protune"

i still don't understand the Subaru community (even though i had one for a bit).

 

I've also been having a very similar issue, I'll see what from that list Kaj provided fixes my issues too.

 

Please let me know.
Plugs were changed 4k miles ago so they shouldn't be fouled yet.

 

plugs don't usually have idle issues, that i can think of.

it sounds like a BISS issue, IMO. that's the most common cause, anyway. i'd start there (it's free to do if you have a tacktrix cable and EvoScan) and go from there.

 

Yea. Sound alike the BISS is adjusted wrong. When is the "test mode" to set the BISS, the engine should idle roughly 100rpm higher than your target idle.

 

i always use the trim for "something something A/C OFF" (i have it near the top of my list, so never really read it LOL) and try to get it as close to "0" as i can. i usually end up at .392 or -.392 before i give up LOL.
i haven't decided which method i like best, but adjusting the trim seems to work a bit better for me.

 

I'll have to schedule a time to get STM to look at the BISS screw. I am new to the Evo community and have never even heard of BISS before, so I think I'll leave that adjustment to the professionals.


What is the worst case scenario if I don't take the car in right away?

 

Worst case, it just continues to do what it's doing. You're not going to hurt it.


The BISS is the Base Idle Screw Setting or something. It sets the idle speed to a "base" setting, then the IACV makes fine adjustments to maintain idle. If you search around there are threads explaining how to adjust.

 

Thank you.
I will do my due diligence and if it's something I feel comfortable adjusting I will take a swing at it. Car doesn't stall or bog at all so I'm not immediately worried, but it does kind of bug me.