It started running lean at idle about a week ago. It usually sits around mid 15 range but now it fluctuates into the low 16s. The only thing ive changed in that time is my radiator to a stock replacement mishimoto. I thought maybe plugs cause my couplers seem ok.

 

Boost leak

 

I thought boost leaks make it rich? Thanks ill test her today

 

at idle, you'll run lean with boost leaks, when boosting you run rich

 

Hendrix, I have encountered this said issue. But my boost gauge actually reads 27psi where my tuner stated should be at 24psi.

Would this be more of a tune issue?

OP, didn't mean to thread jack bud. Relating to the above post.

 

Eh? Don't understand the question, please rephrase. Are you saying that your boost gauge reads different from what the tuner is reading? Does he hook up his own map sensor when he tunes it?

The reason you run lean at idle, when you have boost leaks is because the engine draws in more air from those leaks, hence making you run lean. The maf is at the beginning of the intake so it cant read the air from those leaks coming in. You run rich while boosting because air is being pushed out of those leaks, so less air going in the engine, making you run rich. Pretty simple

 

The samething thing is happening to me. But i know i need a tune bad. 15's normally, but today it jumped around alot 15-17

 

Well now i gotta cel and no afr reading at idle lol. Cars idles smooth no issues still pulled my buddies 350whp sti so its runnin fine lol. Could it be a bad o2 sensor?

 

That could definitely be a possibility, is your wideband o2 sensor tied into your ecu? Or do you have a seperate o2 sensor for your gauge? You might have an exhaust leak as well, read what code your ecu is spitting out, and check for exhaust leaks.

 

Mine runs cruising at 14-15, idle goes blank and wot is 10 and I use an AEM with a new o2 sensor. Pretty sure those are normal numbers I haven't had a problem in the last 4 years

 

Also I know you text'd me earlier, but usually when the AEM sensors go bad they read super lean, like I said before, get the code read and report back

 

I found the problem. My o2 housing bolts backed out. Bolted it back up reset the ecu and its fine. Im gonna bash today and see if the light comes back

 

Awesome! Glad you figured out the problem.

 

Ok I have a 2012 evo x .. just got tuned and has a rough start and idle running 14.5 then dropping to 18 and stays there for a bit to eventually make its way back up .. I also feel there is a loss in power. things I've done to try to fix the problem include :
New upgraded fuel pump and fuel line
Upgraded 1100 Id fuel injectors
Smoke test for leaks -- a leak in intercooler and leak is fixed
Freshly tuned

Codes : misfire in cylinder 4

My coils and sparks are fairly new ..can they really fault that fast

Could this be the reason she is running lean ? And idle bring all over the place .. please help

 

this was an old post but I appreciate this. I had the throttle body leak, fixed both my lean-ness at idle and richness on power. didn't know about the lean bit though. sounds like what I had was a pretty bad leak.