Sudden lean 17AFR, but then sudden rich 10AFR??? No codes
 

Hello all! I found a few threads that were similar to this one but none were exactly alike. Two weeks ago my car suddenly started running really lean. I was at 17.0 at idle, (+35% STFT!)and attempting to accelerate it would only drop to about 13.3 then shoot down to 11.7 at 4800 RPM which was the target. Previously this tune worked just fine for a year so I know it's not the tune.

After about a week of searching for exhaust leaks and intake leaks finding nothing, the AFR's suddenly went back to normal. Sweet! 10 minutes of driving later the AFR's suddenly shot rich...? 10AFR (-35%STFTWTH!!!) I figured it had to be one of the sensors going bad, but I really didnt know where to start, so I figured to just Band-Aid it with a tune for now. I turned off the O2 to eliminate it as a suspect by lowering the reference values (I dont know what thats called) then took out 15% fueling from the troubled areas via the MAF table which just made idle horrible but with 12.5 AFR. I took out another 10% fueling from the MAF table just to see what would happen, yeah naw misfire. Got a code for cylinder #3 misfire and #3 coil function issue.

At this point I'm figuring maybe it's just a bad coil? But why is it that the AFR's go back to hitting target right past 4800 RPM and giving me full power? At this point I switched coils 1 and 3 and I'm going to reduce fueling Again by another 15% to see if the code follows to coil number one now. While I was in there I also checked the number 3 spark plug which was not wet with fuel.

Any help would be amazing.

Current mods:

Speed Density (starts @ 20g/s, car idles at 3g/s. Still using MAF below 20g/s)

Kraftwerks c38 kit high boost pulley

Header back exhaust, mid pipe cat only

DW 300 Fuel pump

DW700cc injectors

No intake leaks, no exhaust leaks, no codes unless I mess with the fueling

Ive seen the front 02 sensors go bad and sweep full rich to full lean. Usually you can see it at idle the afr swings wildly full rich then full lean. Log the afr and the injector pulse widths see which happens first. Ie if you see afr go way rich or lean before the injector pulse widths change it likely thte 02 sensor.


Also cgeck you DI copmuter one on side of engine is well earthed with all three mount bolts if you have an aftermarket bracket worth runnong a separate earth to it

Hey Steve. It was pegged lean for a week with no sweep. I cleaned the MAF sensor, saw the STFT go a little more negative. About 3 days later I filled up the tank with gas then shot complete negative (-35%) for 5 days until I turned off the O2 sensor. AFR's stayed the same. 10.0 AFR at idle no sweep.

About the DI computer: I've actually been running it for about 4 years with only 2 points of contact. I wonder if its done with me. I have another one I can test it against. I've got a logging issue now too that I'm trying to resolve. I 'll do some more testing once I get this going again. Thanks!

I should check this. Car has been slowly running lean only at idle. As soon as I take off, LTFT and STFT's are pretty spot on with occasional bumps but nothing crazy.

Mine was a fuel computer issue. I was VERY lucky to have a spare one to try. I guess after 3 years of driving it with only two bolts holding it down, the computer fritz out. Whatever, I don't understand it, but its fixed now.