Is the wideband running 02 feedback into your ecu? This is the only way what your wideband is reading would influence the ecu fuel trims. A faulty wideband 02 sensor could play havoc on your running in this scenario. If you are not running 02 feedback and experiencing poor running, then its not the widebands fault, its merely reflecting back to you what the actual afr is in the poor running situation. I run a stand alone aem ugeo so ive never dealt with needing to calibrate it, regardless if the calibration was off, it would only effect the actual running of the engine if your ecu has 02 feedback enabled through it.

 

I can say from experience that going from an aftermarket exhaust back to stock (header, HFC all stayed the same), that I did need my tune readjusted. That is very likely the issue. Even if you have closed loop enabled in your engine management, you could be past the preset limits that it will adjust to.

 

Thanks for everyone's help. I hope I'm not returning back to this thread too soon as I think I've fixed the problem.

I believe it was a case of the old dirty IACV. I cleaned that sucker out this morning and drove it straight after, the car idled a lot better and most importantly it didn't shudder after I stepped off the gas.

I hesitantly decided to mash it in 1st and 2nd and the car pulls as hard as it did when it came off the rollers.

Fingers crossed it stays that way, thanks to all who helped out

 

Bad news everyone, the problem is still there. I think there is an issue with the tune. I cleaned the IACV again and the throttle body too this time.

The problem only occurs when the engine has reached operating temperature (e.g. when the fans come on) and the car sits for a while. So I'm leaning towards heat soak. If I let the engine run for a while the AFRs will gradually return to ~14.7:1

The good news is that it has motivated me to do all these jobs that I probably would have put off for a while (rewire fuel pump, replace fuel pump, clean throttle body/IACV, change spark plugs)

I'll report back when I get the car retuned.