To Hopefully End all Rich Threads

[Speed Junkie]

This "SR20DET running rich" thing is all over the forums and they all end up being forgotten and no one ever posts a solution. Or they post they typical "check your plugs" comments that lead nowhere.

Context:

I have a kouki S14 SR with S15 turbo, ehxhaust and typical IC piping, S15 injectors, Walboro 255, Z32 MAF, Kouki S14 ECU with Nistune board, and wiring specialties pro harness which is about a year old as well. The car was tuned by Matt Shue and has ran correctly for about a year.

I have replaced O2 sensor with an OEM bosch one that is a few months old. I've also replaced the ecu temp sensor. The engine and surrounding engine-related wiring is in good condition and not hacked up or jimmy rigged in any away. The spark plugs are less than a year old and correctly gaped. Coils are also good.


The Issue:

Five minutes into a road trip I started noticing that anywhere between 10 in/Hg and atmospheric the AFRs gauge would max out to 10.0 (so actual AFR is lower). Between 20 in/Hg and 10 in/Hg (steady cruising load) the AFRs would hover properly at around 14.3-14.7. The car WAS NOT tuned to run like this. Not to mention 10 AFR anywhere under atmospheric is way too rich.

While the engine is running like this, it would idle at 11-10 AFR and if you let it idle long enough it would max out at 10 and struggle to stay on. And once I got going to cruising speed and between 20-10 in/Hg the AFRs would go back to 14.3-14.7.

I later discovered that some times, if I shut the car off and immediately turned it back on, it would go back to normal. But just a few minutes back on the road it would slowly get worse and eventually end up behaving as it did before.

I hooked up my laptop to it using nistune trial software just to see if I could find any obvious faults but it's hard to tell when I dont really know what normal behavior is. BTW, I had and cleared a typical knock sensor trouble code but these codes are know to be common on these engines. Not to mention I didnt have the rich issue before regardless of the code. I did notice that while it was running rich, the short term fuel trim would drop to max -20% over the course of a few seconds, hold -20% for another few seconds, then snap back to 0- -5%. During this time, AFR at the gauge did not change from 10. This would repeat about once a minute. I remember looking at it last year when it ran right and it was definitely not doing this. It seems like the ECU is trying to cut back fuel to hold stoichiometric but something is not letting it. While this is happening, injectors are at 2.xx ms PW (compared to 1.xx when it's idling correctly). I also compared it running with my friends S13 SR (using free consult software) and all the main readings such as MAF v, O2 v, injector PW etc, were showing similar readings and behaviors (during normal idle).

Are there any other things I can look at in nistune that could point me in the right direction? The other free consult software is pretty limited and wont let me see things like short and long term fuel trim but I can always get a new free trial for nistune.

Maybe this weekend I will record a video of the nistune software open while the car is idling rich. and post it as reference.

Let me know what you guys think.