Sudden Lean Condition in Boost?
 

Last night I took the Miata (2000, ms2pnp,2560r) to work as normal and on the way home, noticed the car running much leaner in boost than it ever has. Normally the car runs at about 11.3-11.4 once in boost, but the AFR's were creeping toward 13.0 suddenly, it wasn't random, every pull it would work its way up. In a search to find what might be the cause, I tried testing fuel pressure right off of the pump assembly just looping lines from one port to the other with a tee. I know there was fuel (key on it would prime, and the car was still getting fuel, just not enough?) but the gauge would read 0 psi(also jumped diag box to run fuel pump (could hear it). Because I don't have a kit with the quick connect lines, I couldn't find much anywhere else to put it. I then thought the fuel filter might be original from 103k miles ago, and it would be chocking the fuel up top, so I replaced that tonight, with no avail, same exact issue. No tune changes have happened to possibly cause this, so I'm pretty much ruling out tune issues. I'm just wondering where else to check before I decided to buy another pump and see what that does (easy to do and cheap, then I will have a spare if it isn't the issue?). Just looking to see if maybe anybody else has had a similar issue or if there might be other things I'm overlooking. Tomorrow will be attempting to keep my computer alive on battery enough to see if the map signal is coming through correct, and it's seeing boost. Any other ideas or recommendations would be great!

Stock fuel pump? How much power are you making? Are your injectors large enough to not worry about going over 100% duty cycle? Does it only do it at WOT or will it go lean under boost and partial throttle? Since you say every pull it creeps it's way to 13, that sounds like fuel pressure dropping out at high flow rates (high power). I'd say if it's a stock pump that'd be a good place to start, since you've already replaced the filter.

My bad, the fuel pump is a Walbro 190LPH with about 3.5k miles on it. The injectors are OEM rx8 yellows, and at around 220-230 whp, I believe they are still into their operating range. It's primarily wide open, but it seems to be staying a tad more lean in partial throttle as well. I also noticed that MS is reading a peak of about 10.5 lbs of boost, while my gauge is reading on the high sides of 11. (11.5-12). I got ahold of DIYautotune, they said my MAP and Baro sensors are correct, so it is sort of strange. I don't know what the ms was reading before as I usually don't have that gauge on the dashboard. I think its either going to be the pump or factory FPR... With no way (yet) to test fuel pressure, it's kind of a shot in the dark. The car has been running great on this tune for about 3k miles, it just randomly decided to go lean. I did notice about a month ago that the Walboro seemed to get a bit louder, but from my understanding that is pretty normal...

Drive it and post a log of the event

Update: Ordered up another fuel pump, issue was fixed. Sadly my computer randomly shuts off (need to take it back to best buy and find out why... just been procrastinating) so it makes logging extremely difficult or I would have. Often shuts off mid log and leaves me nothing. Called a few trustworthy local tuners to see what they thought and the consensus was fuel pump, and it happened to be. Thanks for the reply's guys! Wouldn't have known where to go if the pump wasn't the issue.

What fuel pump did you replace it with? 3500 miles on a walboro seems pretty disappointing.

LOL. The same thing, mainly for sake of tuning. I am in no way confident in myself or my computer that randomly shuts off to tune the car well. That and differential gear stuff are the big two things I will leave to the professionals. I don't plan on going ethanol just because there is almost none near me, so size wise it will be plenty for where I plan to go in the near future, but if it dies I will likely be going to a DW. Obviously if it dies that fast though, I will be looking for other issues first, but will still probably switch it up and see what happens! I really need to start a build tread... LOL