wazhere219

my car is running really lean and breaking up at high rpm the afr is 13.7-14 at wot which is not ok i cant figure it out and im thinking its a sensor issue because it was out of random i replaced fuel pump fuel filter spark plugs spark plug coils and i cant figure it out sometimes when the car is sitting for awhile like 8 hours + and i get in it the afrs are perfect until its warmed up but other than that its running lean and this just happened out of random any answers???? the car is a 13b rx7 fd with pfc ecu

rotaryengineer

I'll take a stab at this... I'm by no means an expert, but I figure I can throw some ideas out there to get the creative juices flowing.

I saw your other post in which a prolific respondent mentioned that a misfire can show up as a lean spike. That may actually be a very helpful if not vague hint as to what might be going on. A lean spike due to a misfire results from the fact that the O2 sensor is measuring excess oxygen. Any time there is a misfire there is excess oxygen in the exhaust stream, regardless of how much unburned fuel there is (which the O2 sensor doesn't respond to). That being said, consider what might cause a misfire, or manifest itself as a misfire at high RPM / high boost:

0) MAP sensor - Since your problem happens only at high RPM / high power, I doubt it is the MAP sensor, otherwise you'd be having issues across the board.
1) over-boost (fuel cut). You can search the forums for fuel cut parameters - I can't recall if you said in your other post whether you are running the stock ECU and/or mods, but this kind of information is helpful when asking for help on something like this.
2) ignition failure. Not sure if this is a typical problem on FDs but if a coil is going bad, it will typically begin to lose spark at the high end. I vaguely recall this has something to do with the ability of the spark to jump the gap in these conditions within the combustion chamber, i.e. if the coil isn't generating high enough voltage it will apparently work fine at low power / low RPM, but tend to cut out at the top end (because more voltage is required to generate spark in these conditions).
3) it may actually be leaning out, and the only thing I can think of in that case is loss of fuel flow/pressure. As the other posters were hinting at, not only fuel pump issues, but any problem within the fuel system could cause a lean condition at the high end: a) undersized injectors (i.e. using stock injectors with more than 14psi or so boost or midpipe or other heavy mods) b) dirty filters c) fuel pressure regulator? d) fuel pressure overall (tired fuel pump, wiring problems, etc) e) fuel pulsation damper (known to fail prematurely) f) dirty, weak or failing injectors?

I believe I saw somewhere in the Service Highlights book the fuel cut curve. If you go to foxed and click on the link for 1993 service highlights (doesn't matter what year your car is - its all good info), you will get a popup with two links. the top link will take you to TSBs, but the bottom link will get you this goldmine of information.

Good luck, and I hope this helps.
TB