Hey guys, looking for some suggestions here. I've been battling this problem for a few months now and can't find the culprit. I've got an Evo IX about 110k on it.


The symptoms are letting off the throttle causes the car to go super rich, more than 10:1 most times (10:1 the richest my AEM reads). Also at cruise or idle the AFRs will intermittently jump all over the place from richer than 10:1 to 15.


I've tested changing the following:


MAP
MAF
Front o2
IACV
Ignition
Stock tune
Boost leak test
TPS


The car did throw a code P0121 Throttle Position Sensor (main) Circuit Range/Performance Problem so the last thing I did was change the TPS but the symptoms remain. I used EvoScan and confirmed the TPS was at 12.5% position with the car off and hits 100% when I put the pedal on the floor.


I've honestly run out of ideas here, also flashed a full open loop tune as a troubleshooting measure but the problem is the same. Anyone got some ideas?


Thanks in advance

 

I should mention over mods include;
Grimm Speed 3port BCS
3" TBE
Walbro 255
AEM intercooler
Injen upper and lower piping
Tial BOV (100% atmosphere)

 

If you pull up on the gas pedal with your foot does it swing back lean. If so check the tension of your throttle cable. Might need to loosen it up some. Its 2 bolts on a goldish slide bracket right behind the motor/valve cover

 

It's the vta bov. There's like a million threads outlining the rich condition caused venting the bov to atmosphere. You're still running a maf. Either go sd, or recirculate it. Tial makes a recirc valve so you can keep the flange.

 

^^ This.

You'll need to either go Speed Density, or recirculate the BPV to get rid of your rich condition.

 

It's not the throttle cable, checked that.


The car was running great for months with the vta. I mean the car will even idle rich without the BOV ever opening, unfortunately I gave my stock uicp to a buddy but I might find one to use to at least rule that out.

 

If you are not vta tuned, it will cause your symptoms. Stock tune with vta is no good. You have those mods on a stock tune?

 

So I'll acknowledge that VTA is not ideal, and it can stumble letting off at high throttle however this problem is more than just rich off throttle as I first described. The car will start cold at 14.7 and without touching the throttle just slowing get richer and richer into 10:1. As bad as VTA might be I can't see that creating this problem.


And then even at a stable cruise speed its super rich, no codes being thrown

 

Are you sure the bov isn't opening while idle when my sti was in speed density I found out my bov was opening even at idle

 

Hey guys, so like I was saying the car is also going super rich after a few seconds of starting it. I did another boost leak test last night and found that the injectors are leaking on the intake manifold. The middle two significantly I would say. I'm getting some new orings ordered asap.


Would a big enough leak at the injectors cause the car to run rich like that or am I getting my hopes up that this might finally be the problem?

 

It could. Only way to find out is to replace the seals and go for a drive. Let us know your results.

 

I had this same issue, I replaced my o2 senor and everything went back to normal. I hope that helps

 

lol never mind you already checked it I didn't read that part ��

 

im pretty sure blow off valves are OPENED with VACCUM