Engine Running Lean

Brsmustng93 said:

Need a little help with a lean condition. Recently installed a top end kit on my fox. The car is running lean at idle according to the wideband. In the area 17-18. I installed 24lb injectors, C&L maf calibrated for 24lb injectors. New fuel filter and 255lph fuel pump. I currently have the fuel pressure regulator maxed out to 55-60 psi. If I crank down the pressure to 39-40. The car runs rough & the wideband goes so lean it doesn't give an AFR. Any suggestions or possible causes?

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Dump codes sticky

Look at the top of the 5.0 Tech forum where the sticky threads are posted. One of them is how to dump the computer codes. Codes may be present even if the CEL (Check Engine Light) isn’t on. You don’t need a code reader or scanner – all you need is a paper clip, or if your lady friend has a hair pin, that will do the job.
I highly suggest that you read it and follow the instructions to dump the codes. http://www.stangnet.com/mustang-forums/threads/how-to-pull-codes-from-eec4.889006/

- improperly calibrated MAF (yes I saw what you wrote)
- turbulence in the intake tract resulting in improper reading at the MAF filament
- vacuum leak
- bad O2 sensors forcing the computer to improperly lean out mixture

I'd start with the codes, as suggested. Then, dealing with all codes, try to find a vacuum leak. I also like to have extra stuff on hand to troubleshoot. So, I personally have extra sets of O2 sensors to try out when I'm diagnosing something like this. Perhaps having an extra set is a good investment as you'll need to replace them at some point, regardless, and narrow-band O2s for stock computers are cheap. Then, I'd try "clocking" the MAF if turbulent air is possible. Finally, I'd try replacing the MAF with a known good 24 lbs MAF. I think the last one is the most likely culprit.

I left out fuel pressure because it looks like you've tested it, but it's possible that your gauge is wrong.

That's all I can think of. Good luck.