fc3spat

Reviving a dead thread, but did anyone ever figure out what the cause was? As I am experiencing the exact same symptoms of erratic idle and rich AFR, with the occasional lean reading. Whilst its doing all that, the revs would be hunting all over the place, and then one day, the car would run completely sweet, turn off and back on and it will be a bag of s*** again.

Totally driving me nuts! My car is a JDM E7 with the following performance mods:
HKS GTII 7460R Kai
HKS EVC6 EBC
HKS Racing Suction
Tomei O2 housing
Tomei front pipe
Tomei decat pipe
Trust cat back
Greddy Type RS BOV
Greddy Type LS Intercooler with piping kit
Ralliart ECU
AEM 320LPH fuel pump rewired

Advance thanks for anyone that maybe able to help!!

fc3spat

Just to add the following have been replaced and are brand new Mits parts:
MAF
IACV
Coils
NGK Plugs

georg1970

Sometimes the fouled spark plugs or wires ( or ignition coils) can cause such engine behavior. If the spark is not constant, not all the oxygen burns out in the fuel mixture. As the result of that there is extra oxygen presence in the exhaust gases recognized by stock narrow band oxygen sensor as the lean AFR so the ECU adds up extra fuel to compensate which creates the circle- mixture becomes more reach as the lambda keeps reading extra oxygen.
The other reason can be dead stock oxygen sensor. When it happens the ECU goes to the safe mode and adds maximum fuel - up to 12.5 percent more to stay on the safe side. Sometimes it is difficult to find if the oxygen sensor is really dead as it would keep sending some signal to ECU - it is good enough for ECU not to turn the check engine light on but not good enough to maintein the fuel supply at the right level. An the last reason I can think of is a hole in the intake or exhaust side - that can lead to incorrect fuel supply as well.

fc3spat

Thanks for the reply.

When this first happened, the first thing that I did was to check the plugs, but they were completely fine, but I changed them anyhow, and the coils at the same time, but the problem still persisted, so that negates the fouled plugs or spark issues. So I doubt the circle-mixture that you mention of is the cause.

I know for sure I dont have any leaks on the intake, as myself and my mechanic have been going through it very thoroughly and have found nothing, I may have a small exhaust gasket leak around the de-cat area, but that is after any form of sensors, so dont think it could be that?

I am leaning towards the o2 sensor giving up actually, though I have not seen the CEL light come on at all, (no its not broken, as it still illuminates pre-startup, and I did see it come on a long long time ago before this problem surfaced). But if it is the o2 sensor, would the Ralliart ECU I'm running go into the safe mode as you mentioned as a OEM ECU would do so? If so, then the o2 could very well be the culprit.

Lastly, my idle issue can sometimes be so bad that it would make the car stall, also maybe when I am coming to a stop or moving slowly like parking up. Cruising the car seems to run smoothly, with AFR's looking correct, but the second I let off the throttle, it will dip to rich very briefly and then back to lean, where bucking "may" happen, or may not, but will not shut the car off, unless I am nearly at a standstill.

Thanks

georg1970

Are you sure that your ICSV is OK?

fc3spat

Sorry, I'm not too clued up on the terminologies, ICSV = ?

I am going to replace the o2 Friday to see if that is the end to my problems.

V8HAHA

iTrader: (5)

Man my car just started doing the same exact thing. I thought it was from sitting for long periods of time, then I thought boost leak but Im pretty sure I don't have a leak and now im thinking O2 sensor. I comes and goes. It will drive fine for a few days then boom 11.2 at idle and 10.0 when driving except 4th and 5th gear but I think cause the load is low.

georg1970

Idle Control Step Valve - it controls your idle stability along with ignition. If the motor is failed you will get all sorts of troubles with afr at idle. Also, can you check what your fuel trims are like ( if all three are at the maximum positive values your oxygen sensor is dead for sure)?

fc3spat

Ah, the idle control stepper, changed that for a brand new oem Mitsubishi part, no change.

No means to check my trims unfortunately...=(
Anyhow, new o2 going in tomorrow, so hope that solves it all, else how would I go about checking the trims?

ridenrunwv

Disconnecting the battery resets fuel trims. Without data logging you can check your AFR at idle, reset fuel trims by disconnecting the battery, and if there's a huge change after you reconnect the battery then your fuel trims were far from zero.

fc3spat

Thanks for the explanation, I actually have disconnected 2 maybe 3 times before and have not solved the problem after connecting back. Problem is exactly the same, rpm stumbles/hunts as before, AFR is always dropping into the rich side, slowly going back to the 14's but would randomly go back rich and/or maybe 1-2 seconds of lean also, but mainly dives into rich.

Does that mean I could safely assume that my trims are fine, and that the culprit is 99% the o2 sensor needing replacement, but just not to the point where it would throw a constant CEL and code out?

georg1970

The other reason can be slipped timing belt- check it out as well.

fc3spat

Another question, could faulty/temperamental injectors cause this also?

georg1970

In theory yes but I have never seen a faulty stock injectors.

fc3spat

Turns out the o2 was not at fault, and the problem is still present.

Regarding the injectors, I was reading in another thread that even down to perished/perishing o-ring seals could also cause this, though the mechanic is speculating that it could be dirty or inconsistent injectors at this point now...