The car is a stock Evo 9 2006 (has an exhaust but the issue was there without the exhaust as well so don't consider that)

The issue:

The car when cold starts normally, has a few hiccups with idle but generally works fine.

If you try to give it a bit of gas it starts to "stall" at 1800-2000 rpm - sounds like a soft revlimit going brrrrrrr but not increasing RPM.

If you drive off - it does the same - not always but it's not operating smoothly.

The symptom however over time goes "away" when the car warms up. It still bucks a bit but A LOT less when it's warmed up. Also the car feels quite laggy for a stock 9 with a 3" exhaust.

CEL goes on but only for EGR errors, which I've already corrected.

What could be the cause of this? Map related changes don't affect it much, we've replaced the sparkplugs and coils.

Also - to note - above 3500 RPM when on boost, the car pulls and drives normally so I'm really stomped... I was thinking MIVEC but that wouldn't cause the actual bucking and breaking up at low RPM, probably just lag - or am I mistaken?

If anyone has had a similar issue I'd really like to hear an idea.

 

Post a log of TPS, RPM, Ign Timing, LTFT Low, LTFT Mid, STFT, Battery Voltage, WB AFR, IPW and Mivec and I'll take a look.

 

I solved the issue temporarily ... Hooked up a wideband on the car and it was running 20s AFR on idle, 16s cruise... FT were +25%... The temporary fix is to scale down the injectors by 25% and it was running perfectly, thinking it's got a lot smaller injectors, however I'm yet to find the underlying cause. Car has a walbro and stock injectors and stock scaling will make them go lean as f*ck...

What order would you recommend for troubleshooting? I'm pretty sure I can start with the MAF sensor, but if it's not - where to go from there?

 

check the stock o2 sensor to see if it is bad